Detecting two-party quantum correlations in quantum-key-distribution protocols

M Curty, O Guhne, M Lewenstein, N Lutkenhaus

A necessary precondition for secure quantum key distribution is that sender and receiver can prove the presence of entanglement in a quantum state that is effectively distributed between them. In order to deliver this entanglement proof one can use the class of entanglement witness (EW) operators that can be constructed from the available measurements results. This class of EWs can be used to provide a necessary and sufficient condition for the existence of quantum correlations even when a quantum state cannot be completely reconstructed. The set of optimal EWs for two well-known entanglement-based (EB) schemes, the six-state and the four-state EB protocols, has been obtained recently [M. Curty , Phys. Rev. Lett. 92, 217903 (2004).] Here we complete these results, now showing specifically the analysis for the case of prepare and measure (PM) schemes. For this, we investigate the signal states and detection methods of the four-state and the two-state PM schemes. For each of these protocols we obtain a reduced set of EWs. More importantly, each set of EWs can be used to derive a necessary and sufficient condition to prove that quantum correlations are present in these protocols.

Photonic-crystal fibers

Philip St. J. Russell

The history, fabrication, theory, numerical modeling, optical properties, guidance mechanisms, and applications of photonic-crystal fibers are reviewed.

Cavity enhanced absorption spectroscopy of multiple trace gas species using a supercontinuum radiation source

J. M. Langridge, T. Laurila, R. S. Watt, R. L. Jones, C. F. Kaminski, J. Hult

Supercontinuum radiation sources are attractive for spectroscopic applications owing to their broad wavelength coverage, which enables spectral signatures of multiple species to be detected simultaneously. Here we report the first use of a supercontinuum radiation source for broadband trace gas detection using a cavity enhanced absorption technique. Spectra were recorded at bandwidths of up to 100 nm, encompassing multiple absorption bands of H2O, O-2 and O-2-O-2. The same instrument was also used to make quantitative measurements of NO2 and NO3. For NO3 a detection limit of 3 parts-per-trillion in 2 s was achieved, which corresponds to an effective 3 sigma sensitivity of 2.4 x 10(-9) cm(-1)Hz(-1/2). Our results demonstrate that a conceptually simple and robust instrument is capable of highly sensitive broadband absorption measurements. (C) 2008 Optical Society of America.

Demonstration of a Quantum Nondemolition Sum Gate

Jun-ichi Yoshikawa, Yoshichika Miwa, Alexander Huck, Ulrik L. Andersen, Peter van Loock, Akira Furusawa

The sum gate is the canonical two-mode gate for universal quantum computation based on continuous quantum variables. It represents the natural analogue to a qubit C-NOT gate. In addition, the continuous-variable gate describes a quantum nondemolition (QND) interaction between the quadrature components of two light modes. We experimentally demonstrate a QND sum gate, employing the scheme by R. Filip, P. Marek, and U. L. Andersen [Phys. Rev. A 71, 042308 (2005)], solely based on off-line squeezed states, homodyne measurements, and feedforward. The results are verified by simultaneously satisfying the criteria for QND measurements in both conjugate quadratures.

The effect of dissipation on nonclassical states of the radiation field

G Leuchs, UL Andersen

We point out similarities in the evolution of different types of nonclassical light fields. Generally, Fock and cat states are considered to decay much faster under dissipation than do squeezed states. We connect measurements of the intensity correlation function of nonclassical light to the second moment of the photon number distribution function. It is shown that all these nonclassical fields behave in a similar manner when one is looking at an appropriate property. Finally it is shown how the electronic-detection noise floor, typically present in measurements of the variance of a photocurrent, can be eliminated based upon a procedure that is well-established for intensity correlation measurements.

Observation of optomechanical multistability in a high-Q torsion balance oscillator

F. Mueller, S. Heugel, L. J. Wang

We observe the optomechanical multistability of a macroscopic torsion balance oscillator. The torsion oscillator forms the moving mirror of a hemispherical laser light cavity. When a laser beam is coupled into this cavity, the radiation pressure force of the intracavity beam adds to the torsion wire's restoring force, forming an optomechanical potential. In the absence of optical damping, up to 23 stable trapping regions were observed due to local light potential minima over a range of 4 mu m oscillator displacement. Each of these trapping positions exhibits optical spring properties. Hysteresis behavior between neighboring trapping positions is also observed. We discuss the prospect of observing optomechanical stochastic resonance, aiming at enhancing the signal-to-noise ratio (SNR) in gravity experiments.

Transmission properties of selectively gold-filled polarization-maintaining PCF

H. W. Lee, M. A. Schmidt, H. Tyagi, L. Prill Sempere, P. St. J. Russell

We report on the optical properties of a polarization-preserving PCF in which two enlarged hollow channels on opposite sides of the core are filled with gold. Surface plasmon resonances and intriguing polarisation effects are observed. (C)2008 Optical Society of America

Narrow linewidth light source for an ultraviolet optical frequency standard

T. Liu, Y. H. Wang, R. Dumke, A. Stejskal, Y. N. Zhao, J. Zhang, Z. H. Lu, L. J. Wang, Th. Becker, H. Walther, et al.

We report an ultra-narrow linewidth light source applicable for a frequency standard in the ultraviolet. The laser is a Nd:YAG laser that emits at 946 nm with 300-mW output power. It is locked to a high-finesse cavity. The minimum Allan deviation is 1.3 x 10(-14) for an integration time of 1 s, which corresponds to a laser linewidth of 2.8 Hz. The cavity drift is measured by a frequency comb and a single-ion spectrum for different time scales. In order to investigate broadening mechanisms due to the fiber transport and doubling systems, the laser light is frequency doubled with two independent systems and compared. The measured minimum beat-note between the two laser fields is less than 1 Hz. By carrying out a high-resolution scan on a trapped single indium ion, we observe a linewidth of 260 Hz on the ion clock transition. Possible reasons for the broadening effects are discussed.

3.9-dB OSNR gain by an NOLM-based 2-R regenerator

M Meissner, K Spionsel, K Cvecek, A Benz, S Weisser, B Schmauss, G Leuchs

We experimentally demonstrate bit-error-rate (BER) improvement of more than nine decades by an asymmetric nonlinear optical loop mirror (NOLM). This can be related to an optical signal-to-noise ratio (OSNR) gain of up to 3.9 dB with respect to the NOLM input OSNR at a bit rate of 40 GB/s. The principle of operation of NOLM-based 2-R regeneration with respect to BER improvement is investigated and it is experimentally shown that BER improvement cannot be detected directly at the regenerator's output.

Atomic beam focusing with a curved magnetic mirror

H. Merimeche

Reflective atom optics has been experimentally realized with a continuous flux of cold atoms by using a magnetized videotape. We have studied the behaviour of a slow atomic beam with a velocity of 20 m s(-1) incident at different angles on the surface of a curved magnetic mirror. Focusing of the beam has been demonstrated and this induces an amplification up to a factor 3 of the fluorescence emitted by the atoms around the focal point. We deduced the shape of our reflector from the focal length measurements and show that the result is completely consistent with those obtained from the reflection of a red diode laser beam on the curved mirror.

Experimental purification of coherent states

Ulrik Andersen, R Filip, J Fiurasek, V Josse, Gerd Leuchs

We propose a scheme for optimal Gaussian purification of coherent states from several imperfect copies. The proposal is experimentally demonstrated for the case of two copies of a coherent state sent through independent noisy channels. Our purification protocol relies on only linear optics and an ancilla vacuum state, rendering this approach an interesting alternative to the more complex protocols of entanglement distillation and quantum error correction.

Polarization properties of PCF with Ge-nanowire

M. A. Schmidt, H. Tyagi, L. Prill Sempere, P. St. J. Russell

A broad-band in-fiber polarizer with suppression >25dB over 300nm bandwidth is reported. It is made by introducing a high quality Ge nanowire into one of the hollow channels of a photonic crystal fiber. (C)2008 Optical Society of America

Radiation Pressure Force Measurement at the Thermal Noise Limit

F. Mueller, S. Heugel, L. J. Wang

We report radiation force measurements to a precision of 100 femto-Newton using a high Q torsion balance oscillator. The opto-mechanically coupled oscillator can be cooled down to a temperature of 300 milli-Kelvin. (C) 2007 Optical Society of America

Models for guidance in kagome-structured hollow-core photonic crystal fibres

G. J. Pearce, G. S. Wiederhecker, C. G. Poulton, S. Burger, P. St. J. Russell

We demonstrate by numerical simulation that the general features of the loss spectrum of photonic crystal fibres (PCF) with a kagome structure can be explained by simple models consisting of thin concentric hexagons or rings of glass in air. These easily analysed models provide increased understanding of the mechanism of guidance in kagome PCF, and suggest ways in which the high-loss resonances in the loss spectrum may be shifted. (C) 2007 Optical Society of America.

Spectral shifts of general beams in turbulent media

H. T. Eyyuboglu, Y. Cai, Y. K. Baykal

Using the concept of generalized beam, by applying the extended Huygens-Fresnel principle, we derive the spectrum for partially coherent cosh-Gaussian, cos-Gaussian, Gaussian and annular Gaussian beams propagating in turbulent media. From this formulation, graphical outputs are constructed illustrating the variation of receiver plane spectra against propagation distance, turbulence strength, degree of partial coherence and transverse coordinate. The dependence of spectral shifts on source and propagation parameters is observed. Spectrum invariance conditions are deduced analytically and discussed for the particular case of a fundamental Gaussian beam.

Reduction from Eu3+ to Eu2+ in BaAl2O4 : Eu phosphor prepared in an oxidizing atmosphere and luminescent properties of BaAl2O4 : Eu

Mingying Peng, Guangyan Hong

A reduction phenomenon of Eu3+ -> Eu2+ was observed for the first time when Eu3+ ions were doped into an AlO4-tetrahedron-containing compound BaAl2O4 in an oxidizing atmosphere of air by high-temperature solid-state reaction. X-ray powder diffraction patterns and photoluminescent spectra are used to confirm the compound structure and detect the simultaneous existence of both divalent and trivalent europium ions, respectively. The abnormal Eu3+ -> Eu2+ reduction is explained by a charge compensation model. Spectroscopic properties of BaAl2O4:Eu are discussed and Eu2+ emission spectrum shows consistence with the results reported by Katsumata et a]. [J. Cryst. Growth 198/199 (1999) 869.] and Lin et al. [Mater. Chem. Phys. 70 (2001) 156.]. (C) 2007 Elsevier B.V. All rights reserved.

Waveguiding and plasmon resonances in two-dimensional photonic lattices of gold and silver nanowires

M. A. Schmidt, L. N. Prill Sempere, H. K. Tyagi, C. G. Poulton, P. St. J. Russell

We report the fabrication of triangular lattices of parallel gold and silver nanowires of high optical quality, with diameters down to 500 nm and length-to-diameter ratios as high as 100 000. The nanowires are supported by a silica glass matrix and are disposed around a central solid glass core, i.e., a missing nanowire. These centimeter-long structures make it possible to trap light within an array of nanowires and characterize the plasmon resonances that form at specific optical frequencies. Such nanowire arrays have many potential applications, e.g., imaging on the subwavelength scale.

Coherent-state information concentration and purification in atomic memory

Jiri Herec, Radim Filip

We propose a feasible method of coherent-state information concentration and purification utilizing quantum memory. The method allows us to optimally concentrate and purify information carried by many noisy copies of an unknown coherent state (randomly distributed in time) to a single copy. Thus nonclassical resources and operations can be saved, if we compare information processing with many noisy copies and a single copy with concentrated and purified information.

2R-Regeneration of an 80-Gb/s RZ-DQPSK signal by a nonlinear amplifying loop mirror

K. Cvecek, K. Sponsel, R. Ludwig, C. Schubert, C. Stephan, G. Onishchukov, B. Schmauss, G. Leuchs

The performance of a nonlinear amplifying loop mirror as a 2R-regenerator for an 80-Gb/s return-to-zero differential-quadrature-phase-shift-keyed signal has been investigated. experimentally. A significant eye-opening improvement and a negative power penalty of up to 2.6 dB were obtained.

A periodic structure mimics a metamaterial

Carsten Rockstuhl, Ulf Peschel, Falk Lederer

We show that properties attributed to planar metamaterials that support resonances due to an appropriately shaped unit cell can also be identified in a medium that exhibits a resonance evoked by its period only. By choosing a subwavelength period, the effective material parameters of such a medium can be retrieved from the complex reflected and transmitted amplitude. The parameters exhibit Lorentzian line shapes in the spectral vicinity of the resonances associated with the period. If this material is stacked to form a three-dimensional medium, a stop gap is observed in transmission in the frequency range where the real part of one effective material parameter becomes negative. The resonance at the origin of the response is related to the excitation of a higher-order Bloch mode. Because their negligible excitation is a prerequisite for deriving effective material parameters, the analyzed structure mimics only the response of a metamaterial but cannot be regarded as a metamaterial. (c) 2007 Optical Society of America.

Experimental Demonstration of Continuous Variable Entanglement Distillation

R. F. Dong, M. Lassen, Ch. Marquardt, R. Filip, U. L. Andersen, G. Leuchs

We experimentally demonstrate distillation of continuous variable entanglement through an attenuating quantum channel. Through a simple protocol based entirely on linear optics and homodyne detection we probabilistically filter out a highly entangled state from a less non-entangled mixed state. (C) 2008 Optical Society of America

Using a quantum well heterostructure to study the longitudinal and transverse electric field components of a strongly focused laser beam

G. Kihara Rurimo, M. Schardt, S. Quabis, S. Malzer, Ch. Dotzler, A. Winkler, G. Leuchs, G. H. Dohler, D. Driscoll, M. Hanson, et al.

We report a method to measure the electric energy density of longitudinal and transverse electric field components of strongly focused polarized laser beams. We used a quantum well photodetector and exploited the polarization dependent optical transitions of light holes and heavy holes to probe the electric field distribution in the focal region. A comparison of the measured photocurrent spectra for radially and azimuthally polarized beams at the light and heavy hole absorption peaks provides a measure of the amount of the longitudinal electric field component. (c) 2006 American Institute of Physics.

2R-regeneration of an RZ-DPSK signal using a nonlinear amplifying loop mirror

K. Cvecek, K. Sponsel, G. Onishchukov, B. Schmauss, G. Leuchs

The performance of a nonlinear amplifying loop mirror as a 2R-regenerator for return-to-zero differential-phase-shift-keyed signals has been investigated experimentally. The measured power characteristics and phase functions show that the signal amplitude is regenerated while the signal phase is preserved in the setup. A significant eye-opening improvement and a negative power penalty of about 1.5 dB were obtained.

Velocity of heat dissipative solitons in optical fibers

A. Ankiewicz, Wenjing Chen, P. St. J. Russell, M. Taki, N. Akhmediev

In the fiber fuse, a pulse of high temperature travels toward the input end of the fiber, where high-power laser light is launched into the fiber. At any point along the fiber, the soliton can be ignited. The fiber core is damaged in the process so that light cannot propagate beyond the hot spot. This phenomenon is an example of a dissipative soliton that can exist only in the presence of an external energy supply and internal loss, We analyze this phenomenon, derive an expression for the velocity of the soliton, and determine its width as functions of the physical parameters of the laser and the fiber material. (C) 2008 Optical Society of America

Quantum repeaters using coherent-state communication

Peter van Loock, Norbert Lutkenhaus, W. J. Munro, Kae Nemoto

We investigate quantum repeater protocols based upon atomic qubit-entanglement distribution through optical coherent-state communication. Various measurement schemes for an optical mode entangled with two spatially separated atomic qubits are considered in order to nonlocally prepare conditional two-qubit entangled states. In particular, generalized measurements for unambiguous state discrimination enable one to completely eliminate spin-flip errors in the resulting qubit states, as they would occur in a homodyne-based scheme due to the finite overlap of the optical states in phase space. As a result, by using weaker coherent states, high initial fidelities can still be achieved for larger repeater spacing, at the expense of lower entanglement generation rates. In this regime, the coherent-state-based protocols start resembling single-photon-based repeater schemes.

Efficient terahertz emission from ballistic transport enhanced n-i-p-n-i-p superlattice photomixers

S. Preu, F. H. Renner, S. Malzer, G. H. Doehler, L. J. Wang, M. Hanson, A. C. Gossard, T. L. J. Wilkinson, E. R. Brown

The authors report on photomixing terahertz sources that overcome the transit time versus RC-time trade-off and allow for independent optimization of both of them, using a n-i-p-n-i-p superlattice. Furthermore, they take advantage of ballistic transport for reduced transit times. Apart from more favorable material parameters, In(Al)GaAs photomixers benefit from the advanced telecommunication laser technology around 1.55 mu m as compared to GaAs. In such devices, a terahertz-power output of 1 mu W has been achieved at 0.4 THz at a photocurrent of 3.8 mA. A comparison between corresponding GaAs- and InGaAs-based n-i-p-n-i-p photomixers reveals an improvement of performance by at least an order of magnitude for the latter one. (c) 2007 American Institute of Physics.

Radiation modes and roughness loss in high index-contrast waveguides

Christopher G. Poulton, Christian Koos, Masafumi Fujii, Andreas Pfrang, Thomas Schimmel, Juerg Leuthold, Wolfgang Freude

We predict the scattering loss in rectangular high index-contrast waveguides, using a new variation of the classical approach of coupled-mode theory. The loss predicted by this three-dimensional (3-D) model is considerably larger than that calculated using previous treatments that approximate the true 3-D radiation modes with their two-dimensional counterparts. The 3-D radiation modes of the ideal waveguide are expanded in a series of cylindrical harmonics, and the coupling between the guided and radiation modes due to the sidewall perturbation is computed. The waveguide attenuation can then be calculated semianalytically. It is found that the dominant loss mechanism is radiation rather than reflection, and that the transverse electric polarization exhibits much larger attenuation than transverse magnetic polarization. The method also gives simple rules that can be used in the design of low-loss optical waveguides. The structural properties of sidewall roughness of an InGaAs/InP pedestal waveguide are measured using atomic force microscopy, and the measured attenuation is found to compare well with that predicted by the model.

Experimental investigation of a modified NOLM for phase-encoded signal regeneration

K. Cvecek, G. Onishchukov, K. Sponsel, A. G. Striegler, B. Schmauss, G. Leuchs

We experimentally investigate the amplitude and phase transfer characteristics of a modified nonlinear optical loop mirror (NOLM) with a directional attenuator (DA-NOLM) optimized for differential phase-shift keying signal regeneration. The results show that the phase relation is preserved in the setup and thus the DA-NOLM is suitable for amplitude regeneration of phase-shift-keyed signals.

Temperature dependence of indirect-exciton luminescence in in-plane magnetic field

M. Orlita, G. H. Doehler, R. Grill, P. Hlidek, S. Malzer, J. Prochazka, M. Zvara

We report on a magneto-luminescence on a double quantum well subject to an in-plane magnetic field. The attention is paid to the properties of interwell excitons, which are indirect in the real space and which become indirect in the reciprocal space as well when a finite in-plane magnetic field is applied. Such indirect exciton states become optically inactive unless some relaxation mechanisms of their momentum appear. The experiment is carried out on a sample where, as reported previously, the radiative recombination of indirect excitons is possible due to their localization or via collisions with structural defects. The experimental data presented here, measured at various temperatures, favour the latter mechanism which is less sensitive to the system temperature in comparison with the former one. (c) 2008 Elsevier B.V. All rights reserved.

Modified hollow Gaussian beam and its paraxial propagation

Yangjian Cai, Chiyi Chen, Fei Wang

A model named modified hollow Gaussian beam (HGB) is proposed to describe a dark hollow beam with adjustable beam spot size, central dark size and darkness factor. In this modified model, both the beam spot size and the central dark size will be convergent to finite constants as the beam order approaches infinity, which are much different from that of the previous unmodified model, where the beam spot size and the central dark size will not be convergent as the beam order approaches infinity. The dependences of the propagation factor of modified and unmodified HGBs on the beam order are found to be the same. Based on the Collins integral, analytical formulas for the modified HGB propagating through aligned and misaligned optical system are derived. Some numerical examples are given. (c) 2007 Elsevier B.V. All rights reserved.

Coherent Control of Ultra-High Frequency Acoustic Resonances in Photonic Crystal Fibers

G. S. Wiederhecker, A. Brenn, H. L. Fragnito, P. St. J. Russell

Acoustic resonances trapped within the glass core (1 mu m diameter) of a photonic crystal fibre are excited electrostrictively using laser pulses. Using pulse sequences they can be coherently controlled leading to a 100-fold increase in their amplitude. (C) 2008 Optical Society of America

Absolute calibration in grazing incidence interferometry via rotational averaging

Klaus Mantel, Juergen Lamprecht, Norbert Lindlein, Johannes Schwider

Interferometry in grazing incidence can be used to test cylindrical mantle surfaces. The absolute accuracy of the resulting surface profiles is limited by systematic wavefront aberrations caused in the interferometer, in particular due to an inversion of the test wavefront in an interferometer using diffractive beam splitters. For cylindrical specimens, a calibration method using four positions has therefore been investigated. This test is combined with another method of optical metrology: the rotational averaging procedure. The implementation for grazing incidence is described and measurement results for hollow cylinders are presented. The gain in accuracy is demonstrated. (c) 2006 Optical Society of America.

Optical excitation and characterization of gigahertz acoustic resonances in optical fiber tapers

Myeong Soo Kang, Andre Brenn, Gustavo S. Wiederhecker, Philip St. J. Russell

Transverse acoustic resonances at gigahertz frequencies are excited by electrostriction in the few-micrometer-thick waists of low-loss optical fiber tapers of up to 40 cm long. A pump-probe technique is used in which the resonances are excited by a train of optical pulses and probed in a Sagnac interferometer. Strong radially symmetric acoustic resonances are observed and the dependence of their frequencies on taper thickness is investigated. Such easily reconfigurable acousto-optic interactions may have applications in the high-frequency mode locking of fiber lasers. (C) 2008 American Institute of Physics.

Verifying continuous-variable entanglement of intense light pulses

O Glockl, UL Andersen, G Leuchs

Three different methods have been discussed to verify continuous variable entanglement of intense light beams. We demonstrate all three methods using the same setup to facilitate the comparison. The nonlinearity used to generate entanglement is the Kerr effect in optical fibers. Due to the brightness of the entangled pulses, standard homodyne detection is not an appropriate tool for the verification. However, we show that by using large asymmetric interferometers on each beam individually, two noncommuting variables can be accessed and the presence of entanglement verified via joint measurements on the two beams. Alternatively, we witness entanglement by combining the two beams on a beam splitter that yields certain linear combinations of quadrature amplitudes which suffice to prove the presence of entanglement.

Nonunity gain quantum nondemolition measurements based on measurement and repreparation

Jessica Schneider, Oliver Gloeckl, Gerd Leuchs, Ulrik L. Andersen

We demonstrate experimentally a nonunity gain quantum nondemolition measurement based on a simple homodyne measurement and recreation strategy. Although the output state is an amplified version of the input state, the device meets standard criteria for QND measurements: the transfer coefficient was measured to 1,78, and the conditional variance was measured to 0.66. (c) 2006 Optical Society of America.

Waveguide properties of single subwavelength holes demonstrated with radially and azimuthally polarized light

J. Müller, Peter Banzer, Susanne Quabis, Ulf Peschel, Gerd Leuchs

We investigate the transmission of focused beams through single subwavelength holes in a silver film. We use radially and azimuthally polarized light to excite higher-order waveguide modes as well as to match the radial symmetry of the aperture geometry. Remarkably, the transmission properties can be described by a classical waveguide model even for thicknesses of the silver film as thin as a quarter of a wavelength.

Polarization optimized focusing of light and coupling to sub-wavelength antennae

Gerd Leuchs, Susanne Quabis

This chapter describes the polarization optimized focusing of light and coupling to sub-wavelength antennae. In a set-up for focusing of light, the lens system transforms the transverse field distribution at the input into the transverse field distribution at the focal plane. The parameters of the input beam are the transverse intensity and phase distribution as well as its state of polarization. The various optical rays directed by the lens to the focal spot typically carry different polarizations even if the input beam has a homogeneous polarization distribution across the beam. Locally the polarization is linear everywhere but the direction of the electric field vector depends on the transverse coordinates within the cross section of the beam in such a way that the electric field vectors oscillate in the radial direction. In the limit of high numerical aperture, focusing the field distribution at and near the focus is calculated using the Debye approximation. The effects of the vector properties of light on the structure of the focus are best observed if two special input beams are compared with identical intensity distribution but different polarization properties

Demonstration of the spatial separation of the entangled quantum sidebands of an optical field

EH Huntington, GN Milford, C Robilliard, TC Ralph, O Glockl, Ulrik L. Andersen, S Lorenz, Gerd Leuchs

Quantum optics experiments on "bright" beams are based on the spectral analysis of field fluctuations and typically probe correlations between radio-frequency sideband modes. However, the extra degree of freedom represented by this dual-mode picture is generally ignored. We demonstrate the experimental operation of a device which can be used to separate the quantum sidebands of an optical field. We use this device to explicitly demonstrate the quantum entanglement between the sidebands of a squeezed beam.

Degree of polarization for partially coherent general beams in turbulent atmosphere

H. T. Eyyuboglu, Y. Baykal, Y. Cai

The degree of polarization is found for optical excitations of cosh-Gaussian, cos-Gaussian and annular-Gaussian beams in a turbulent atmosphere. The related formulation is based on the beam coherence polarization matrix. The self and mutual coherence functions appearing in the beam coherence polarization matrix are evaluated, when the above mentioned excitations exhibit partial source coherence for self and cross fields. Plots showing the variation of the degree of polarization are provided versus the propagation length when the source size, displacement parameter, structure constant and the degree of source coherence for self and cross fields change.

Active laser radar systems with stochastic electromagnetic beams in turbulent atmosphere

Yangjian Cai, Olga Korotkova, Halil T. Eyyuboglu, Yahya Baykal

Propagation of stochastic electromagnetic beams through paraxial ABCD optical systems operating through turbulent atmosphere is investigated with the help of the ABCD matrices and the generalized Huygens-Fresnel integral. In particular, the analytic formula is derived for the cross-spectral density matrix of an electromagnetic Gaussian Schell-model (EGSM) beam. We applied our analysis for the ABCD system with a single lens located on the propagation path, representing, in a particular case, the unfolded double-pass propagation scenario of active laser radar. Through a number of numerical examples we investigated the effect of local turbulence strength and lens' parameters on spectral, coherence and polarization properties of the EGSM beam. (C) 2008 Optical Society of America

Dead time correction in coincidence counting of photon pairs

M. S. Kang, D-H Lee, J. Lee, J. Y. Lee, S-K Choi, H. S. Park

We describe two methods for evaluating the dead time of a time-to-amplitude converter (TAC). The dead time is obtained by measuring either the corresponding time interval in an oscilloscope trace or the relation between the single count rate and the coincidence count rate. Values for the TAC dead time are obtained in the range from 3.4 mu s to 14.3 mu s for the two methods with respective standard uncertainties of 2.9 x 10(-8) s and 3.3 x 10(-9) s. The TAC dead time is applied to the calibration of coincidence-counting measurements of optical transmission and photon-heralding efficiency.

Photonic crystal fibres: A new era in the control of light

P. St. J. Russell

Through their unique properties - often offering orders of magnitude improvement over previous technologies - photonic crystal fibres are giving rise to numerous new applications spanning many areas of science.

Propagation properties of anomalous hollow beams in a turbulent atmosphere

Yangjian Cai, Halil T. Eyyuboglu, Yahya Baykal

Propagation of circular and elliptical anomalous hollow beams in a turbulent atmosphere is investigated in detail. Based on the extended Huygens-Fresnel integral, analytical formulae for the average irradiance of circular and elliptical anomalous hollow beams propagating in a turbulent atmosphere are derived. The irradiance and spreading properties of circular and elliptical anomalous hollow beams in a turbulent atmosphere and in free space are studied numerically. It is found that circular and elliptical anomalous hollow beams at short propagation distance in turbulent atmosphere have similar propagation properties to those of free space, while at long propagation distance, circular and elliptical anomalous hollow beams eventually become circular Gaussian beams in a turbulent atmosphere, which is much different from their propagation properties in free space. The conversion from an anomalous hollow beam to a circular Gaussian beam becomes quicker and the beam spot spreads more rapidly for a larger structure constant, a shorter wavelength and a smaller waist size of the initial beam. (C) 2008 Elsevier B.V. All rights reserved.

Spectrally smooth supercontinuum from 350 nm to 3 mu m in sub-centimeter lengths of soft-glass photonic crystal fibers.

FG Omenetto, NA Wolchover, MR Wehner, M Ross, A Efimov, AJ Taylor, VVRK Kumar, AK George, JC Knight, NY Joly, et al.

The conversion of light fields in photonic crystal fibers ( PCFs) capitalizes on the dramatic enhancement of several optical nonlinearities. We present here spectrally smooth, highly broadband supercontinuum radiation in a short piece of high-nonlinearity soft-glass PCF. This supercontinuum spans several optical octaves, with a spectral range extending from 350 nm to beyond 3000 nm. The selection of an appropriate propagation-length determines the spectral quality of the supercontinuum generated. Experimentally, we clearly identify two regimes of nonlinear pulse transformation: when the fiber length is much shorter than the dispersion length, soliton propagation is not important and a symmetric supercontinuum spectrum arises from almost pure self-phase modulation. For longer fiber lengths the supercontinuum is formed by the breakup of multiple Raman-shifting solitons. In both regions very broad supercontinuum radiation is produced. (c) 2006 Optical Society of America.

Diamagnetic shift of disorder-localized excitons in narrow GaAs/AlGaAs quantum wells

M. Erdmann, C. Ropers, M. Wenderoth, R. G. Ulbrich, S. Malzer, G. H. Doehler

A correlation between the diamagnetic shift and transition energy of disorder-localized excitons is observed in magnetomicrophotoluminescence (mu PL) on narrow GaAs/Al0.3Ga0.7As quantum wells (QW's). The QW's were grown by molecular-beam epitaxy without growth interruption at the interfaces. mu PL spectra were obtained in a confocal setup with the magnetic field applied normal to the QW plane. The lowest-energy exciton states have the smallest diamagnetic coefficients; the exciton diamagnetic shift in the localized exciton tail of the QW emission spectra increases by a factor of 2 as a function of transition energy. The positive correlation between diamagnetic shift and emission energy reveals exciton localization by short-range correlated interface disorder.

Atomic scale structure and optical emission of AlxGa1-xAs/GaAs quantum wells

C. Ropers, M. Wenderoth, L. Winking, T. C. G. Reusch, M. Erdmann, R. G. Ulbrich, M. Grochol, F. Grosse, R. Zimmermann, S. Malzer, et al.

A combined study of the optical and structural properties of AlGaAs/GaAs quantum wells is presented. Microphotoluminescence experiments, magnetomicrophotoluminescence, and atomically resolved cross-sectional scanning tunneling microscopy were performed on the same quantum well sample. Constant-current topographs with aluminum and/or gallium sensitivity are used to directly extract disorder potentials. Using these potentials, exciton absorption spectra, microphotoluminescence spectra, and diamagnetic shifts of individual exciton states are calculated in an envelope function approximation. Very good agreement between the theoretical and experimental results is found.

Photonic molecules in the Terahertz - Mode splitting in coupled dielectric whispering gallery mode resonators

S. Preu, H. G. L. Schwefel, S. Malzer, G. H. Doehler, L. J. Wang, M. Hanson, J. D. Zimmerman, A. C. Gossard

We present an investigation of photonic molecules in the (sub-) Terahertz range, We couple two dielectric whispering gallery mode resonators with matched resonance spectra over tens of resonances. The (loaded) quality (Q)-factors of the individual disks range from 50 - 800. We observe mode splitting for all modes in the range of 0. 15 - 0.33 (0.29) THz for a set of polyethylene (quartz) disks with radius = 3 mm ( 1.8 mm). With increasing distance of the resonators, the splitting decreases due to reduced mode overlap, in very good agreement with numerical calculations. The frequency splitting, of different modes is also in good agreement with the numerical calculation.

Experimental entanglement distillation of mesoscopic quantum states

Ruifang Dong, Mikael Lassen, Joel Heersink, Christoph Marquardt, Radim Filip, Gerd Leuchs, Ulrik L. Andersen

The distribution of entangled states between distant parties in an optical network is crucial for the successful implementation of various quantum communication protocols such as quantum cryptography, teleportation and dense coding(1-3). However, owing to the unavoidable loss in any real optical channel, the distribution of loss-intolerant entangled states is inevitably afflicted by decoherence, which causes a degradation of the transmitted entanglement. To combat the decoherence, entanglement distillation, a process of extracting a small set of highly entangled states from a large set of less entangled states, can be used(4-14). Here we report on the distillation of deterministically prepared light pulses entangled in continuous variables that have undergone non-Gaussian noise. The entangled light pulses(15-17) are sent through a lossy channel, where the transmission is varying in time similarly to light propagation in the atmosphere. By using linear optical components and global classical communication, the entanglement is probabilistically increased.

Design of a mode converter for efficient light-atom coupling in free space

M. Sondermann, R. Maiwald, H. Konermann, N. Lindlein, U. Peschel, G. Leuchs

In this article, we describe how to develop a mode converter that transforms a plane electromagnetic wave into an inward-moving dipole wave. The latter one is intended to bring a single atom or ion from its ground state to an excited state by absorption of a single photon wave packet with near-100% efficiency.

Stimulated Brillouin scattering from multi-GHz-guided acoustic phonons in nanostructured photonic crystal fibres

P. Dainese, P. St. J. Russell, N. Joly, J. C. Knight, G. S. Wiederhecker, H. L. Fragnito, V. Laude, A. Khelif

Wavelength-scale periodic microstructuring dramatically alters the optical properties of materials. An example is glass photonic crystal fibre(1) ( PCF), which guides light by means of a lattice of hollow micro/nanochannels running axially along its length. In this letter, we explore stimulated Brillouin scattering in PCFs with subwavelength-scale solid silica glass cores. The large refractive-index difference between air and glass allows much tighter confinement of light than is possible in all-solid single-mode glass optical fibres made using conventional techniques. When the silica-air PCF has a core diameter of around 70% of the vacuum wavelength of the launched laser light, we find that the spontaneous Brillouin signal develops a highly unusual multi-peaked spectrum with Stokes frequency shifts in the 10-GHz range. We attribute these peaks to several families of guided acoustic modes each with different proportions of longitudinal and shear strain, strongly localized to the core(2,3). At the same time, the threshold power for stimulated Brillouin scattering(4) increases fivefold. The results show that Brillouin scattering is strongly affected by nanoscale microstructuring, opening new opportunities for controlling light-sound interactions in optical fibres.

Numerical study of guided modes in arrays of metallic nanowires

C. G. Poulton, M. A. Schmidt, G. J. Pearce, G. Kakarantzas, P. St. J. Russell

We numerically investigate the band structure and guided modes within arrays of metallic nanowires. We show that bandgaps appear for a range of array geometries and that these can be used to guide light in these structures. Values of attenuation as low as 1.7 dB/cm are predicted for arrays of silver wires at communications wavelengths. This is more than 100 times smaller than the attenuation of the surface plasmon polariton modes on a single silver nanowire. (c) 2007 Optical Society of America.

THz collective oscillations of ballistic electrons in wide potential wells: Bridging classical transport with quantum dynamics

M Eckardt, M Betz, A Schwanhausser, S Trumm, F Sotier, L Robledo, S Malzer, S Muller, K Unterrainer, A Leitenstorfer, et al.

Parabolic shaped potential wells of a width between 120 nm and 250 nm are defined in a band gap engineered p(+)-n-p(+) heterostructure. After femtosecond photoinjection of carriers near the boundary of the well, electrons are found to coherently oscillate across the well with the classical harmonic-oscillator frequency of a few THz instead of performing the intuitively expected unidirectional relaxation towards the bottom of the well. Most strikingly, the coherence of this periodic electron motion is maintained despite multiple phonon scattering events. This novel transport regime is predicted by detailed Monte Carlo simulations and verified by analyzing the femtosecond transmission of the heterostructure as well as by directly detecting the THz radiation emitted by the oscillating electron-hole dipole. By decreasing the well width, this semi-classical ballistic transport regime is expected to converge towards a quantum-beat regime.

Non-reciprocal transmission and Schmitt trigger operation in strongly modulated asymmetric WBGs

Masafumi Fujii, Ayan Maitra, Christopher Poulton, Juerg Leuthold, Wolfgang Freude

We investigate numerically a non-reciprocal switching behavior in strongly modulated waveguide Bragg gratings (WBGs) having a longitudinally asymmetric stopband configuration. The minimum power predicted for a stable switching operation is found to be approximately 77 mW for a realistic waveguide structure made of prospective materials; we assume in this paper a nano-strip InGaAsP/InP waveguide having longitudinally asymmetric modulation of the waveguide width. The analysis has been performed with our in-house nonlinear finite-difference time-domain (FDTD) code adapted to parallel computing. The numerical results clearly show low-threshold Schmitt trigger operation, as well as non-reciprocal transmission property where the switching threshold for one propagation direction is lower than that for the other direction. In addition, we discuss the modulation-like instability phenomena in such nonlinear periodic devices by employing both an instantaneous Kerr nonlinearity and a more involved saturable nonlinearity model. (C) 2006 Optical Society of America.

Single-photon quantum key distribution in the presence of loss

Marcos Curty, Tobias Moroder

We investigate two-way and one-way single-photon quantum key distribution (QKD) protocols in the presence of loss introduced by the quantum channel. Our analysis is based on a simple precondition for secure QKD in each case. In particular, the legitimate users need to prove that there exists no separable state (in the case of two-way QKD), or that there exists no quantum state having a symmetric extension (one-way QKD), that is compatible with the available measurements results. We show that both criteria can be formulated as a convex optimization problem known as a semidefinite program, which can be efficiently solved. Moreover, we prove that the solution to the dual optimization corresponds to the evaluation of an optimal witness operator that belongs to the minimal verification set of them for the given two-way (or one-way) QKD protocol. A positive expectation value of this optimal witness operator states that no secret key can be distilled from the available measurements results. We apply such analysis to several well-known single-photon QKD protocols under losses.

Quasi-phase-matched high harmonic generation in hollow core photonic crystal fibers

H. Ren, A. Nazarkin, J. Nold, P. St. J. Russell

The potential of hollow core photonic crystal fiber as a nonlinear gas cell for efficient high harmonic generation is discussed. The feasibility of phase-matching this process by modulating the phase of ionization electrons using a counter-propagating laser field is shown. In this way, harmonics with energies of several hundreds of eV can be produced using fs-laser pump pulses of mu J energy. (C) 2008 Optical Society of America

Exact frequency comb mode number determination in precision optical frequency measurements

J. Zhang, Z. H. Lu, Y. H. Wang, T. Liu, A. Stejskal, Y. N. Zhao, R. Dumke, Q. H. Gong, L. J. Wang

We demonstrate a method to determine the absolute mode number of a frequency comb when it is used for high-precision laser frequency measurement, without the help of wavemeters. Our technique involves changing the repetition rate of the frequency comb in a two-step process. Guidelines for choosing the correct repetition rates for different laser linewidths are given. As a demonstration, the absolute frequencies of two lasers with different linewidths are measured with our method.

Squeezing based on nondegenerate frequency doubling internal to a realistic laser

UL Andersen, P Tidemand-Lichtenberg, P Buchhave

We investigate theoretically the quantum fluctuations of the fundamental field in the output of a nondegenerate second-harmonic generation process occurring inside a laser cavity. Due to the nondegenerate character of the nonlinear medium, a field orthogonal to the laser field is for some operating conditions independent of the fluctuations produced by the laser medium. We show that this fact may lead to perfect squeezing for a certain polarization mode of the fundamental field. The experimental feasibility of the system is also discussed.

Forward Brillouin Scattering in Tapered Optical Fibers

M. S. Kang, A. Brenn, G. S. Wiederhecker, P. St. J. Russell

We experimentally study forward Brillouin scattering in tapered fibers. Circularly symmetric acoustic phonons resonant in fibers are observed by using a novel photoacoustic measurement technique. Strong acousto-optic interaction can take place in highly tapered fibers. (C) 2008 Optical Society of America

Optimal unambiguous state discrimination of two density matrices: Lower bound and class of exact solutions

P Raynal, N Lutkenhaus

Recently the problem of unambiguous state discrimination of mixed quantum states has attracted much attention. So far, bounds on the optimum success probability have been derived [T. Rudolph, R. W. Spekkens, and P. S. Turner, Phys. Rev. A 68, 010301(R) (2003)]. For two mixed states they are given in terms of the fidelity. Here we give tighter bounds as well as necessary and sufficient conditions for two mixed states to reach these bounds. Moreover we construct the corresponding optimal measurement strategies. With this result, we provide analytical solutions for unambiguous discrimination of a class of generic mixed states. This goes beyond known results which are all reducible to some pure state case. Additionally, we show that examples exist where the bounds cannot be reached.

Quantum reconstruction of an intense polarization squeezed optical state

Ch. Marquardt, J. Heersink, R. Dong, M. V. Chekhova, A. B. Klimov, L. L. Sanchez-Soto, U. L. Andersen, G. Leuchs

We perform a reconstruction of the polarization sector of the density matrix of an intense polarization squeezed beam starting from a complete set of Stokes measurements. By using an appropriate quasidistribution, we map this onto the Poincare space, providing a full quantum mechanical characterization of the measured polarization state.

Long-range spiralling surface plasmon modes on metallic nanowires

M. A. Schmidt, P. St. J. Russell

We discuss the characteristics of surface plasmon modes guided on metallic nanowires of circular cross-section embedded in silica glass. Under certain conditions such wires allow low-loss guided modes, full account being taken of ohmic losses in the metal. We find that these modes can be bound to the wire even when the real part of their axial refractive index is less than that of the surrounding dielectric. We assess in detail the accuracy of a simple model in which SPs are viewed as spiralling around the nanowire in a helical path, forming modes at certain angles of pitch. The results are relevant for understanding the behavior of light in twodimensional arrays of metallic nanowires in fiber form. (c) 2008 Optical Society of America.

Phase-preserving 2R regeneration of a WDM RZ-DPSK signal using a nonlinear amplifying loop mirror

K. Cvecek, K. Sponsel, C. Stephan, G. Onishchukov, R. Ludwig, C. Schubert, B. Schmauss, G. Leuchs

A modified NALM setup for phase-preserving amplitude regeneration of WDM-DPSK signals is proposed. Demultiplexing-multiplexing within the NALM fiber loop is used to avoid WDM channel crosstalk. A negative power penalty of 1.2 dB was obtained. (C) 2007 Optical Society of America.

Efficiency of coherent-state quantum cryptography in the presence of loss: Influence of realistic error correction

Matthias Heid, Norbert Luetkenhaus

We investigate the performance of a continuous-variable quantum key distribution scheme in a practical setting. More specifically, we take a nonideal error reconciliation procedure into account. The quantum channel connecting the two honest parties is assumed to be lossy but noiseless. Secret key rates are given for the case that the measurement outcomes are postselected or a reverse reconciliation scheme is applied. The reverse reconciliation scheme loses its initial advantage in the practical setting. If one combines postselection with reverse reconciliation, however, much of this advantage can be recovered.

Coherent control of ultrahigh-frequency acoustic resonances in photonic crystal fibers

G. S. Wiederhecker, A. Brenn, H. L. Fragnito, P. St. J. Russell

Ultrahigh frequency acoustic resonances (2 GHz) trapped within the glass core (1 mu m diameter) of a photonic crystal fiber are selectively excited through electrostriction using laser pulses of duration 100 ps and energy 500 pJ. Using precisely timed sequences of such driving pulses, we achieve coherent control of the acoustic resonances by constructive or destructive interference, demonstrating both enhancement and suppression of the vibrations. A sequence of 27 resonantly-timed pulses provides a 100-fold increase in the amplitude of the vibrational mode. The results are explained and interpreted using a semianalytical theory, and supported by precise numerical simulations of the complex light-matter interaction.

Heat dissipative solitons in optical fibers

N. Akhmediev, P. St. J. Russell, M. Taki, J. M. Soto-Crespo

We propose a one-dimensional model governing the propagation of heat waves in an optical fiber (the "fiber fuse"). The model has solutions in the form of high temperature localized waves moving towards the input end of the fiber, fueled by the laser power. These waves can be ignited by local heating at any point along the fiber. The effect of such a wave is irreversible damage to the fiber core. The phenomenon was observed earlier by Hand and Russell, when locally heating a fiber through which CW light of modest intensity was propagating. This induced self-destruction of the optical fiber core. (c) 2007 Elsevier B.V. All rights reserved.

BER improvement using a 2-R regenerator based on an asymmetric nonlinear optical loop mirror

M Meissner, K Sponsel, K Cvecek, A Benz, S Weisser, B Schmauss, G Leuchs

BER improvement of 15 decades is observed using an asymmetric NOLM at a bitrate of 40Gbit/s and an input OSNR of 28.7dB. The BER improvement can be converted into 3.9dB of OSNR gain. Both were achieved by optimizing NOLM input power and splitting ratio. This allows for longer spans and thus reduces the over all amount of amplifiers or allows for an increased system reach.

Precision measurement of the refractive index of carbon dioxide with a frequency comb

J. Zhang, Z. H. Lu, L. J. Wang

We report a higher precision measurement of the refractive index of carbon dioxide using a frequency comb as the light source in a Mach-Zehnder interferometer setup. The experimental sensitivity can reach the level of 8.8 X 10(-9). Taking into account the measurement accuracy of temperature and pressure, the experimental accuracy has a value of 1.2 X 10(-8). The measurement result has a deviation from the commonly quoted result [Old et al., J. Opt. Soc. Am. 61, 89 (1971)] by 6.4 X 10(-7) at 800 nm. (c) 2007 Optical Society of America.

Interference between two coherently driven monochromatic terahertz sources

S. Preu, S. Malzer, G. H. Doehler, Q. Z. Zhao, M. Hanson, J. D. Zimmerman, A. C. Gossard, L. J. Wang

We report the interference between two phase-locked, continuous-wave, and coherent terahertz (Thz) photomixers. Both spatial and temporal coherence are observed with a narrow linewidth of less than 10 MHz. Phase locking is achieved by using one pair of lasers for both sources. The concept of optical narrow linewidth (fiber) distribution of the mixing signal opens various applications. We outline the necessary steps to extend this scheme for very long base line interferometric THz detection in the tens of kilometer range and for the improvement of spatial resolution for active stand-off THz imaging. (C) 2008 American Institute of Physics.

Bound soliton pairs in photonic crystal fiber

A. Podlipensky, P. Szarniak, N. Y. Joly, C. G. Poulton, P. St. J. Russell

We demonstrate experimentally the formation and stable propagation of bound soliton pairs in a highly nonlinear photonic crystal fiber. The bound pairs occur at a particular power as the consequence of high-order soliton fission. They propagate over long distances with constant inter-soliton frequency and time separation. During propagation, the soliton self-frequency shift causes the central frequency of the pairs to move towards longer wavelength. The formation and characteristics of the bound soliton pairs are confirmed numerically. We believe this to be the first experimental observation of such bound soliton pairs. (c) 2007 Optical Society of America.

Direct experimental observation of the single reflection optical Goos-Hanchen shift

H. G. L. Schwefel, W. Koehler, Z. H. Lu, J. Fan, L. J. Wang

We report a precise direct measurement of the Goos-Hanchen shift after one reflection off a dielectric interface coated with periodic metal stripes. The spatial displacement of the shift is determined by image analysis. A maximal absolute shift of 5.18 and 23.39 mu m for TE and TM polarized light, respectively, is determined. This technique is simple to implement and can be used for a large range of incident angles. (C) 2008 Optical Society of America.

Unconditional quantum cloning of coherent states with linear optics

Ulrik L. Andersen, V Josse, Gerd Leuchs

A scheme for optimal Gaussian cloning of optical coherent states is proposed and experimentally demonstrated. Its optical realization is based entirely on simple linear optical elements and homodyne detection. The optimality of the presented scheme is limited only by detection inefficiencies. Experimentally, we achieved a cloning fidelity of about 65%, which almost touches the optimal value of 2/3.

Temporal dynamics of the alpha factor in semiconductor optical amplifiers

Jin Wang, Ayan Maitra, Chris G. Poulton, Wolfgang Freude, Juerg Leuthold

The temporal evolution of the a-factor during pump-probe experiments in semiconductor optical amplifiers (SOAs) is not constant but varies strongly with time. It even takes on negative values for short periods of time. As a consequence, cross-phase modulation (XPM) effects usually lag behind cross-gain modulation effects by several picoseconds. This delay has important consequences for ultrafast operation of all-optical devices based on SOAs. It. actually means that not every scheme will be suited for ultrafast operation. In order to properly model the XPM and phase variations within an SOA during a pump-probe experiment, a new parameterization for the alpha-factor is introduced. Inclusion of this model leads to an excellent agreement with the recent 160-Gb/s experiments for both phase and amplitude evolutions of the respective signals with time.

Femto-Newton light force measurement at the thermal noise limit

F. Mueller, S. Heugel, L. J. Wang

The measurement of very small light forces has wide applications in many fields of physics. A common measurement method for small force detection is the determination of changes in the dynamic behavior of mechanical oscillators, either in amplitude or in frequency. The detection of slowly varying forces mostly requires long period oscillators, such as a torsion pendulum. We demonstrate the application of a macroscopic, low-noise, torsion balance oscillator for the detection of radiation pressure forces at the femto-Newton level. The system is "precooled" (removing excess seimic noise) to be only thermal noise limited. The demonstrated force sensitivity reaches the thermal limit. (C) 2008 Optical Society of America.

Polarization effect in the transmission through a single nanoscopic aperture

J. Mueller, P. Banzer, S. Quabis, G. Leuchs

Transmission of a strongly focused light beam through a nanoscopic aperture differs for radial and azimuthal polarization and for holes and coaxial structures. Experimental results are compared with FDTD-calculations showing the relevance of surface plasmons. (c) 2006 Optical Society of America

Guided acoustic wave Brillouin scattering in photonic crystal fibers

D. Elser, Ch Wittmann, U. L. Andersen, O. Gloeckl, S. Lorenz, Ch Marquardt, G. Leuchs

In silica glass fibers, thermally excited acoustic phonons scatter light into the beam propagating in the forward direction. At acoustic frequencies up to several hundreds of megahertz, the wave vectors of the phonons interacting with the light propagate essentially transversally to the fiber axis. This effect is known as Guided Acoustic Wave Brillouin Scattering (GAWBS) and leads to phase and polarization noise in the guided light. For fiber-based quantum optics experiments, this excess noise is a major limitation. In Photonic Crystal Fibers (PCFs), light is guided by a microstructure simultaneously acting as a 2D transversal phononic crystal which modifies the acoustic noise spectrum. We demonstrate a GAWBS-noise reduction in commercially available PCFs. This gives rise to the prospect of fiber-based quantum optic devices exhibiting less excess noise, thus resulting in higher quantum state purity. Further improvement can be achieved by tailoring the photonic microstructure such that a reduction of phonon noise by design is achieved.

Quasi-absolute measurement of aspheres with a combined diffractive optical element as reference

Frank Simon, Gufran Khan, Klaus Mantel, Norbert Lindlein, Johannes Schwider

We have already reported a method for the quasi-absolute test of rotationally symmetric aspheres by means of combined diffractive optical elements (combo-DOEs). The combo-DOEs carry the information for the ideal shape of an aspheric surface under test as well as a spherical wave for the measurement at the cat's eye position. An experimental demonstration of the procedure is given. Measurements with two different designs of combo-DOEs have been conducted, and their relative advantages and disadvantages are discussed. (c) 2006 Optical Society of America.

Comment on "arbitrated quantum-signature scheme"

Marcos Curty, Norbert Luetkenhaus

We investigate the quantum signature scheme proposed by and Zeng and Keitel [Phys. Rev. A 65, 042312 (2002)]. It uses Greenberger-Horne-Zeilinger states and the availability of a trusted arbitrator. However, in our opinion the protocol is not clearly operationally defined and several steps are ambiguous. Moreover, we argue that the security statements claimed by the authors are incorrect.

Security of coherent-state quantum cryptography in the presence of Gaussian noise

Matthias Heid, Norbert Luetkenhaus

We investigate the security against collective attacks of a continuous variable quantum key distribution scheme in the asymptotic key limit for a realistic setting. The quantum channel connecting the two honest parties is assumed to be lossy and imposes Gaussian noise on the observed quadrature distributions. Secret key rates are given for direct and reverse reconciliation schemes including post-selection in the collective attack scenario. The effect of a nonideal error correction and two-way communication in the classical post-processing step is also taken into account.

Off-axis Gaussian Schell-model beam and partially coherent laser array beam in a turbulent atmosphere

Yangjian Cai, Qiang Lin, Yahya Baykal, Halil T. Eyyuboglu

The propagation of an off-axis Gaussian Schell-model (GSM) beam in a turbulent atmosphere is investigated based on the extended Huygens-Fresnel integral formula. Analytical formulae for the cross-spectral density and corresponding partially coherent complex curvature tensor of an off-axis GSM beam propagating in a turbulent atmosphere are derived. Based on these formulae, the propagation properties of such kind of beam in a turbulent atmosphere are investigated in detail. Furthermore, the methods are extended to investigate the propagation properties of a partially coherent laser array beam in a turbulent atmosphere. The properties of an off-axis GSM beam and a partially coherent laser array beam in a turbulent atmosphere are closely related with the beam parameters and the structure constant of the turbulent atmosphere. (c) 2007 Elsevier B.V. All rights reserved.

Probing semiconductor gap states with resonant tunneling

S Loth, M Wenderoth, L Winking, RG Ulbrich, S Malzer, GH Dohler

Tunneling transport through the depletion layer under a GaAs {110} surface is studied with a low temperature scanning tunneling microscope (STM). The observed negative differential conductivity is due to a resonant enhancement of the tunneling probability through the depletion layer mediated by individual shallow acceptors. The STM experiment probes, for appropriate bias voltages, evanescent states in the GaAs band gap. Energetically and spatially resolved spectra show that the pronounced anisotropic contrast pattern of shallow acceptors occurs exclusively for this specific transport channel. Our findings suggest that the complex band structure causes the observed anisotropies connected with the zinc blende symmetry.

Experimental observation of truncated fractional Fourier transform for a partially coherent Gaussian Schell-model beam

Fei Wang, Yangjian Cai, Qiang Lin

The truncated fractional Fourier transform (FRT) is applied to a partially coherent Gaussian Schell-model (GSM) beam. The analytical propagation formula for a partially coherent GSM beam propagating through a truncated FRT optical system is derived by using a tensor method. Furthermore, we report the experimental observation of the truncated FRT for a partially coherent GSM beam. The experimental results are consistent with the theoretical results. Our results show that initial source coherence, fractional order, and aperture width (i.e.. truncation parameter) have strong influences on the intensity and coherence properties of the partially coherent beam in the FRT plane. When the aperture width is large, both the intensity and the spectral degree of coherence in the FRT plane are of Gaussian distribution. As the aperture width decreases, the diffraction pattern gradually appears in the FRT plane, and the spectral degree of coherence becomes of non-Gaussian distribution. As the coherence of the initial GSM beam decreases, the diffraction pattern for the case of small aperture widths gradually disappears. (c) 2008 Optical Society of America.

Photoluminescence of n-doped double quantum well - electron subbands under influence of in-plane magnetic fields

M. Orlita, M. Byszewski, G. H. Doehler, M. Grill, P. Hlidek, S. Malzer, M. Zvara

We report on photoluminescence (PL) measurements of a GaAs/AlGaAs double quantum well (DQW) in high magnetic fields. Measurements were carried out on a selectively contacted symmetric p-delta n-DQW-delta n-p structure, which allows a variation of the electron density in DQW by a p-n bias and simultaneously a tilting of DQW, when a p-p bias is applied. Attention was paid to phenomena in in-plane magnetic fields, theoretically studied by Huang and Lyo (HL), [Phys. Rev. B 59, (1999) 7600]. In this paper, we compare our results for both symmetric and asymmetric DQWs with the theoretical model made by HL. Whereas the spectra from a symmetric DQW fully confirmed the theoretical predictions, the results gained from DQW with an electric-field-induced asymmetry did not allow a proper study of anticipated effects. The reasons for that are discussed. (c) 2006 Elsevier B.V. All rights reserved.

Application of frequency combs in the measurement of the refractive index of air

J. Zhang, Z. H. Lu, B. Menegozzi, L. J. Wang

We report a new method in the precision measurement of the refractive index of air using a highly unbalanced Michelson interferometer with a femtosecond optical frequency comb as the light source. Standard dry air is filled into a 30 m multipass cell, serving as the long arm of the interferometer, while a short arm acts as the reference path. Both time and frequency domain interferograms are recorded to measure the refractive index of air. The deviation of our experimental results with Edlen's formula is 1.4x10(-9) at 800 nm. Our experiment has a standard error of 5.2x10(-9) at fixed parameters (pressure and temperature). This is achieved by putting the multipass cell into a temperature-stabilized box, and also by locking the interferometer path length with a He-Ne laser. We achieved a temperature stabilization of 0.8 mK for 25 h. This corresponds to 0.4 mu m multipass cell length change. The locking of the He-Ne interferometer enables us to achieve 7 nm path-length change outside the multipass cell. Combined with accurate measurement of temperature and pressure, we were able to achieve an accuracy of 7.7x10(-9). (c) 2006 American Institute of Physics.

Sequential attacks against differential-phase-shift quantum key distribution with weak coherent states

Marcos Curty, Lucy Liuxuan Zhang, Hoi-Kwong Lo, Norbert Luetkenhaus

We investigate limitations imposed by sequential attacks on the performance of differential-phase-shift quantum key distribution protocols that use pulsed coherent light. In particular, we analyze two sequential attacks based on unambiguous state discrimination and minimum error discrimination, respectively, of the signal states emitted by the source. Sequential attacks represent;I special type of intercept-resend attacks and, therefore, they provide ultimate upper bounds on the maximal distance achievable by quantum key distribution schemes.

Calibration for cylindrical specimens in grazing-incidence interferometry via integration of difference measurements

Klaus Mantel, Juergen Lamprecht, Norbert Lindlein, Johannes Schwider

Cylindrical specimens may be tested advantageously by using grazing-incidence interferometry. A multiple positions test in combination with rotational averaging has recently been used to separate the surface deviations of the specimen from the interferometric aberrations. To reduce the measuring time and to check whether the results are reliable, a second procedure is now investigated, which uses the principle of the multiple positions test to determine quantities proportional to the difference quotients of the surface deviations. After numerical integration, the results can be compared with those obtained previously by rotational averaging. The measurement principle is described, and calibration results are presented. (c) 2006 Optical Society of America.

Subkelvin cooling of a gram-sized oscillator

F. Mueller, S. Heugel, L. J. Wang

Mechanical oscillators have a long tradition in measuring very small forces, particularly of gravitational nature. Oscillators have thermal energy of k(B)T/2 in each degree of freedom. Here, we demonstrate the dynamical cooling of a gram-sized oscillator to 300 mK in equivalent temperature, a reduction of noise by a factor of 10(6) compared to the seismic background level. A simple physical model is provided for the cooling process. We also demonstrate the dynamic control of the oscillator's eigenfrequency, aiming at providing a stationary reference point for position measurements. The method may find applications in precision measurements of weak forces. (C) 2008 American Institute of Physics.

Reduction of guided acoustic wave Brillouin scattering in photonic crystal fibers

D. Elser, U. L. Andersen, A. Korn, O. Gloeckl, S. Lorenz, Ch. Marquardt, G. Leuchs

Guided acoustic wave Brillouin scattering (GAWBS) generates phase and polarization noise of light propagating in glass fibers. This excess noise affects the performance of various experiments operating at the quantum noise limit. We experimentally demonstrate the reduction of GAWBS noise in a photonic crystal fiber in a broad frequency range by tailoring the acoustic modes using the photonic also as a phononic crystal. We compare the noise spectrum to the one of a standard fiber and observe a tenfold noise reduction in the frequency range up to 200 MHz. Based on our measurement results as well as on numerical simulations, we establish a model for the reduction of GAWBS noise in photonic crystal fibers.

Scintillation properties of non-circular flat-topped beams

Yangjian Cai

The scintillation properties of a flat-topped (FT) beam of non-circular (rectangular or elliptical) symmetry propagating in a weak turbulent atmosphere are investigated. Explicit expressions for the on-axis scintillation index of rectangular and elliptical FT beams in a weak turbulent atmosphere are derived. It is found that the scintillation index value of a rectangular or elliptical FT beam can be smaller than that of a circular Gaussian beam under certain conditions. The scintillation properties of non-circular FT beams are closely related to the beam parameters.

All-fibre source of continuous variable entangled light

A Nandan, M Sabuncu, J Heersink, O Glockl, G Leuchs, UL Andersen

We experimentally demonstrate the production of continuous variable entanglement of femto second laser pulses from an all-fibre asymmetric Sagnac interferometer exploiting the Kerr effect. Contrary to the experiment of Silberhorn et at. [Phys. Rev. Lett. 86,4267 (2001)], the asymmetric coupler in the Sagnac loop is fully integrated in the fibre, making the source extremely compact, reliable and robust. Employing a simple detection scheme, comprising a beam splitter and two intensity detectors, we clearly observe quantum correlations of conjugate quadratures, hereby witnessing entanglement.

Paraxial propagation of a partially coherent flattened Gaussian beam through apertured ABCD optical systems

Yangjian Cai, Xiang Lue, Halil T. Eyyuboglu, Yahya Baykal

By expanding the hard aperture function into a finite sum of complex Gaussian functions, some approximate analytical formulae for the cross-spectral density of a partially coherent flattened Gaussian beam (FGB) propagating through apertured aligned and misaligned ABCD optical systems are derived based on the generalized Collins formula. The results obtained by using the approximate analytical formula are in good agreement with those obtained by using the numerical integral calculation. As a numerical example, the focusing properties (including average irradiance distribution and focal shift) of a partially coherent FGB focused by an apertured thin lens are studied, and it is found that the focusing properties of a partially coherent FGB are closely related to its initial coherence and the aperture width. Our results provide an effective and fast way for studying the paraxial propagation of a partially coherent FGB through apertured ABCD optical systems. (C) 2008 Elsevier B.V. All rights reserved.

Progress toward an optical frequency standard based on a single Indium ion

Y. H. Wang, T. Liu, A. Stejskal, Y. N. Zhao, J. Zhang, Z. H. Lu, R. Dumke, L. J. Wang, Th. Becker, H. Walther, et al.

We report on the current status of the optical frequency standard based on a single In-115(+) ion. A single Indium ion is being trapped in a Paul-Strauble type trap and cooled in combination of Doppler cooling and sideband cooling to its lowest vibrational states. To interrogate the S-1(0)-(3)p(0) clock transition with high accuracy we demonstrate a Hz-level laser system resonant to this transition. With the current setup we are able to achieve a measured linewidth of the clock transition of 260 Hz. The statistical uncertainty of the absolute frequency measurement is 3 x 10(-13), limited mainly by our reference frequency standard.

Automatic coarse registration of three-dimensional surfaces by information theoretic selection of salient points

Nikolaus Schoen, Gerd Haeusler

We describe a new method to register surface data measured by optical three-dimensional (3-D) sensors from various views of an object. With our method, complete 3-D models of objects can be generated without user interaction. Circumferential acquisition of 3-D objects is done by taking several views from different directions. To generate a complete 3-D-model, the views must be aligned with each other. This process is called registration and is commonly done interactively by searching for so-called corresponding points in the different views and by use of these points to calculate the appropriate rotation and translation. Our approach is based on automatically finding points that are eye catching or salient compared with other surface points. We derive a quantitative measure of point salience and a feature definition for free-form surfaces by introducing a concept to measure pragmatic information. Experiments confirm that our salient points can be robustly located on general free-form surfaces, even if there are no corners or edges. Furthermore, the neighborhoods of the salient points are highly distinguishable from each other. This results in a large reduction of the complexity of the subsequent geometric matching. The computing time is only a few seconds. We present results from various fields of application. (c) 2006 Optical Society of America

Nonlinear entanglement witnesses

O Guhne, N Lutkenhaus

Entanglement detection typically relies on linear inequalities for mean values of certain observables (entanglement witnesses), where violation indicates entanglement. We provide a general method to improve any of these inequalities for bipartite systems via nonlinear expressions. The nonlinearities are of different orders and can be directly measured in experiments, often without any extra effort.

Design considerations for the absolute testing approach of aspherics using combined diffractive optical elements

Gufran S. Khan, Klaus Mantel, Irina Harder, Norbert Lindlein, Johannes Schwider

Aspheric optical surfaces are often tested using diffractive optics as null elements. For precise measurements, the errors caused by the diffractive optical element must be calibrated. Recently, we reported first experimental results of a three position quasi-absolute test for rotationally invariant aspherics by using combined-diffractive optical elements (combo-DOEs). Here we investigate the effects of the DOE substrate errors on the proposed calibration procedure and present a set of criteria for designing an optimized combo-DOE. It is demonstrated that this optimized design enhances the overall consistency of the procedure. Furthermore, the rotationally varying part of the surface deviations is compared with the rotationally varying deviations obtained by an N-position averaging procedure and is found to be in good agreement. (c) 2007 Optical Society of America.

Quantitative Broadband Chemical Sensing in Air-Suspended Solid-Core Fibers

T. G. Euser, J. S. Y. Chen, N. J. Farrer, M. Scharrer, P. J. Sadler, P. St. J. Russell

We report a sensitive evanescent field sensor using air-suspended solid-core fibers. Excellent agreement between measured and calculated mode profiles allows us to measure quantitative broadband absorption spectra with sample volumes as low as 1 mu L. (C)2008 Optical Society of America

Enhancement of optical absorption and photocurrent of 6H-SiC by laser surface nanostructuring

Q. Z. Zhao, F. Ciobanu, S. Malzer, L. J. Wang

Nanoscale ripple structures with spatial periods of up to 155 nm are generated by a single beam ultrashort pulsed laser structuring of 6H-SiC surface. The period of nanoripples does not depend on the laser fluence and the number of pulses. Optical absorption and photocurrent measurements on SiC samples are investigated before and after laser nanostructuring. Nearly 40% enhancement of optical absorption and a factor of 3 increase of photocurrent are achieved after laser nanostructuring. (c) 2007 American Institute of Physics.

Quantum key distribution with passive decoy state selection

Wolfgang Mauerer, Christine Silberhorn

We propose a quantum key distribution scheme which closely matches the performance of a perfect single photon source. It nearly attains the physical upper bound in terms of key generation rate and maximally achievable distance. Our scheme relies on a practical setup based on a parametric downconversion source and present day, nonideal photon-number detection. Arbitrary experimental imperfections which lead to bit errors are included. We select decoy states by classical postprocessing. This allows one to improve the effective signal statistics and achievable distance.

Characterization of cylindrical micro-lenses in transmitted light and with grazing incidence interferometry in reflected light

Juergen Lamprecht, Norbert Lindlein, Johannes Schwider

High numerical aperture cylindrical micro-lenses are needed in collimating the laser light from laser bars in the near infrared. Diffraction limited performance of such collimation lenses can only be obtained if the surface shape deviates strongly from circular symmetry. Therefore, null tests make the use of diffractive optical elements (DOE) necessary. For performance test at the design wavelength the reference DOE produces an ideal cylinder wave which enables the compensation of the wave front coming from the micro-lens to a plane wave. The use of a DOE-master enables beside the null test geometry also a removal of the anamorphic distortion due to the cylinder geometry. The DOEs having a numerical aperture of 0.8 are produced on an e-beam machine. The measurement of the wave aberrations is done with the help of the phase shifting technique. Alignment aberrations are eliminated by a least square fit of suitable misalignment functionals derived from an analytic approximation. The shape of the micro-lenses is tested in reflected light showing surface defects directly. The cylinder symmetry allows for a grazing incidence test using two DOE with nearly constant spatial frequency. There are no limitations concerning the test of high numerical aperture surfaces since the structure of the diffractive elements are parallel curves to the profile curve. The mean spatial frequency of the DOE defines the effective wavelength. Since we use a diffractive interferometer the effective wavelength is identical to the pitch of the DOE. Usually pitches between 4-10 mu m are used resulting in a fringe sensitivity of 2-5 mu m. The test delivers the deviation of the surface from the ideal form. In the case of non-circular symmetry the DOE deviates from the axicon type DOE. In addition to the shape deviations also the radius of curvature at the vertex can be measured. The stage for the cylindrical micro-lens is equipped with a length measuring device using grating references from Renishaw providing a length increment of 0.1 mu m. With the axicon DOE there are two positions where nearly fluffed out fringes can be observed. Starting from the basic test position the lens can be moved until the vertex of the lens coincides with the focal line of the wave generated by the central part of the DOE. The distance between these two positions gives the radius of curvature for the vertex. Modem manufacturing of micro-lenses comprises also hot embossing in plastics or even into glass. Because of the small dimensions of the lenses and the required accuracy of the surface shape also tests of the impressing mould have to be carried out. The DOE approach enables also the test of the embossing form with the same sensitivity as the final test of the lens. Measuring results for the two test methods will be given.

THz-photomixer based on quasi-ballistic transport

GH Dohler, F Renner, O Klar, M Eckardt, A Schwanhausser, S Malzer, D Driscoll, M Hanson, AC Gossard, G Loata, et al.

We report on a novel concept for THz photomixers with high conversion efficiency up to several THz. In contrast to the conventional pin photomixer we can overcome the trade-off between either optimizing transit-time or RC-roll-off. Using quasi-ballistic transport in nano-pin-diodes the transport path can be optimized regarding both path length and transit time. Independently, the capacitance can be kept small by using a sufficiently large number of optimized nano-pin-diodes in series. The concept is presented in detail and first experimental results are reported which corroborate our theoretical expectations.

Iterations of nonlinear entanglement witnesses

Tobias Moroder, Otfried Guehne, Norbert Luetkenhaus

We describe a generic way to improve a given linear entanglement witness by a quadratic, nonlinear term. This method can be iterated, leading to a whole sequence of nonlinear witnesses, which become stronger in each step of the iteration. We show how to optimize this iteration with respect to a given state and prove that in the limit of the iteration the nonlinear witness detects all states that can be detected by the positive map corresponding to the original linear witness.

Depth resolved scanning tunneling spectroscopy of shallow acceptors in gallium arsenide

S Loth, M Wenderoth, L Winking, RG Ulbrich, S Malzer, GH Dohler

Scanning tunneling spectroscopy (STS) at 8 K is used to study single shallow acceptors embedded near {110}-surfaces ill gallium arsenide (GaAs). At appropriate bias voltages the circularly symmetric contrast normally observed for charged defects evolves into a pronounced triangular shaped protrusion. Comparing dopants at different depths under the surface, we find a linear shift of the associated conductivity maximum along (112) directions. Comparative Studies of Carbon and Zinc acceptors in a modulation-doped heterostructure reveal that both dopants act similarly. The experimental findings Suggest that the highly anisotropic features induced by acceptors resemble a bulk property of the GaAs crystal prominently demonstrating its Ziricblende symmetry.

Achieving Gaussian outputs from large-mode-area higher-order-mode fibers

Norbert Lindlein, Gerd Leuchs, Siddharth Ramachandran

We describe an alternative to fiber-gratings for converting higher-order LP0m (m > 1) fiber modes into a nearly fundamental Gaussian shape at the output of a fiber. This schematic enables the use of light propagation in higher-order modes of a fiber, a fiber-platform that has recently shown great promise for achieving very large mode areas needed for future high-power lasers and amplifiers. The conversion will be done by using a binary phase plate in the near field of the fiber, which emits the LP0m mode. Since the binary phase plate alone cannot increase the quality factor M-2 of the laser beam because of some broad sidebands, a filtering of the sidebands is done in the Fourier plane of a telescope. Of course, this will cost some of the total light power, but on the other side the M-2 factor can be reduced to nearly the ideal value near 1.0, and it is shown that similar to 76% of the total light power can be conserved for all investigated modes (2 <= m <= 8). A tolerance analysis for the phase plate and its adjustment is made, and different optical imaging systems to form a magnified image of the fiber mode on the phase plate are discussed in order to have more tolerance for the adjustment of the phase plate. (c) 2007 Optical Society of America.

Experimental demonstration of continuous variable quantum erasing

UL Andersen, O Glockl, S Lorenz, G Leuchs, R Filip

We experimentally demonstrate the concept of continuous variable quantum erasing. The amplitude quadrature of the signal state is labeled to another state via a quantum nondemolition interaction, leading to a large uncertainty in the determination of the phase quadrature due to the inextricable complementarity of the two observables. We show that by erasing the amplitude quadrature information we are able to recover the phase quadrature information of the signal state.

Scintillation properties of dark hollow beams in a weak turbulent atmosphere

Y. Chen, Y. Cai, H. T. Eyyuboglu, Y. Baykal

The on-axis scintillation index for a circular dark hollow beam (DHB) propagating in a weak turbulent atmosphere is formulated, and the scintillation properties of a DHB are investigated in detail. The scintillation index for a DHB reduces to the scintillation index for a Gaussian beam, an annular beam and a flat-topped beam under certain conditions. It is found that the scintillation index of a DHB is closely related to the beam parameters and can be lower than that of a Gaussian beam, an annular beam and a flat-topped beam in a weak turbulent atmosphere at smaller waist sizes and longer propagation lengths.

State of polarization of a stochastic electromagnetic beam in an optical resonator

Olga Korotkova, Min Yao, Yangjian Cai, Halil T. Eyyuboglu, Yahya Baykal

On the basis of the unified theory of coherence and polarization, we investigate the behavior of the state of polarization of a stochastic electromagnetic beam in a Gaussian cavity. Formulations both in terms of Stokes parameters and in terms of polarization ellipse are given. We show that the state of polarization stabilizes, except in the case of a lossless cavity, after several passages between the mirrors, exhibiting monotonic or oscillatory behavior depending on the parameters of the resonator. We also find that an initially (spatially) uniformly polarized beam remains nonuniformly polarized even for a large number of passages between the mirrors of the cavity. (C) 2008 Optical Society of America

Second-harmonic generation by an astigmatic partially coherent beam

Yangjian Cai, Ulf Peschel

We investigate second-harmonic generation by an astigmatic partially coherent beam. An explicit expression for the second-order correlation function of the second-harmonic field is obtained. The properties of the generated field and the conversion efficiency for second-harmonic generation are studied numerically. We find that using an astigmatic instead of astigmatic partially coherent pump beam can increase the conversion efficiency of the second-harmonic generation. (c) 2007 Optical Society of America.

Upper bound on the secret key rate distillable from effective quantum correlations with imperfect detectors

T Moroder, M Curty, N Lutkenhaus

We provide a simple method to obtain an upper bound on the secret key rate that is particularly suited to analyze practical realizations of quantum key distribution protocols with imperfect devices. We consider the so-called trusted device scenario where Eve cannot modify the actual detection devices employed by Alice and Bob. The upper bound obtained is based on the available measurements results, but it includes the effect of the noise and losses present in the detectors of the legitimate users.

Quenching of the deuteron in flight

M. Dillig, C. Rothleitner

We investigate the Lorentz contraction of a deuteron in flight. Starting from the Blankenbecler-Sugar projection of the Bethe-Salpeter equation to a 3-dimensional quasi potential equation, we apply it for the deuteron bound in an harmonic oscillator potential (for an analytical result) or by the Bonn NN potential for a more realistic estimate. We find substantial quenching with increasing external momenta and a significant modi. cation of the high momentum spectrum of the deuteron.

Intensity fluctuations in J-Bessel-Gaussian beams of all orders propagating in turbulent atmosphere

H. T. Eyyuboglu, E. Sermutlu, Y. Baykal, Y. Cai, O. Korotkova

The scintillation index of a J (n) -Bessel-Gaussian beam of any order propagating in turbulent atmosphere is derived and numerically evaluated at transverse cross-sections with the aid of a specially designed triple integral routine. The graphical outputs indicate that, just like the previously investigated J (0)-Bessel-Gaussian beam, higher-order members of the family also offer favorable scintillation characteristics at large source sizes. This advantage is maintained against rising beam orders. Viewed along the propagation axis, beams with lower orders and smaller widths exhibit smaller values of the scintillation index at shorter propagation distances and large values at longer propagation distances. Further, it is shown that the scintillation index of the J (n) -Bessel-Gaussian beams (n > 0) is larger than that of the fundamental Gaussian and the J (0)-Bessel-Gaussian beams only near the on-axis points, while remaining smaller towards the edges of the beam.

Microdeflectometry - a novel tool to acquire three-dimensional microtopography with nanometer height resolution

Gerd Haeusler, Claus Richter, Karl-Heinz Leitz, Markus C. Knauer

We introduce "microdeflectometry," a novel technique for measuring the microtopography of specular surfaces. The primary data are the local slope of the surface under test. Measuring the slope instead of the height implies high information efficiency and extreme sensitivity to local shape irregularities. The lateral resolution can be better than 1 mu m, whereas the resulting height resolution is in the range of 1 nm. Microdeflectometry can be supplemented by methods to expand the depth of field, with the potential to provide quantitative 3D imaging with scanning-electron-microscope-like features. (C) 2008 Optical Society of America.

Experimental generation of four-mode continuous-variable cluster states

Mitsuyoshi Yukawa, Ryuji Ukai, Peter van Loock, Akira Furusawa

Continuous-variable Gaussian cluster states are a potential resource for universal quantum computation. They can be efficiently and unconditionally built from sources of squeezed light using beam splitters. Here we report on the generation of three different kinds of continuous-variable four-mode cluster states. In our realization, the resulting cluster-type correlations are such that no corrections other than simple phase-space displacements would be needed when quantum information propagates through these states. At the same time, the inevitable imperfections from the finitely squeezed resource states and from additional thermal noise are minimized, as no antisqueezing components are left in the cluster states.

Interference experiments with a coherent tunable THz nipnip superlattic photomixer

S. Preu, F. H. Renner, S. Malzer, G. H. Doehler, L. J. Wang, M. Hanson, A. C. Gossard, T. L. J. Wilkinson, E. R. Brown

Anyon statistics with continuous variables

Jing Zhang, Changde Xie, Kunchi Peng, Peter van Loock

We describe a continuous-variable scheme for simulating the Kitaev lattice model and for detecting statistics of Abelian anyons. The corresponding quantum optical implementation is solely based upon Gaussian resource states and Gaussian operations, hence allowing for a highly efficient creation, manipulation, and detection of anyons. This approach extends our understanding of the control and application of anyons and it leads to the possibility for experimental proof-of-principle demonstrations of anyonic statistics using continuous-variable systems.

Quantum dot microdrop laser

J. Schaefer, J. P. Mondia, R. Sharma, Z. H. Lu, A. S. Susha, A. L. Rogach, L. J. Wang

We report single-mode and multimode lasing from isolated spherical liquid microcavities containing CdSe/ZnS nanocrystal quantum dots. Lasing is observed at densities more than 2 orders of magnitude lower than previously demonstrated or theoretically predicted, assuming a uniform nanocrystal quantum dot distribution. Charged droplets, between 10 and 40 mu m in size, are electrodynamically levitated and optically pumped. Substantial laser signals at low thresholds are measured from the directional emission normal to the pump beam, owing to the high Q cavity modes.

Design of highly transparent organic photodiodes

Edgar S. Zaus, Sandro Tedde, Tobias Rauch, Jens Fuerst, Gottfried H. Doehler

In this brief, various approaches for the realization of transparent photodiodes based on bulk heterojunction blends of poly-3-hexylthiophene and [6,6]-phenyl C-61-butyric acid methyl ester are studied. The choice of the constituents of the device is discussed concerning transmittance and light-detecting properties as dark current and external quantum efficiency (EQE). Blending several light-absorbing materials makes tailoring of the transmittance spectrum possible. Transmittance of 36% of the incident light together with 46% EQE at a wavelength of 530 nm are promising results and show the potential for highly transparent, photodiodes based on organic layers.

Security of quantum key distribution with imperfect devices

D Gottesman, HK Lo, N Lutkenhaus, J Preskill

We prove the security of the Bennett-Brassard (BB84) quantum key distribution protocol in the case where the source and detector are under the limited control of an adversary. Our proof applies when both the source and the detector have small basis-dependent flaws, as is typical in practical implementations of the protocol. We derive a general lower bound on the asymptotic key generation rate for weakly basis-dependent eavesdropping attacks, and also estimate the rate in some special cases: sources that emit weak coherent states with random phases, detectors with basis-dependent efficiency, and misaligned sources and detectors.

Hybrid quantum computation in quantum optics

P. van Loock, W. J. Munro, Kae Nemoto, T. P. Spiller, T. D. Ladd, Samuel L. Braunstein, G. J. Milburn

We propose a hybrid quantum computing scheme where qubit degrees of freedom for computation are combined with quantum continuous variables for communication. In particular, universal two-qubit gates can be implemented deterministically through qubit-qubit communication, mediated by a continuous-variable bus mode ("qubus"), without direct interaction between the qubits and without any measurement of the qubus. The key ingredients are controlled rotations of the qubus and unconditional qubus displacements. The controlled rotations are realizable through typical atom-light interactions in quantum optics. For such interactions, our scheme is universal and works in any regime, including the limits of weak and strong nonlinearities.

Demonstration of deterministic and high fidelity squeezing of quantum information

Jun-ichi Yoshikawa, Toshiki Hayashi, Takayuki Akiyama, Nobuyuki Takei, Alexander Huck, Ulrik L. Andersen, Akira Furusawa

By employing a recent proposal [R. Filip, P. Marek, and U.L. Andersen, Phys. Rev. A 71, 042308 (2005)] we experimentally demonstrate a universal, deterministic, and high-fidelity squeezing transformation of an optical field. It relies only on linear optics, homodyne detection, feedforward, and an ancillary squeezed vacuum state, thus direct interaction between a strong pump and the quantum state is circumvented. We demonstrate three different squeezing levels for a coherent state input. This scheme is highly suitable for the fault-tolerant squeezing transformation in a continuous variable quantum computer.

Evanescent wave broadband cavity enhanced absorption spectroscopy using supercontinuum radiation: A new probe of electrochemical processes

Mathias Schnippering, Patrick R. Unwin, Johan Hult, Toni Laurila, Clemens F. Kaminski, Justin M. Langridge, Roderic L. Jones, Mikhail Mazurenka, Stuart R. Mackenzie

An evanescent wave variant of broadband cavity enhanced absorption spectroscopy using a supercontinuum light source has been used to detect electrogenerated species at the silica-water interface. In proof-of-concept experiments [IrCl(6)](2-) was produced by electro-oxidation of [IrCl(6)](3-) in a thin layer electrochemical cell. Diffusion of the Ir(IV) across the cell to a silica interface was monitored yielding real-time concentrations within an evanescent field region at the interface. The optical response was compared with the electrochemical response during chronoamperometric step and cyclic voltammetric experiments and both were simulated by finite element modeling. The experiment is highly sensitive to interfacial processes and its wide spectral width and fast time resolution make it a potentially powerful tool for in situ spectroscopic monitoring of processes and intermediates in dynamical electrochemistry. (C) 2008 Elsevier B.V. All rights reserved.

Dispersive properties of rocking filters in highly birefringent photonic crystal fiber

L. Y. Zang, M. S. Kang, G. J. Pearce, M. Scharrer, S. Rammler, P. St. J. Russell

We study the dispersive properties of the eigenmodes of rocking filters produced in highly birefringent photonic crystal fibers. The dispersion changes dramatically close to filter resonance. Rocking filters provide more flexibility in dispersion engineering.

Photonic crystal fibers: Basics and applications

Philip St John Russell

Experimental generation of a partially coherent flat-topped beam

Fei Wang, Yangjian Cai

An efficient optical system is proposed to generate a partially coherent flat-topped (FT) beam theoretically. Furthermore, we report the experimental generation of a partially coherent FT beam based on the proposed optical system. The intensity distribution and the modulus of the square of the spectral degree of coherence of the generated partially coherent FT beam are measured. The experimental results are consistent with the theoretical results. (c) 2008 Optical Society of America.

Anomalous pulse breakup in small-core photonic crystal fibers

A. Podlipensky, P. Szarniak, N. Y. Joly, P. St. J. Russell

Detailed numerical and experimental studies of propagation of 110 fs laser pulses at 800 am in small-core photonic crystal fibers (gamma = 100 W (1) km(-1)) reveal that pulse breakup occurs in two distinct regimes defined by the input power. At low peak power (soliton order N <= g7) higher-order soliton fission occurs: individual solitons being ejected from the input pulse one after the other and are at-ranged in wavelength and in time by peak power. At higher levels of peak power (N>8), pulse breakup results in ejection of bound soliton pairs and the formation of single solitons that collide during propagation. (C) 2008 Optical Society of America

Experimental Demonstration of Macroscopic Quantum Coherence in Gaussian States

Ch. Marquardt, U. L. Andersen, G. Leuchs, Y. Takeno, M. Yukawa, H. Yonezawa, A. Furusawa

We experimentally proved the existence of macroscopic coherences in Gaussian quantum states using a recently proposed criterion [I]. We show interferences with remarkable distances in phase space for squeezed, entangled and even for coherent states. (C) 2008 Optical Society of America

Ballistic transport in semiconductor nanostructures: From quasi-classical oscillations to novel THz-emitters

G. H. Doehler, M. Eckardt, A. Schwanhaeusser, F. Renner, S. Malzer, S. Trumm, M. Betz, F. Sotier, A. Leitenstorfer, G. Loata, et al.

By suitable design it is possible to achieve quasi-ballistic transport in semiconductor nanostructures over times up to the ps-range. Monte-Carlo simulations reveal that under these conditions phase-coherent real-space oscillations of an electron ensemble, generated by fs-pulses become possible in wide potential wells. Using a two-color pump-and-probe technique we have been able to observe this new phenomenon in excellent agreement with the theoretical predictions. Apart from its fundamental significance, ballistic transport in nanostructures can also be used for high-efficiency coherent THz-sources. The concept of these THz-emitters and its experimental confirmation will also be presented.

A monolithically integrated intensity-independent polarization-sensitive switch operating at 1.3 mu m based on ordering in InGaAsP

S Kramer, S Neumann, W Prost, FJ Tegude, S Malzer, GH Dohler

Recently we have shown that the quaternary semiconductor InGaAsP spontaneously forms a mono-atomic superlattice oriented in [111](B)-direction if grown under suitable conditions by MOVPE. The superlattice-induced reduction of the crystal-symmetry leads to a much larger polarisation anisotropy as in ternary III/V-semiconductors, as the ordering affects both the group-III- and the group-V-sublattice. In this contribution we report on an optoelectronic switch, based on the absorption anisotropy for [110]- and [1 (1) over bar0]-polarised light. Switching between "on" and "off" depends on the polarisation angle only, but hardly on the intensity. (c) 2006 Elsevier B.V. All rights reserved.

Sub-shot-noise phase quadrature measurement of intense light beams

O Glockl, Ulrik L. Andersen, S Lorenz, Christine Silberhorn, N Korolkova, Gerd Leuchs

We present a setup for performing sub-shot-noise measurements of the phase quadrature of intense pulsed light without the use of a separate local oscillator. A Mach-Zehnder interferometer with an unbalanced arm length is used to detect the fluctuations of the phase quadrature at a single sideband frequency. With this setup, the nonseparability of a pair of quadrature-entangled beams is demonstrated experimentally. (C) 2004 Optical Society of America.

Quantum information with continuous variables

SL Braunstein, P van Loock

Quantum information is a rapidly advancing area of interdisciplinary research. It may lead to real-world applications for communication and computation unavailable without the exploitation of quantum properties such as nonorthogonality or entanglement. This article reviews the progress in quantum information based on continuous quantum variables, with emphasis on quantum optical implementations in terms of the quadrature amplitudes of the electromagnetic field.

Ultrafast all-optical switching of 3D photonic band gap crystals - art. no. 66400G

T. G. Euser, H. Wei, J. Kalkman, Y. Jun, M. Guina, S. Suomalainen, A. Polman, D. J. Norris, W. L. Vos

We present ultrafast optical switching experiments on 3D photonic band gap crystals. Switching the Si inverse opal is achieved by optically exciting free carriers by a two-photon process. We probe reflectivity in the frequency range of second order Bragg diffraction where the photonic band gap is predicted. We observe a large frequency shift of up to 1.5% of all spectral features including the peak that corresponds to the photonic band gap. We also demonstrate large, ultrafast shifts of stop bands of planar GaAs/AlAs photonic structures. We briefly discuss how our results can be used in future switching and modulation applications.

Analysis and optimization of coupling to external cavities in feedback experiments with vertical-cavity surface-emitting lasers

K. F. Jentsch, M. Sondermann, T. Ackemann

The feedback strength is a crucial parameter for feedback experiments using semiconductor lasers. In this article, the coupling efficiency of the field of vertical-cavity surface-emitting lasers (VCSELs) to external cavities containing one collimating lens has been analyzed in detail using ABCD-matrix methods. It is found that for a given set of parameters there are two distinct, experimentally realizable positions of the collimating lens which allow for optimal coupling, if the cavity length is sufficiently small. The predictions are verified in experiments using single-transverse-mode VCSELs. The obtained coupling efficiencies exceed 70%. (c) 2007 Elsevier B.V. All rights reserved.

Effective Negative Nonlinearity of a Nonlinear Amplifying Loop Mirror for Compensating Nonlinearity-Induced Signal Distortions

Klaus Sponsel, Kristian Cvecek, Christian Stephan, Georgy Onishchukov, Bernhard Schmauss, Gerd Leuchs

Conditions for realizing effective negative nonlinearity using a NALM are analyzed. Efficient compensation of SPM-induced nonlinear phase noise in a 100Gb/s DPSK system is confirmed in numerical simulations.

Phase-preserving amplitude regeneration for a WDM RZ-DPSK signal using a nonlinear amplifying loop mirror

K. Cvecek, K. Sponsel, C. Stephan, G. Onishchukov, R. Ludwig, C. Schubert, B. Schmauss, G. Leuchs

We propose a modified nonlinear amplifying loop mirror (NALM) for phase-preserving 2R regeneration of wavelength division multiplexed (WDM) return-to-zero differential phase-shift-keyed signals. As proof of principle the regeneration capability of this NALM setup has been investigated experimentally for two 10 Gbit/s wavelength channels. A significant eye-opening improvement and a negative power penalty of 1.2 dB have been observed in both channels. (C) 2008 Optical Society of America.

Recent developments in quantum key distribution: Theory and practice

Wolfgang Mauerer, Wolfram Helwig, Christine Silberhorn

Quantum key distribution is among the foremost applications of quantum mechanics, both in terms of fundamental physics and as a technology on the brink of commercial deployment. Starting from principal schemes and initial proofs of unconditional security for perfect systems, much effort has gone into providing secure schemes which can cope with numerous experimental imperfections unavoidable in real world implementations. In this paper, we provide a comparison of various schemes and protocols. We analyse their efficiency and performance when implemented with imperfect physical components. We consider how experimental faults are accounted for using effective parameters. We compare various recent protocols and provide guidelines as to which components propose best advances when being improved. (c) 2008 WILEY-VCH Verlag GmbH & Co. KGaA. Weinheim.

Propagation of laser array beams in a turbulent atmosphere

Y. Cai, Y. Chen, H. T. Eyyuboglu, Y. Baykal

The propagation of phase-locked and non-phase-locked laser array beams of radial and rectangular symmetries in a turbulent atmosphere are investigated based on the extended Huygens-Fresnel integral. The beamlet used in our paper for constructing the laser array beams is of elliptical Gaussian mode. Analytical formulae for the average irradiance of phase-locked and non-phase-locked radial and rectangular laser array beams are derived through vector integration and tensor operation. The irradiance properties of these laser array beams in a turbulent atmosphere are studied numerically. It is found that both phase-locked and non-phase-locked radial and rectangular laser array beams eventually become circular Gaussian beams in a turbulent atmosphere, which is much different from their propagation properties in free space. The propagation properties are closely related to the parameters of laser array beams and the structure constant of the turbulent atmosphere.

Photonic sensing based on variation of propagation properties of photonic crystal fibres

John H. Rothwell, Donal A. Flavin, William N. MacPherson, Julian D. C. Jones, Jonathan C. Knight, Philip St. J. Russell

We report on a low-coherence interferometric scheme for the measurement of the strain and temperature dependences of group delay and dispersion in short, index-guiding, 'endlessly-single-mode' photonic crystal fibre elements in the 840 nm and 1550 nm regions. Based on the measurements, we propose two schemes for simultaneous strain and temperature measurement using a single unmodified PCF element, without a requirement for any compensating components, and we project the measurement accuracies of these schemes.

Optical properties of photonic crystal fiber with integral micron-sized Ge wire

H. K. Tyagi, M. A. Schmidt, L. Prill Sempere, P. St. J. Russell

Using a selective hole closure technique, individual hollow channels in silica-air photonic crystal fibers are filled with pure Ge by pumping in molten material at high pressure. The smallest channels filled so far are 600 nm in diameter, which is 10x smaller than in previous work. Electrical conductivity and micro-Raman measurements indicate that the resulting cm-long wires have a high degree of crystallinity. Optical transmission spectra are measured in a sample with a single wire placed adjacent to the core of an endlessly single-mode photonic crystal fiber. This renders the fiber birefringent, as well as causing strongly polarization-dependent transmission losses, with extinction ratios as high as 30 dB in the visible. In the IR, anti-crossings between the glass-core mode and resonances on the high index Ge wire create a series of clear dips in the spectrum transmitted through the fiber. The measurements agree closely with the results of finite-element simulations in which the wavelength dependence of the dielectric constants is taken fully into account. A toy model based on a multilayer structure is used to help interpret the results. Finally, the temperature dependence of the anti-crossing wavelengths is measured, the preliminary results suggesting that the structure might form the basis of a compact optical thermometer. Since Ge provides electrical conductance together with low-loss guidance in the mid-IR, Ge-filled PCF seems likely to lead to new kinds of in-fiber detector and sensor, as well as having potential uses in ultra-low-threshold nonlinear optical devices. (C) 2008 Optical Society of America

Partially coherent anomalous hollow beam and its paraxial propagation

Yangjian Cai, Fei Wang

Anomalous hollow beam is extended to the partially coherent case. Analytical propagation formulae for a partially coherent anomalous hollow beam passing through a paraxial ABCD optical system are derived. The propagation properties of a partially coherent anomalous hollow beam in free space and the focusing properties of a partially coherent anomalous hollow beam are studied numerically It is found that the. propagation and focusing properties of the partially coherent anomalous hollow beam are closely related to its initial coherence. (C) 2008 Elsevier B.V. All rights reserved.

Electronic noise-free measurements of squeezed light

Leonid A. Krivitsky, Ulrik L. Andersen, Ruifang Dong, Alexander Huck, Christoffer Wittmann, Gerd Leuchs

We study the implementation of a correlation measurement technique for the characterization of squeezed light. We show that the sign of the covariance coefficient revealed from the time-resolved correlation data allow us to distinguish among squeezed, coherent, and thermal states. In contrast to the traditional method of characterizing squeezed light, involving measurement of the variation of the difference photocurrent, the correlation measurement method allows one to eliminate the contribution of the electronic noise, which becomes a crucial issue in experiments with dim sources of squeezed light. (C) 2008 Optical Society of America

Polarization-dependent coupling to plasmon modes on submicron gold wire in photonic crystal fiber

H. W. Lee, M. A. Schmidt, H. K. Tyagi, L. Prill Sempere, P. St. J. Russell

We present experimental results on coupling to surface plasmon modes on gold nanowires selectively introduced into polarization-maintaining photonic crystal fibers. Highly polarization- and wavelength-dependent transmission is observed. In one sample 24.5 mm long, the transmission on and off resonance differs by as much as 45 dB. Near-field optical images of the light emerging from such a gold-filled fiber show light guided on the wire at surface plasmon resonances. Finite element simulations are in good agreement with the experimental results. These gold-filled fibers can be potentially used as in-fiber wavelength-dependent filters and polarizers and as near-field tips for sub-wavelength-scale imaging. (C) 2008 American Institute of Physics.

Optical amplification at the quantum limit

Ulrik L. Andersen, Gerd Leuchs

In this paper we evaluate experimentally measured signal-to-noise ratio performance of different types of optical amplifiers [V. Josse, M. Sabuncu, N. Cerf, et al., Phys. Rev. Lett. 96 163602 ( 2006); Z. Y. Ou, S. F. Pereira and H. J. Kimble, Phys. Rev. Lett. 70 3239 ( 1993)]. We pay particular attention to the low gain regime where the quantum limit of the amplifier varies with gain. Independent of the amplifier type the noise performance is remarkably close to the quantum limit. Low gain optical amplifiers are of interest for quantum communication.

An efficient Fredholm method for the calculation of highly excited states of billiards

Hakan E. Tureci, Harald G. L. Schwefel

A numerically efficient Fredholm formulation of the billiard problem is presented. The standard solution in the framework of the boundary integral method in terms of a search for roots of a secular determinant is reviewed first. We next reformulate the singularity condition in terms of a flow in the space of an auxiliary one-parameter family of eigenproblems and argue that the eigenvalues and eigenfunctions are analytic functions within a certain domain. Based on this analytic behavior, we present a numerical algorithm to compute a range of billiard eigenvalues and associated eigenvectors by only two diagonalizations.

Phase-locked anti-Stokes Raman generation in gas-filled hollow-core photonic crystal fibers

A. Nazarkin, P. St. J. Russell

We show that the efficient non-phase-matched anti-Stokes Raman generation observed in gas-filled hollow-core PCF can be explained by "phase locking" of the interacting fields leading to the establishment of a phase difference independent of the optical path.

Absolute frequency and isotope shift measurements of the cooling transition in singly ionized indium

Y. H. Wang, R. Dumke, J. Zhang, T. Liu, A. Stejskal, Y. N. Zhao, Z. H. Lu, L. J. Wing, Th. Becker, H. Walther, et al.

We report greater than two orders of magnitude improvements in the absolute frequency and isotope shift measurements of the In(+) 5s(2) (1)S(0) (F - 9/2)-5s5p (3)P(1) (F = 11/2) transition near 230.6 urn. The laser-induced fluorescence from a single In(+) in a radio-frequency trap is detected. The fourth-harmonic of a semiconductor laser is used as the light source. The absolute frequency is measured with the help of a frequency comb referenced to a Cs atomic clock. The resulting transition frequencies for isotopes 115In(+) and 113In(+) are measured to be 1299 648 954.54(10) MHz and 1299 649 585.36(16) MHz, respectively. The deduced cooling transition frequency difference is 630.82(19) MHz. By taking into account of the hyperfine interaction, the isotope shift is calculated to be 695.76(1.68) MHz.

Connection of anisotropic conductivity to tip-induced space-charge layers in scanning tunneling spectroscopy of p-doped GaAs

S. Loth, M. Wenderoth, R. G. Ulbrich, S. Malzer, G. H. Doehler

The electronic properties of shallow acceptors in p-doped GaAs{110} are investigated with scanning tunneling microscopy (STM) at low temperature. Shallow acceptors are known to exhibit distinct triangular contrasts in STM images for certain bias voltages. Spatially resolved I(V) spectroscopy is performed to identify their energetic origin and behavior. A crucial parameter-the scanning tunneling microscope tip's work function-is determined experimentally. The voltage dependent potential configuration and band bending situation are derived. Ways to validate the calculations with the experiment are discussed. Differential conductivity maps reveal that the triangular contrasts are only observed with a depletion layer present under the STM tip. The tunnel process leading to the anisotropic contrasts calls for electrons to tunnel through vacuum gap and a finite region in the semiconductor.

Dielectric microcavities for THz radiation: identical mode spectrum and coupling in poly-ethylene disks

S. Preu, H. G. L. Schwefel, S. Maker, G. H. Doehler, L. J. Wang

We report the fabrication and characterization of identical dielectric whispering gallery microcavites for teraherz (THz) radiation. The THz radiation is coupled via a tapered waveguide into the coupled cavities. Mode splitting is observed. (C) 2008 Optical Society of America

Improved absolute frequency measurement of the In-115(+) 5s(2) S-1(0)-5s5p P-3(0) narrowline transition: Progress towards an optical frequency standard

Y. H. Wang, T. Liu, R. Dumke, A. Stejskal, Y. N. Zhao, J. Zhang, Z. H. Lu, L. J. Wang, Th. Becker, H. Walther, et al.

We report on an improved absolute frequency measurement of the 5s(2 1)S(0)-5s5p(3) P-0 narrowline clock transition at 236.5 nm, for a single, trapped, and laser-cooled In-115 ion. Using a narrowline laser as the local oscillator, a linewidth of 43 Hz for the transition is resolved. The uncertainty of the transition frequency's centroid is 18 Hz, leading to a fractional uncertainty of 1.4 x 10(-14). For absolute frequency measurement, we use an optical frequency comb locked to a cesium clock as the reference. The transition frequency is found to be 1267402452900967(63) Hz, averaged over 13 days of separate measurements. The accuracy is about 5.0 x 10(14). We discuss possibilities for further improvement.

Intercept-resend attacks in the Bennett-Brassard 1984 quantum-key-distribution protocol with weak coherent pulses

M Curty, N Lutkenhaus

Unconditional security proofs of the Bennett-Brassard 1984 protocol of quantum key distribution have been obtained recently. These proofs cover also practical implementations that utilize weak coherent pulses in the four signal polarizations. Proven secure rates leave open the possibility that new proofs or new public discussion protocols will obtain larger rates over increased distance. In this paper we investigate limits to the error rate and signal losses that can be tolerated by future protocols and proofs.

Scintillation of astigmatic dark hollow beams in weak atmospheric turbulence

Yangjian Cai, Halil T. Eyyuboglu, Yahya Baykal

The scintillation properties of astigmatic dark hollow beams (DHBs) in weak atmospheric turbulence were investigated in detail. An explicit expression for the on-axis scintillation index of an astigmatic DH13 propagating in weak atmospheric turbulence was derived. It was found that the scintillation index value of an astigmatic DH13 with suitable astigmatism (i.e., ratio of the beam waist size in the x direction to that in the y direction), dark size, beam waist size, and wavelength can be smaller than that of a stigmatic DH13 and that of stigmatic and astigmatic flat-topped, annular, and Gaussian beams in weak atmospheric turbulence particularly at long propagation ranges. Our results will be useful in long-distance free-space optical communications. (c) 2008 Optical Society of America

Distillation of squeezing from non-Gaussian quantum states

J. Heersink, Ch. Marquardt, R. Dong, R. Filip, S. Lorenz, G. Leuchs, U. L. Andersen

We show that single copy distillation of squeezing from continuous variable non-Gaussian states is possible using linear optics and conditional homodyne detection. A specific non-Gaussian noise source, corresponding to a random linear displacement, is investigated experimentally. Conditioning the signal on a tap measurement, we observe probabilistic recovery of squeezing.

Up-tapering of Optical Fibers Using a Conventional Flame Tapering Rig

G. Kakarantzas, L. Prill-Sempere, P. St. J. Russell

We demonstrate the fabrication of low-loss up-tapers in SMF-28 using a conventional tapering rig. Waist diameters of 240 pm, uniform over several cm, have been produced. The technique also works for photonic crystal fibers. (C)2007 Optical Society of America

Bismuth-activated luminescent materials for broadband optical amplifier in WDM system

Mingying Peng, Botao Wu, Ning Da, Chen Wang, Danping Chen, Congshan Zhu, Jianrong Qiu

New broadband near infrared luminescence covering the whole work windows (1260-1625 nm) of the current wavelength division multiplexing (WDM) system was found from bismuth-activated M2O-Al2O3-SiO2 (M = Li, Na) and Li2O-Ta2O5-SiO2 glasses at room temperature in the case of 808 nm-laser excitation. But the near infrared luminescence mechanism of the bismuth-activated glasses is not well understood up to now. The figure-of-merits of bandwidth and gain of the glasses are better than those of Er3+-doped silicate glasses and Ti3+ doped sapphire, implying they are the promising gain-medium candidates for the broadband amplifiers and the widely tunable laser sources. (c) 2007 Elsevier B.V. All rights reserved.

Optimization of a nonlinear amplifying loop mirror for amplitude regeneration in phase-shift-keyed transmission

Klaus Sponsel, Kristian Cvecek, Christian Stephan, Georgy Onishchukov, Bernhard Schmauss, Gerd Leuchs

We present the numerical optimization of the transmission characteristics of a nonlinear amplifying loop mirror for amplitude regeneration of phase-encoded optical transmission formats. Adjusting the splitting factor, the amplifier gain and the phase bias, minimal phase distortions can be achieved while strong amplitude fluctuations are regenerated. The limiting effects of noise from the built-in amplifier and of amplified Rayleigh backscattering are also discussed.

Truncated su(2) moment problem for spin and polarization states

Tobias Moroder, Michael Keyl, Norbert Luetkenhaus

We address the problem whether a given set of expectation values is compatible with the first and second moments of the generic spin operators of a system with total spin j. Those operators appear as the Stokes operator in quantum optics, as well as the total angular momentum operators in the atomic ensemble literature. We link this problem to a particular extension problem for bipartite qubit states; this problem is closely related to the symmetric extension problem that has recently drawn much attention in different contexts of the quantum information literature. We are able to provide operational, approximate solutions for very large spin numbers, and in fact the solution becomes exact in the limiting case of infinite spin numbers. Solutions for low spin numbers are formulated in terms of a hyperplane characterization, similar to entanglement witnesses, which can be efficiently solved with semidefinite programming.

A new 4 pi geometry optimized for focusing on an atom with a dipole-like radiation pattern

N. Lindlein, R. Maiwald, H. Konermann, M. Sondermann, U. Peschel, G. Leuchs

Focusing electromagnetic radiation efficiently onto an atom requires an open geometry, which is as close to the full solid angle as possible. Additionally, the radiant intensity should be as close as possible to a dipole radiation in order to have a similar field distribution as in the emission process. Here, we propose to make use of a novel combination of a parabolic mirror and a diffractive optical element.

Metal nanowire arrays in photonic crystal fibers

L. N. Prill Sempere, M. A. Schmidt, H. K. Tyagi, C. G. Poulton, P. St. J. Russell

Metallic nanowire arrays are created by pumping molten metal into the hollow channels of silica glass PCF. Measurements show that, at certain wavelengths, the core-guided light couples to leaky surface plasmon modes on the nanowires.

Scintillation index of elliptical Gaussian beam in turbulent atmosphere

Yangjian Cai, Yuntian Chen, Halil T. Eyyuboglu, Yahya Baykal

A tensor method is used to formulate the on-axis scintillation index for an elliptical Gaussian beam (EGB; astigmatic Gaussian beam) propagating in a weak turbulent atmosphere. Variations of the on-axis scintillation of an EGB are studied. It is interesting to find that the scintillation index of an EGB can be smaller than that of a circular Gaussian beam in a weakly turbulent atmosphere under certain conditions and is closely related to the ratio of the beam waist size along the long axis to that along the short axis of the EGB, the wavelength, and the structure constant of the turbulent atmosphere. (C) 2007 Optical Society of America.

Average irradiance and polarization properties of a radially or azimuthally polarized beam in a turbulent atmosphere

Yangjian Cai, Qiang Lin, Halil T. Eyyuboglu, Yahya Baykal

Analytical formulas are derived for the average irradiance and the degree of polarization of a radially or azimuthally polarized doughnut beam (PDB) propagating in a turbulent atmosphere by adopting a beam coherence-polarization matrix. It is found that the radial or azimuthal polarization structure of a radially or azimuthally PDB will be destroyed (i.e., a radially or azimuthally PDB is depolarized and becomes a partially polarized beam) and the doughnut beam spot becomes a circularly Gaussian beam spot during propagation in a turbulent atmosphere. The propagation properties are closely related to the parameters of the beam and the structure constant of the atmospheric turbulence. (C) 2008 Optical Society of America.

FDTD-modelling of dispersive nonlinear ring resonators: Accuracy studies and experiments

Christian Koos, Masafumi Fujii, Christopher G. Poulton, Ralf Steingrueber, Juerg Leuthold, Wolfgang Freude

The accuracy of nonlinear finite-difference time-domain (FDTD) methods is investigated by modeling nonlinear optical interaction in a ring resonator. We have developed a parallelized 3-D FDTD algorithm which incorporates material dispersion, chi((3))-nonlinearities and stair-casing error correction. The results of this implementation are compared and experiments, and intrinsic errors of the FDTD algorithn, Or', from geometrical uncertainties arising from,the falif. icition tolerances of the device. A series of progressively less c0n0ex FDTD models is investigated, omitting material dispersior abandoning the stair-casing error correction, and approximating the structure by a 2-D effective index model. We compare the results of the different algorithms and give guidelines as to which degree of complexity is needed in order to obtain reliable simulation results in the linear and the nonlinear regime. In both cases, incorporating stair-casing error correction and material dispersion into a 2-D effective index model turns out to be computationally much cheaper and more effective than performing a fully three-dimensional simulation without these features.

Upper bounds on success probabilities in linear optics

S Scheel, N Lutkenhaus

We develop an abstract way of de. ning linear-optics networks designed to perform quantum information tasks such as quantum gates. We will be mainly concerned with the non-linear sign shift (NSS) gate, but it will become obvious that all other gates can be treated in a similar manner. The abstract scheme is extremely well suited for analytical as well as numerical investigations since it reduces the number of parameters for a general setting. With that, we show numerically and partially analytically for a wide class of states that the success probability of generating a NSS gate does not exceed 1/4, which to our knowledge is the strongest bound to date.

High numerical aperture imaging with different polarization patterns

N. Lindlein, S. Quabis, U. Peschel, G. Leuchs

The modulation transfer function (MTF) is calculated for imaging with linearly, circularly and radially polarized light as well as for different numerical apertures and aperture shapes. Special detectors are only sensitive to one component of the electric energy density, e. g. the longitudinal component. For certain parameters this has advantages concerning the resolution when comparing to polarization insensitive detectors. It is also shown that in the latter case zeros of the MTF may appear which are purely due to polarization effects and which depend on the aperture angle. Finally some ideas are presented how to use these results for improving the resolution in lithography. (c) 2007 Optical Society of America

NOLM-based RZ-DPSK signal regeneration

AG Striegler, M Meissner, K Cvecek, K Sponsel, G Leuchs, B Schmauss

We present a nonlinear optical loop mirror (NOLM)-based 2R-regenerator setup, which signals modulated in phase-sensitive modulation formats. In a conventional NOLM, fluctuations of the signal amplitude are converted into phase fluctuations. Therefore, it is not suitable for regeneration of signals, modulated in formats such as differential phase-shift keying (DPSK) or duobinary. In this letter, we present a modified NOLM setup for 2R-regeneration taking return-to-zero DPSK as an example.

Optimum splitting ratio for amplifier noise reduction by an asymmetric nonlinear optical loop mirror

M Meissner, M Rosch, B Schmauss, G Leuchs

We theoretically and experimentally analyze the influence of the splitting ratio and the input power on the noise reduction capability of an asymmetric nonlinear optical loop mirror (NOLM) for different input noise levels. An easy method to calculate the optimum parameters for noise reduction is also presented. The best noise reduction is found at NOLM input powers at which the nonlinear transfer function has a slope close to zero. Additionally, the splitting ratio of the NOLM has to be adapted to its input noise level to suppress amplitude fluctuations effectively. Since the noise reduction by the NOLM is due to the Kerr nonlinearity, which has a timescale below a few femtoseconds, the noise reduction is applicable to short pulses in the picosecond and femtosecond range. This makes the NOLM applicable as an optical regenerator in an optical data transmission system at high bit rates, such as 160 GBit/s.

Propagation of Bessel and Bessel-Gaussian beams through an unapertured or apertured misaligned paraxial optical systems

Yangjian Cai, Xiang Lu

Propagation of Bessel and Bessel-Gaussian beams through an unapertured or apertured misaligned paraxial optical system is investigated. Analytical formulas for Bessel and Bessel-Gaussian beams propagating through an unapertured misaligned paraxial optical system are derived based on the generalized diffraction integral formula for treating the propagation of a laser beam through a misaligned paraxial optical system in the cylindrical coordinate system. By expanding the hard aperture function into a finite sum of complex Gaussian functions, some approximate analytical formulas are derived for Bessel and Bessel-Gaussian beams propagating through an apertured misaligned paraxial optical system. Some numerical examples are illustrated. The present analytical formulas provide a convenient and effective way for studying the propagation and transformation of Bessel and Bessel-Gaussian beams through an unapertured or apertured misaligned paraxial optical system. (C) 2007 Elsevier B.V. All rights reserved.

An alternative theoretical model for an anomalous hollow beam

Yangjian Cai, Zhaoying Wang, Qiang Lin

An alternative and convenient theoretical model is proposed to describe a flexible anomalous hollow beam of elliptical symmetry with an elliptical solid core, which was observed in experiment recently (Phys. Rev. Lett, 94 (2005) 134802). In this model, the electric field of anomalous hollow beam is expressed as a finite sum of elliptical Gaussian modes. Flat-topped beams, dark hollow beams and Gaussian beams are special cases of our model. Analytical propagation formulae for coherent and partially coherent anomalous hollow beams passing through astigmatic ABCD optical systems are derived. Some numerical examples are calculated to show the propagation and focusing properties of coherent and partially coherent anomalous hollow beams. (C) 2008 Optical Society of America.

Evolution of the degree of polarization of an electromagnetic Gaussian Schell-model. beam in a Gaussian cavity

Min Yao, Yangjian Cai, Halil T. Eyyuboglu, Yahya Baykal, Olga Korotkova

The interaction of an electromagnetic Gaussian Schell-model (EGSM) beam with a Gaussian cavity is analyzed. In particular, the evolution of the degree of polarization of the EGSM beam is investigated. The results show that the behavior of the degree of polarization depends on both the statistical properties of the source that generates the EGSM beam and the parameter of the cavity. (C) 2008 Optical Society of America

Discreteness in time

Ulf Peschel, Christoph Bersch, Georgy Onishchukov

In this paper we discuss the joint propagation of a periodically modulated field and a pulse of different frequency in an optical fiber. The pulse experiences the action of an index lattice induced via cross-phase modulation by the periodic field. We predict effects of discreteness to show up both in the temporal and in the spatial domain. For large walk-off between the two fields one should observe Bloch oscillations in frequency space, where discrete diffraction is expected to occur for equal velocities of both waves.

Scintillation advantages of lowest order Bessel-Gaussian beams

H. T. Eyyuboglu, Y. Baykal, E. Sermutlu, Y. Cai

For a weak turbulence propagation environment, the scintillation index of the lowest order Bessel-Gaussian beams is formulated. Its triple and single integral versions are presented. Numerical evaluations show that at large source sizes and large width parameters, when compared at the same source size, Bessel-Gaussian beams tend to exhibit lower scintillations than the Gaussian beam scintillations. This advantage is lost however for excessively large width parameters and beyond certain propagation lengths. Large width parameters also cause rises and falls in the scintillation index of off-axis positions toward the edges of the received beam. Comparisons against the fundamental Gaussian beam are made on equal source size and equal power basis.

On the improvement of two-dimensional curvature computation and its application to turbulent premixed flame correlations

R. S. M. Chrystie, I. S. Burns, J. Hult, C. F. Kaminski

Measurement of curvature of the flamefront of premixed turbulent flames is important for the validation of numerical models for combustion. In this work, curvature is measured from contours that outline the flamefront, which are generated from laser-induced fluorescence images. The contours are inherently digitized, resulting in pixelation effects that lead to difficulties in computing curvature of the flamefront accurately. A common approach is to fit functions locally to short sections along the flame contour, and this approach is also followed in this work; the method helps smoothen the pixelation before curvature is measured. However, the length and degree of the polynomial, and hence the amount of smoothing, must be correctly set in order to maximize the precision and accuracy of the curvature measurements. Other researchers have applied polynomials of different orders and over different segment lengths to circles of known curvature as a test to determine the appropriate choice of polynomial; it is shown here that this method results in a sub-optimal choice of polynomial function. Here, we determine more suitable polynomial functions through use of a circle whose radius is sinusoidally modulated. We show that this leads to a more consistent and reliable choice for the local polynomial functions fitted to experimental data. A polynomial function thus determined is then applied to flame contour data to measure curvature of experimentally acquired flame contours. The results show that there is an enhancement in local flame speed at sections of the flamefront with a non-zero curvature, and this agrees with numerical models.

Spin lifetimes and strain-controlled spin precession of drifting electrons in GaAs

M. Beck, C. Metzner, S. Malzer, G. H. Doehler

We study the transport of spin-polarized electrons in n-GaAs using spatially resolved continuous-wave Faraday rotation. From the measured steady-state distribution, we determine spin relaxation times under drift conditions and, in the presence of strain, the induced spin splitting from the observed spin precession. Controlled variation of strain along [110] allows us to deduce the deformation potential causing this effect, while strain along [100] has no effect. The electric-field dependence of the spin lifetime is explained quantitatively in terms of an increase of the electron temperature.

TE- and TM-polarization-resolved spectroscopy on quantum wells under normal incidence

M Schardt, A Winkler, G Rurimo, M Hanson, D Driscoll, S Quabis, S Malzer, G Leuchs, GH Doehler, AC Gossard, et al.

We present TE- and TM-polarization-resolved photocurrent measurements on quantum well pin diodes under normal incidence. Usually, optical experiments performed in such a geometry yield information only about transitions involving in-plane (p, and p,) components of the hole wave functions because of the in-plane (TE) polarization of the light. Information on transitions sensitive to the p, components can be obtained by focussing a radially polarized laser beam through a microscope objective with high numerical aperture (NA = 0.9). With our setup, the electrical field vector at the focal tail has a significant component along the optical axis (TM-polarization!) which enables excitation of transitions sensitive to p, components also. Additionally, the existence of a degenerate (azimuthally polarized) optical mode enables switching these p. components on and off easily. Experimental evidence of these features has been achieved by exploiting the selection rules for e-hh and e-lh transitions in a quantum well structure. We present a comparison of our recorded spectra with theoretical predictions obtained from simple geometric optics assumptions. For our quantum wells the polarization effects are small because our measurement averages the intensity distribution of the whole focal plane. We plan to extend our measurements to polarization resolved single quantum dot spectroscopy. By restricting the detection region to the spatial extent of a single dot, one can exploit the almost pure TM-polarization on the optical axis for obtaining high contrast between heavy- and light-hole exciton absorption. (c) 2006 Published by Elsevier B.V.

Highly collimated and directional continous-wave Terahertz emission by photomixing in semiconductor device arrays - art. no. 61940F

S. Preu, S. Malzer, G. H. Doehler, J. Zhang, Z. H. Lu, L. J. Wang

CW-photomixing semiconductor devices have hardly exceeded an output power of 10 mu W around 1 THz. Availability of a few mNV, however, would stimulate the demand for THz-imaging, -scanning, and spectroscopy. Increasing the poor power conversion efficiency from the optical pump to THz-output is most desirable. On the other hand, the thermal threshold "per pixel" is limited to about 100 mW of pump laser power. So both limits have to be pushed towards higher performance. In this paper we report on arrays of photomixing devices to overcome the thermal threshold limit. If each individual photomixer in the array can be driven to the same thermal threshold power, the overall THz output can be larger by a factor N x AY for an array. The power of directed emission, however, can be increased even by a factor (N x M)' compared to the individual device. In addition, by adjusting the two laser beams slightly noncollinear, a directional control of the emitted THz-beam is achieved. The angular difference of the incident beams is enhanced by the ratio of the THz-wavelength (approximate to 300 mu m) and the optical wavelength (approximate to 0.85 mu m) with regard to direction of the emitted THz-beam. Thus, a full steering of the THz beam can be achieved by tuning this angle by less than 1 degree (17.5 mrad).

Entanglement verification for quantum-key-distribution systems with an underlying bipartite qubit-mode structure

J Rigas, O Guhne, N Lutkenhaus

We consider entanglement detection for quantum-key-distribution systems that use two signal states and continuous-variable measurements. This problem can be formulated as a separability problem in a qubit-mode system. To verify entanglement, we introduce an object that combines the covariance matrix of the mode with the density matrix of the qubit. We derive necessary separability criteria for this scenario. These criteria can be readily evaluated using semidefinite programming and we apply them to the specific quantum key distribution protocol.

Experimentally realizable quantum comparison of coherent states and its applications

Erika Andersson, Marcos Curty, Igor Jex

When comparing quantum states to each other, it is possible to obtain an unambiguous answer, indicating that the states are definitely different, already after a single measurement. In this paper we investigate comparison of coherent states, which is the simplest example of quantum state comparison for continuous variables. The method we present has a high success probability, and is experimentally feasible to realize as the only required components are beam splitters and photon detectors. An easily realizable method for quantum state comparison could be important for real applications. As examples of such applications we present a "lock and key" scheme and a simple scheme for quantum public key distribution.

Polarized photovoltage spectroscopy study of InAs/GaAs(001) quantum dot ensembles

J Bhattacharyya, S Ghosh, S Malzer, GH Dohler, BM Arora

We have studied self-assembled InAs quantum dot (QD) ensembles on GaAs(001) substrate using polarized photovoltage spectroscopy. The photovoltage spectrum shows four prominent QD related features whose nature differs for probe light incident along [001] and polarized parallel to [110] and [1 (1) over bar0] directions. The polarization anisotropy suggests that for the lowest energy transition there is only a change in the oscillator strength with change in polarization, while for the higher energy transitions there is also an apparent shift in the transition energy. By comparison with simulations of the absorption spectrum, we show that the main features of the observed polarization anisotropy can be understood on the basis of a model where an anisotropic two dimensional harmonic oscillator potential represents the lateral confinement of the carriers within the QD in the (001) plane. (c) 2005 American Institute of Physics.

Tailored polarization patterns for performance optimization of optical devices

Gerd Leuchs, Susanne Quabis

In the case of strong focusing the smallest possible focal spot can be reached, provided one uses a specially designed polarization pattern. Other optical set-ups also employing high numerical aperture imaging are likewise expected to improve performance under polarization optimization. We propose a strategy for calculating the polarization pattern required for system optimization. A single element modulating only the polarization and not amplitude and phase may lead to satisfactory performance in some cases.

Sum and difference frequency generation as diagnostics for leaky eigenmodes in two-dimensional photonic crystal waveguides

AD Bristow, JP Mondia, HM van Driel

We experimentally demonstrate how sum frequency generation (SFG) and difference frequency generation (DFG) of 150 fs pulses can be enhanced and serve as diagnostics for leaky eigenmodes in a two-dimensional GaAs photonic crystal waveguide. SFG at 795 nm is obtained in reflection from s-polarized 1900 nm and p-polarized 1360 nm pulses, with both input beams coupled to leaky eigenmodes; the SFG is enhanced by > 350x compared to that from an untextured GaAs surface. We are able to detect Drude induced subnanometer blueshifts of the SFG, corresponding to refractive index changes of <= 10(-3). DFG of 1360 nm light obtained in reflection from s-polarized 1900 nm and p-polarized 793 nm pulses displays an enhancement of > 500x via three different leaky eigenmodes. As the 793 nm beam polarization is varied from p polarized to right and left circularly polarized, the DFG remains essentially linearly polarized but with a reduced, albeit different, intensity for right and left circularly polarized 795 nm pulses. Futhermore, the plane of polarization also rotates by different amounts for the left and right circularly polarized light, demonstrating interference of the components generated by s- and p-polarized 793 nm pulses. Overall, our results demonstrate how enhanced DFG and SFG from leaky eigenmodes can be used to characterize their properties more precisely than linear optical techniques. (c) 2006 American Institute of Physics.

Quantum information processing and communication - Strategic report on current status, visions and goals for research in Europe

P Zoller, T Beth, D Binosi, R Blatt, H Briegel, D Bruss, T Calarco, JI Cirac, D Deutsch, J Eisert, et al.

We present an excerpt of the document "Quantum Information Processing and Communication: Strategic report on current status, visions and goals for research in Europe", which has been recently published in electronic form at the website of FET (the Future and Emerging Technologies Unit of the Directorate General Information Society of the European Commission, http://www.cordis.lu/ist/fet/qipc-sr.htm). This document has been elaborated, following a former suggestion by FET, by a committee of QIPC scientists to provide input towards the European Commission for the preparation of the Seventh Framework Program. Besides being a document addressed to policy makers and funding agencies (both at the European and national level), the document contains a detailed scientific assessment of the state-of-the-art, main research goals, challenges, strengths, weaknesses, visions and perspectives of all the most relevant QIPC sub-fields, that we report here.

Experimental observation of fractional Fourier transform for a partially coherent optical beam with Gaussian statistics

Fei Wang, Yangjian Cai

We report the experimental observation of the fractional Fourier transform (FRT) for a partially coherent optical beam with Gaussian statistics [i.e., partially coherent Gaussian Schell-model (GSM) beam]. The intensity distribution (or beam width) and the modulus of the square of the spectral degree of coherence (or coherence width) of a partially coherent GSM beam in the FRT plane are measured, and the experimental results are analyzed and agree well with the theoretical results. The FRT optical system provides a convenient way to control the properties, e.g., the intensity distribution, beam width, spectral degree of coherence, and coherence width, of a partially coherent beam. (c) 2007 Optical Society of America.

Dynamic control of higher-order modes in hollow-core photonic crystal fibers

T. G. Euser, G. Whyte, M. Scharrer, J. S. Y. Chen, A. Abdolvand, J. Nold, C. F. Kaminski, P. St. J. Russell

We present a versatile method for selective mode coupling into higher-order modes of photonic crystal fibers, using holograms electronically generated by a spatial light modulator. The method enables non-mechanical and completely repeatable changes in the coupling conditions. We have excited higher order modes up to LP(31) in hollow-core photonic crystal fibers. The reproducibility of the coupling allows direct comparison of the losses of different guided modes in both hollow-core bandgap and kagome-lattice photonic crystal fibers. Our results are also relevant to applications in which the intensity distribution of the light inside the fiber is important, such as particle-or atom-guidance. (C) 2008 Optical Society of America

Unconditional security of the Bennett 1992 quantum key-distribution protocol over a lossy and noisy channel

K Tamaki, N Lutkenhaus

We show that the security proof of the Bennett 1992 protocol over loss-free channel given by Tamaki, Koashi, and Imoto [Phys. Rev. Lett. 90, 167904 (2003)] can be adapted to accommodate loss. We assumed that Bob's detectors discriminate between single-photon states on one hand and vacuum state or multiphoton states on the other hand.

Efficient CW terahertz generation with n-i-pn-i-p photomixers

G. H. Doehler, S. Preu, S. Malzer, L. J. Wang, M. Hanson, J. D. Zimmerman, A. C. Gossard, T. L. J. Wilkinson, E. R. Brown

We report on a concept for highly efficient generation of terahertz (THz) radiation and its verification by experimental studies. The THz generation is based on photomixing in a periodic stack of specially designed pinphotodiodes. This design allows to suppress the slow hole transport, to optimize high-speed ballistic electron transport in the photodiodes and, at the same time, to avoid the degradation of the high-frequency performance due to the usual "RC- roll-off".

One-way quantum key distribution: Simple upper bound on the secret key rate

Tobias Moroder, Marcos Curty, Norbert Luetkenhaus

We present a simple method to obtain an upper bound on the achievable secret key rate in quantum key distribution (QKD) protocols that use only unidirectional classical communication during the public-discussion phase. This method is based on a necessary precondition for one-way secret key distillation; the legitimate users need to prove that there exists no quantum state having a symmetric extension that is compatible with the available measurements results. The main advantage of the obtained upper bound is that it can be formulated as a semidefinite program, which can be efficiently solved. We illustrate our results by analyzing two well-known qubit-based QKD protocols: the four-state protocol and the six-state protocol.

Luminescence of double quantum wells subject to in-plane magnetic fields

M Orlita, R Grill, P Hlidek, M Zvara, GH Dohler, S Malzer, M Byszewski

We report on photoluminescence (PL) measurements of a symmetric GaAs/AlGaAs double quantum well (DQW) in high magnetic fields. For this study, a selectively contacted p-delta n-DQW-delta n-p structure was chosen, allowing an independent tuning of the electron density in the DQW and thus a creation of a two-dimensional electron gas. Our attention was focused on phenomena in in-plane magnetic fields, where the field-induced depopulation of the antibonding subband observable in the PL spectra as a so-called N-type kink was predicted by Huang and Lyo (HL) [Phys. Rev. B 59, 7600 (1999)]. Whereas the equivalent behavior has been observed several times in the electric transport measurements and a proper theoretical description has been found, to the best of our knowledge, no PL experiment in a direct comparison with the theoretical model developed by HL has ever been published. We carried out a self-consistent calculation based on their model and achieved a good agreement with our experimental results. Additionally, the influence of the excitonic interaction on the PL spectra, not taken into account by HL, is also discussed. This enables us to explain small deviations from the HL theory. The interpretation of the in-plane magnetic field measurements is supported by the experiment with the magnetic field in the perpendicular orientation that allows a sufficiently accurate estimation of the electron density in the DQW. Distinctive renormalization effects of DQW subbands at various electron densities are also observed and discussed.

Interference between monochromatic Terahertz sources

S. Preu, M. Hanson, J. D. Zimmerman, S. Malzer, A. C. Gossard, G. H. Doehler, L. J. Wang

We report on a method to improve spatial resolution and available power at the same time by mutual coherent emission of distant CW-Terahertz photomixers spaced by less than the coherence length of the mixing lasers.

White-light interferometry with higher accuracy and more speed

C Richter, B Wiesner, R Gross, G Hausler

Continuous-variable quantum key distribution using polarization encoding and post selection

S Lorenz, N Korolkova, G Leuchs

We present an experimental demonstration of a quantum key distribution protocol using coherent polarization states. Post selection is used to ensure a low error rate and security against beam-splitting attacks even in the presence of high losses. Signal encoding and readout in polarization bases avoids the difficult task of sending a local oscillator with the quantum channel. This makes our setup robust and easy to implement. A shared key was established for losses up to 64%.

Full-field shape measurement of specular surfaces

J Kaminski, S Lowitzsch, MC Knauer, G Hausler

THE QUANTUM PROPERTIES OF MULTIMODE OPTICAL AMPLIFIERS REVISITED

G. Leuchs, U. L. Andersen, C. Fabre

There are a number of physically different realizations of an optical amplifier and yet they all share the same fundamental quantum limit as far as their noise characteristics are concerned. We review the underlying mathematical formalism without the restriction of a minimum number of modes being involved, its physical implications and relate it to the phenomenological models for the various amplifiers.

Raman-like light scattering from acoustic phonons in photonic crystal fiber

P Dainese, PSJ Russell, GS Wiederhecker, N Joly, HL Fragnito, V Laude, A Khelif

Raman and Brillouin scattering are normally quite distinct processes that take place when light is resonantly scattered by, respectively, optical and acoustic phonons. We show how few-GHz acoustic phonons acquire many of the same characteristics as optical phonons when they are tightly trapped, transversely and close to modal cut-off, inside the wavelength-scale core of an air-glass photonic crystal fiber (PCF). The result is an optical scattering effect that closely resembles Raman scattering, though at much lower frequencies. We use photoacoustic techniques to probe the effect experimentally and finite element modelling to explain the results. We also show by numerical modelling that the cladding structure supports two phononic band gaps that contribute to the confinement of sound in the core. (c) 2006 Optical Society of America

A simple and efficient optical 3D-sensor based on "photometric stereo" ("UV-Laser Therapy")

F Wolfsgruber, C Ruhl, J Kaminski, L Kraus, G Hausler, R Lampalzer, EB Haussler, P Kaudewitz, F Klampfl, A Gortler, et al.

Ideal bend contour trajectories for single-mode operation of low-loss overmoded waveguides

Christian Koos, Christopher G. Poulton, Lars Zimmermann, Lenin Jacome, Juerg Leuthold, Wolfgang Freude

Bend designs for single-mode operation of low-loss overmoded waveguides are presented. A class of contour trajectories for minimum radiation loss is derived analytically, and numerical optimization is used to find ideal trajectory parameters. Three-dimensional finite-difference time-domain simulations predict bend losses below 0.1 dB for 180 degrees-bends of overmoded silicon-on-insulator waveguides with a radius of R = 1.5 mu m. These findings are supported experimentally.

Interferometric null test of a parabolic reflector generating a Hertzian dipole field

G. Leuchs, K. Mantel, A. Berger, H. Konermann, M. Sondermann, U. Peschel, N. Lindlein, J. Schwider

Interferometric surface tests of stigmatic aspherics can be carried out in a null test configuration. Null tests require reference null elements either plane or spherical surfaces or both. A parabolic reflector transforms a plane into a spherical wave which converges to the focus of the paraboloid. Therefore, a spherical ball lens or a steel ball can be placed into the focus enabling a double-pass geometry for the null test. Here a Fizeau interferometer geometry has been selected in order to guarantee invariance against polarization distortions under the assumption that radially polarized laser light is used for the interferometer. Radial polarized light is necessary to mimic a Hertzian dipole field. Due to the extreme solid angle produced by the paraboloid the alignment of the setup is very critical and needs auxiliary systems for the control. Aberrations caused by misalignments are removed via fitting of suitable functionals provided through ray-trace simulations. It turned out that the usual vector approximations fail under these extreme circumstances. Test results arc given for a paraboloid with 2mm focal length transforming a plane wave into a near dipole wave comprising a solid angle of about 3,4 pi.

Experimental evidence for Raman-induced limits to efficient squeezing in optical fibers

Ruifang Dong, Joel Heersink, Joel F. Corney, Peter D. Drummond, Ulrik L. Andersen, Gerd Leuchs

We report new experiments on polarization squeezing using ultrashort photonic pulses in a single pass of a birefringent fiber. We measure what is to our knowledge a record squeezing of -6.8 +/- 0.3 dB in optical fibers, which when corrected for linear losses is -10.4 +/- 0.8 dB. The measured polarization squeezing as a function of optical pulse energy, which spans a wide range from 3.5-178.8 pJ, shows a very good agreement with the quantum simulations, and for the first time we see the proof experimentally that Raman effects limit and reduce squeezing at high pulse energy. (c) 2008 Optical Society of America.

Experimental demonstration of macroscopic quantum coherence in Gaussian states

Christoph Marquardt, Ulrik L. Andersen, Gerd Leuchs, Yuishi Takeno, Mitsuyoshi Yukawa, Hidehiro Yonezawa, Akira Furusawa

We witness experimentally the presence of macroscopic coherence in Gaussian quantum states using a recently proposed criterion [E. G. Cavalcanti and M. D. Reid, Phys. Rev. Lett. 97 170405 (2006)]. The macroscopic coherence stems from interference between macroscopically distinct states in phase space, and we prove experimentally that a coherent state contains these features with a distance in phase space of 0.51 +/- 0.02 shot noise units. This is surprising because coherent states are generally considered being at the border between classical and quantum states, not yet displaying any nonclassical effect. For squeezed and entangled states the effect may be larger but depends critically on the state purity.

Paraxial propagation of a partially coherent Hermite-Gaussian beam through aligned and misaligned ABCD optical systems

Yangjian Cai, Chiyi Chen

Paraxial propagation of a partially coherent Hermite-Gaussian beam through aligned and misaligned ABCD optical systems is investigated based on the generalized Collins formula for treating the propagation of a partially coherent beam through such optical systems. Analytical formulas for the cross-spectral density of a partially coherent Hermite-Gaussian beam propagating through such optical systems are derived. As an application example, we derive the propagation formulas for a partially coherent flattened Gaussian beam by expressing it as a superposition of a series of partially coherent Hermite-Gaussian beams by using polynomial expansion. The focusing properties of a partially coherent Hermite-Gaussian beam focused by a thin lens are studied as a numerical example. (c) 2007 Optical Society of America.

Supercontinuum generation in ultra-low-loss silica nanoweb fibres

A. Podlipensky, P. Szarniak, M. Scharrer, N. Y. Joly, P. St. J. Russell

We present fabrication and characterization of ultra-low-loss (0.6 dB/m) glass nanowebs in fibre form. Generation of broad band supercontinuum in the fs regime is reported and discussed.

Experimental quantum cloning with continuous variables

Ulrik L. Andersen, Vincent Josse, Norbert Luetkenhaus, Gerd Leuchs

In this chapter we present a scheme for optimal Gaussian cloning of optical coherent states. Its optical realization is based entirely on simple linear optical elements and homodyne detection. This is in contrast to previous proposals where parametric processes were suggested to be used for optimal Gaussian cloning. The optimality of the presented scheme is only limited by detection inefficiencies. Experimentally we achieved a cloning fidelity of up to 65%, which almost touches the optimal value of 2/3.

Generation of a high-quality partially coherent dark hollow beam with a multimode fiber

Chengliang Zhao, Yangjian Cai, Fei Wang, Xuanhui Lu, Yuzhu Wang

We report the experimental generation of a high-quality partially coherent dark hollow beam (DHB) by coupling a partially coherent beam into a multimode fiber (MMF) with a suitable incidence angle. The interference experiment of the generated partially coherent DHB passing through double slits is demonstrated. It is found that the coupling efficiency of the MMF, the quality, and the coherence of the generated partially coherent DHB are closely controlled by the coherence of the input beam. (c) 2008 Optical Society of America.

An efficient source of continuous variable polarization entanglement

Ruifang Dong, Joel Heersink, Jun-Ichi Yoshikawa, Oliver Gloeckl, Ulrik L. Andersen, Gerd Leuchs

We have experimentally demonstrated the efficient creation of highly entangled bipartite continuous variable polarization states. Exploiting an optimized scheme for the production of squeezing using the Kerr non-linearity of a glass fibre we generated polarization squeezed pulses with a mean classical excitation in (S)over-cap(3). Polarization entanglement was generated by interfering two independent polarization squeezed fields on a symmetric beam splitter. The resultant beams exhibit strong quantum noise correlations in the dark (S)over-cap(1)-(S)over-cap(2) polarization plane. To verify entanglement generation, we characterized the quantum correlations of the system for two different sets of conjugate Stokes parameters. The quantum correlations along the squeezed and the anti-squeezed Stokes parameters were observed to be - 4.1 +/- 0.3 and - 2.6 +/- 0.3 dB below the shot noise level, respectively. The degree of correlations was found to depend critically on the beam-splitting ratio of the entangling beam splitter. Carrying out measurements on a different set of conjugate Stokes parameters, correlations of -3.6 +/- 0.3 and -3.4 +/- 0.3 dB have been observed. This result is more robust against asymmetries in the entangling beam splitter, even in the presence of excess noise.

All-fibre source of amplitude squeezed light pulses

M Meissner, C Marquardt, J Heersink, T Gaber, A Wietfeld, G Leuchs, UL Andersen

An all-fibre source of amplitude squeezed solitons utilizing the self-phase modulation in an asymmetric Sagnac interferometer is experimentally demonstrated. The asymmetry of the interferometer is passively controlled by an integrated fibre coupler, allowing for the optimization of the noise reduction. We have carefully studied the dependence of the amplitude noise on the asymmetry and the power launched into the Sagnac interferometer. Qualitatively, we find good agreement between the experimental results, a semi-classical theory and earlier numerical calculations (Schmitt et al 1998 Phys. Rev. Lett. 81 2446). The stability and flexibility of this all-fibre source makes it particularly well suited to applications in quantum information science.

Absolute testing of the reference surface of a Fizeau interferometer through even/odd decompositions

R. Schreiner, J. Schwider, N. Lindlein, K. Mantel

Absolute testing of spherical surfaces is a technological necessity because of increased accuracy requirements. In a Fizeau setup, the main part of the interferometer deviations thereby comes from the reference surface. We demonstrate the validity of an absolute testing procedure for the reference surface that has been proposed earlier. The procedure relies on the decomposition of the surface deviations into odd and even parts and could be used in partially coherent illumination. The odd deviations are obtained from a basic and a 180 degrees-rotated position of an auxiliary sphere, and the even deviations can be measured with the help of a cat's eye position in double pass using an opaque half screen in the interferometer aperture. (c) 2008 Optical Society of America

Quantitative broadband chemical sensing in air-suspended solid-core fibers

T. G. Euser, J. S. Y. Chen, M. Scharrer, P. St. J. Russell, N. J. Farrer, P. J. Sadler

We demonstrate a quantitative broadband fiber sensor based on evanescent-field sensing in the cladding holes of an air-suspended solid-core photonic crystal fiber. We discuss the fabrication process, together with the structural and optical characterization of a range of different fibers. Measured mode profiles are in good agreement with finite element method calculations made without free parameters. The fraction of the light in the hollow cladding can be tuned via the core diameter of the fiber. Dispersion measurements are in excellent agreement with theory and demonstrate tuning of the zero dispersion wavelength via the core diameter. Optimum design parameters for absorption sensors are discussed using a general parameter diagram. From our analysis, we estimate that a sensitivity increase of three orders of magnitude is feasible compared to standard cuvette measurements. Our study applies to both liquid and gas fiber sensors. We demonstrate the applicability of our results to liquid chemical sensing by measuring the broad absorption peak of an aqueous NiCl(2) solution. We find excellent agreement with the reference spectrum measured in a standard cuvette, even though the sample volume has decreased by three orders of magnitude. Our results demonstrate that air-suspended solid-core photonic crystal fibers can be used in quantitative broadband chemical-sensing measurements. (C) 2008 American Institute of Physics.

Generation of polarization squeezing with periodically poled KTP at 1064 nm

Mikael Lassen, Metin Sabuncu, Preben Buchhave, Ulrik L. Andersen

We report the experimental demonstration of directly produced polarization squeezing at 1064 nm from a type I optical parametric amplifier (OPA) based on a periodically poled KTP crystal (PPKTP). The orthogonal polarization modes of the polarization squeezed state are both defined by the OPA cavity mode, and the birefringence induced by the PPKTP crystal is compensated by a second, but inactive, PPKTP crystal. Stokes parameter squeezing of 3.6 dB and anti squeezing of 9.4 dB is observed. (c) 2007 Optical Society of America.

Coupled whispering gallery mode resonators in the Terahertz frequency range

S. Preu, H. G. L. Schwefel, S. Malzer, G. H. Doehler, L. J. Wang, M. Hanson, J. D. Zimmerman, A. C. Gossard

We report on coupling of two whispering gallery mode resonators in the Terahertz frequency range. Due to the long wavelength in the millimeter to submillimeter range, the resonators can be macroscopic allowing for accurate size and shape control. This is necessary to couple specific modes of two or more resonators. Sets of polyethylene (PE) and quartz disk resonators are demonstrated, with medium (loaded) quality (Q)-factors of 40-800. Both exhibit coinciding resonance frequency spectra over more than ten times the free spectral range. Loading effects of single resonators are investigated which provide strong Q-factor degradation and red-shifts of the resonances in the 0.2% range. By coupling two resonators of the same size, we observe mode splitting, in very good agreement with our numerical calculations. (C) 2008 Optical Society of America.

Quantum filtering of optical coherent states

C. Wittmann, D. Elser, U. L. Andersen, R. Filip, P. Marek, G. Leuchs

We propose and experimentally demonstrate nondestructive and noiseless removal (filtering) of vacuum states from an arbitrary set of coherent states of continuous variable systems. Errors, i.e., vacuum states in the quantum information are diagnosed through a weak measurement, and on that basis, probabilistically filtered out. We consider three different filters based on on-off detection, phase stabilized, and phase randomized homodyne detection. We find that on-off detection, optimal in the ideal theoretical setting, is superior to the homodyne strategy also in a practical setting.

Simple criteria for the implementation of projective measurements with linear optics

P van Loock, N Lutkenhaus

We derive a set of criteria to decide whether a given projection measurement can be, in principle, exactly implemented solely by means of linear optics. The derivation can be adapted to various detection methods, including photon counting and homodyne detection. These criteria enable one to obtain no-go theorems easily for the exact distinguishability of orthogonal quantum states with linear optics, including the use of auxiliary photons and conditional dynamics.

Efficient polarization squeezing in optical fibers

J Heersink, V Josse, G Leuchs, UL Andersen

We report on a novel and efficient source of polarization squeezing that uses a single pass through an optical fiber. Using the fiber's two orthogonal polarization axes produces two identical squeezed beams. Combining these in a Stokes measurement generates polarization squeezing of up to 5.1 &PLUSMN; 0.3 dB. Furthermore, this scheme enables us to directly measure, for both polarizations, the noise of any given quadrature. &COPY; 2005 Optical Society of America.

Bulk and contact-sensitized photocarrier generation in single layer organic devices

Debdutta Ray, Meghan P. Patankar, Gottfried H. Doehler, K. L. Narasimhan

In this paper, we report on the photoelectronic properties of N,N-'-diphenyl-N,N-'-bis(3-methylphenyl)-(1,1(')-biphenyl)-4,4(')-diamine (TPD) studied in sandwich geometry. In particular, we have obtained from both forward and reverse bias measurements the mu tau product for holes in TPD. mu is the hole mobility and tau is the carrier trapping time. The mu tau product is a measure of the electronic quality of the material and allows a quantitative comparison of different samples. We have carried out numerical simulations to understand the photocurrent in these structures. We show that in reverse bias, the photocurrent (PC) is due to carrier generation in the bulk of the sample. The carrier generation is governed by field assisted exciton dissociation at electric fields greater than 10(6) V/cm. At lower fields the generation of carriers occurs spontaneously in the bulk of the sample. In forward bias, the photocurrent is due to exciton dissociation at the indium tin oxide contact. We also obtain a mu tau product for holes from forward bias PC measurements which is in agreement with the value obtained from reverse bias measurements. Based on our experiments, we demonstrate that TPD in a sandwich structure is a good candidate for cheap large area visible blind ultraviolet detector arrays. (c) 2006 American Institute of Physics.

Collision of orthogonally polarized solitons in photonic crystal fiber

A. Podlipensky, P. Szarniak, N. Y. Joly, C. G. Poulton, P. St. J. Russell

We study experimentally the propagation and collision of two orthogonally polarized solitons in a weakly-birefiringent photonic crystal fiber. The collision occurs at a particular power and initiates energy transfer between the colliding solitons. (C) 2007 Optical Society of America

Experimentally feasible quantum erasure-correcting code for continuous variables

J. Niset, U. L. Andersen, N. J. Cerf

We devise a scheme that protects quantum coherent states of light from probabilistic losses, thus achieving the first continuous-variable quantum erasure-correcting code. If the occurrence of erasures can be probed, then the decoder enables, in principle, a perfect recovery of the original light states. Otherwise, if supplemented with postselection based on homodyne detection, this code can be turned into an efficient erasure-filtration scheme. The experimental feasibility of the proposed protocol is carefully addressed.

Wave-optical simulation of a grating lateral shearing interferometer with a periodic incoherent light source

Norbert Lindlein, Sarina Wunderlich, Irina Harder, Klaus Mantel, Maik Lano, Johannes Schwider

A quite simple numerical model for the wave-optical simulation of the interference in a grating lateral shearing interferometer with a periodic light source and a large lateral shear is presented. Aberrations of the collimating lens will generate a spatially varying modulation in the interference pattern. The model assumes that the light source itself is completely spatially incoherent so that only the light from each point of the light source has to be propagated wave-optically through the optical system. Then, the intensity distributions of all light source points in the detector plane can just be added. The simulations are compared to theoretical calculations of partial coherence theory and also to experimental results.

Effect of detector dead times on the security evaluation of differential-phase-shift quantum key distribution against sequential attacks

Marcos Curty, Kiyoshi Tamaki, Tobias Moroder

We investigate limitations imposed by detector dead times on the performance of sequential attacks against a differential-phase-shift (DPS) quantum key distribution (QKD) protocol with weak coherent pulses. In particular; we analyze sequential attacks based on unambiguous state discrimination of the signal states emitted by the source and we obtain ultimate upper bounds on the maximal distance achievable by a DPS QKD scheme both for the case of calibrated and uncalibrated devices, respectively.

Shape reconstruction from gradient data

Svenja Ettl, Juergen Kaminski, Markus C. Knauer, Gerd Haeusler

We present a generalized method for reconstructing the shape of an object from measured gradient data. A certain class of optical sensors does not measure the shape of an object but rather its local slope. These sensors display several advantages, including high information efficiency, sensitivity, and robustness. For many applications, however, it is necessary to acquire the shape, which must be calculated from the slopes by numerical integration. Existing integration techniques show drawbacks that render them unusable in many cases. Our method is based on an approximation employing radial basis functions. It can be applied to irregularly sampled, noisy, and incomplete data, and it reconstructs surfaces both locally and globally with high accuracy. (C) 2008 Optical Society of America.

Guided modes in arrays of metallic nanowires

C. G. Poulton, M. Schmidt, G. Pearce, G. Kakarantzas, P. St. J. Russell

We study numerically the formation of photonic band gaps and guided "defect" modes within two dimensional arrays of metallic nanowires. Attenuations as low as 1.7 dB/cm are predicted for silver wires at 1550 nm wavelength. (C) 2007 Optical Society of America

Scintillations of laser array beams

H. T. Eyyuboglu, Y. Baykal, Y. Cai

The scintillation index of a laser array beam is analytically derived and numerically evaluated for weak turbulence conditions. On-axis as well as off-axis positions of the receiver plane are considered. Our graphical illustrations prove that at longer propagation ranges and at some midrange radial displacement parameters, laser array beams exhibit less scintillations, when compared to a fundamental Gaussian beam. However, when compared among themselves, laser array beams tend to have reduced scintillations with rising numbers of beamlets, longer propagation wavelengths, at midrange radial displacement parameters, at intermediate Gaussian source sizes, at bigger inner scales and smaller outer scales of turbulence. However, in this improvement, the number of beamlets does not seem to have a major role.

Dynamic and steady state current response to light excitation of multilayered organic photodiodes

E. S. Zaus, S. Tedde, J. Fuerst, D. Henseler, G. H. Doehler

Measurements of current transients are used to gain insight into the mechanism of charge transport and extraction of photodiodes based on bulk heterojunction blends of poly-3-hexyl-thiophene and [6,6]-phenyl C-61 butyric acid methyl ester. It is shown that the implementation of an appropriate hole conducting layer leads to a reduction of the dark current in the reverse direction. It is observed that the dynamic response to light excitation is strongly influenced by the thickness of the hole conducting layer, the light intensity, and the applied bias. Charge accumulation at the interface is assumed to result in the characteristic shape of the transients. The shape of the switch-off transient can be understood qualitatively by an equivalent circuit diagram. (c) 2007 American Institute of Physics.

Controlling Acousto-Optic Interactions in Photonic Crystal Fiber with Sub-Wavelength Core-Hole

G. S. Wiederhecker, A. Brenn, H. Hundertmark, C. M. B. Cordeiro, J. C. Knight, P. St. J. Russell, H. L. Fragnito

The quasi-Raman interaction between confined acoustic phonons and light in PCF is strongly altered by the introduction of a sub-wavelength hole running axially through the core. Coupling calculations and forward scattering spectra illustrate the effect. (C)2007 Optical Society of America

Testing quantum devices: Practical entanglement verification in bipartite optical systems

Hauke Haeseler, Tobias Moroder, Norbert Luetkenhaus

We present a method to test quantum behavior of quantum information processing devices, such as quantum memories, teleportation devices, channels, and quantum key distribution protocols. The test of quantum behavior can be phrased as the verification of effective entanglement. Necessary separability criteria are formulated in terms of a matrix of expectation values in conjunction with the partial transposition map. Our method is designed to reduce the resources for entanglement verification. A particular protocol based on coherent states and homodyne detection is used to illustrate the method. A possible test for the quantum nature of memories using two nonorthogonal signal states arises naturally. Furthermore, closer inspection of the measurement process in terms of the Stokes operators reveals a security threat for quantum key distribution involving phase reference beams.

Experimental continuous-variable cloning of partial quantum information

Metin Sabuncu, Gerd Leuchs, Ulrik L. Andersen

The fidelity of a quantum transformation is strongly linked with the prior partial information of the state to be transformed. We illustrate this interesting point by proposing and demonstrating the superior cloning of coherent states with prior partial information. More specifically, we propose two simple transformations that under the Gaussian assumption optimally clone symmetric Gaussian distributions of coherent states as well as coherent states with known phases. Furthermore, we implement for the first time near-optimal state-dependent cloning schemes relying on simple linear optics and feedforward.

Generation of a radially polarized doughnut mode of high quality

Susanne Quabis, Ralf Dorn, Gerd Leuchs

We present an experimental set-up to generate laser beams with locally varying polarization distribution. In a linear set-up, a radially polarized beam of high quality regarding intensity distribution, polarization and phase-front distortion is generated. This beam can be used for tight focusing. Further applications are discussed.