Multimode states in decoy-based quantum-key-distribution protocols

Wolfram Helwig, Wolfgang Mauerer, Christine Silberhorn

Every security analysis of quantum-key distribution (QKD) relies on a faithful modeling of the employed quantum states. Many photon sources, such as for instance a parametric down-conversion (PDC) source, require a multimode description but are usually only considered in a single-mode representation. In general, the important claim in decoy-based QKD protocols for indistinguishability between signal and decoy states does not hold for all sources. We derive bounds on the single-photon transmission probability and error rate for multimode states and apply these bounds to the output state of a PDC source. We observe two opposing effects on the secure key rate. First, the multimode structure of the state gives rise to a new attack that decreases the key rate. Second, more contributing modes change the photon number distribution from a thermal toward a Poissonian distribution, which increases the key rate.

Numerical Analysis of Parametric Downconversion

Wolfgang Mauerer, Christine Silberhorn

Parametric downconversion (PDC) is a popular technique to produce twin beams of photons that are entangled in multiple degrees of freedom. The generated states form the basis for numerous applications that require entanglement. An exact quantification of this resource is therefore essential, for instance for quantum cryptography that relies on a complete knowledge of the correlation contained in the state. While the determination of an entanglement monotone for the PDC process is only possible analytically in special cases, an exact calculation must usually be performed numerically. Recent work by Mikhailova et al. [2] analyses a certain class of PDC states for which the concurrence entanglement measure can be obtained by an analytical approximation. In this contribution, we analyse the validity of the approximation by comparison with exact numerical methods.

Asymptotic security of binary modulated continuous-variable quantum key distribution under collective attacks

Yi-Bo Zhao, Matthias Heid, Johannes Rigas, Norbert Luetkenhaus

We estimate a lower bound to the secret key rate of a binary modulated continuous variable quantum key distribution scheme. We consider the collective attack scenario with quantum channels that impose arbitrary noise on the exchanged signals. The analysis is done in the infinite key limit. Bob performs ideal homodyne measurements on the received states and the two honest parties employ a reverse reconciliation procedure in the classical postprocessing step of the protocol.

Excitation of plasmonic gap waveguides by nanoantennas

Jing Wen, Sergei Romanov, Ulf Peschel

We model and optimize the excitation of a plasmonic gap waveguide by a dipole antenna. The coupling efficiency strongly depends on antenna and waveguide properties where impedanec matching plays a critical role. The optimization of antenna lengths and gap widths shows that concepts of circuit networks can likewise be applied to optical frequencies. Using classical optimization schemes known from electrical engineering we manage to increase the coupling efficiency by a factor of 129 compared with the situation without antennas. (C) 2009 Optical Society of America

Quantum Key Distribution with Multi Letter Continuous Variable Alphabets

Denis Sych, Gerd Leuchs

We present a new protocol for continuous variable quantum key distribution. The novelty of the protocol is a multi letter alphabet represented by coherent states of light with a fixed amplitude and variable phase. Information is encoded in the phase of a coherent state which can be chosen from a regular discrete set consisting, however, of an arbitrary number of letters. We evaluate the security of the protocol against the beam splitting attack. As a result we show the proposed protocol has advantages over the standard two letter coherent state protocol, especially in the case when losses in the communication channel are low.

Continuous Variable Entanglement Distillation of Non-Gaussian States

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

We experimentally demonstrate distillation of continuous variable entangled light that has undergone non-Gaussian attenuation loss. The continuous variable entanglement is generated with optical fibers and sent through a lossy channel, where the transmission is varying in time. By employing simple linear optical components, a measurement induced operation and classical communication, we demonstrate that the entanglement is probabilistically increased.

Novel Nanophotonic Waveguides Based on Metal, Semiconductor or Soft Glass Modified Photonic Crystal Fibres

M. A. Schmidt, H. Tyagi, H. Lee, P. St J. Russell

Here we review on our results of photonic crystal fibres (PCFs) which have been filled with solid materials such as metals, semiconductors and low-melting temperature compound glasses. As filling method we used the high-temperature pressure cell technique. Depending on the material in the air-holes, we observed the excitation of spiralling plasmonic modes, Mie-resonances and coupled waveguide modes. The filling of the air-holes is thus a novel concept to manipulate the optical properties of PCFs in a desired way and to create novel in-fibre and waveguiding devices.

Experimental observation of lensless coincidence fractional Fourier transform with a Gaussian Schell-model beam

Fei Wang, Yangjian Cai, Jun She

A modified lensless optical system for implementing coincidence fractional Fourier transform (FRT) is proposed, and the conditions for the optical system to implement the coincidence FRT with incoherent or partially coherent light are discussed. Furthermore, we report the experimental observation of lensless coincidence FRT of an object (double slits) with a typical partially coherent beam - Gaussian Schell-model (GSM) beam. The experimental results are analyzed and agree reasonably well with the theoretical results. (C) 2008 Elsevier GmbH. All rights reserved.

The information of ambiguity

Ludmila Praxmeyer, Stig Stenholm, Nikolay V. Vitanov

The phase space characteristics of a quantum state are best captured by the Wigner distribution. This displays transparently the diagonality information of the density matrix. The complementary function offering transparently the off-diagonal elements is captured by a function called the S-function, or the ambiguity. In carrying the maximal information about the quantum coherences it represents the uncertainties or ambiguity of the diagonal information. Mathematically this is manifested in its role as the phase space moment generating function. Formally it complements the information in the Wigner function. These formal relations provide the starting point for the present investigations. As a measure of quantum uncertainties, ambiguity may be used to define a probability measure on the off-diagonality. The mathematical and physical consistency of this view is presented in this paper. For a pure state, we find the extraordinary result that such distributions are their own Fourier transforms. The physical interpretation of this distribution as a carrier of classical signal fuzziness suggests the introduction of heuristic approximations to the observational uncertainties. We illustrate the properties and interpretation of the ambiguity function by some specific examples. We find that for smooth, 'Gaussian-like' distributions, the heuristic considerations provide good approximations. On the other hand, representing quantum interferences, the ambiguity serves as the most positive probe for the ultimate quantum structures which have been called sub-Planckian. They are interesting because it has been argued that such structures are physically observable.

All-solid bandgap guiding in tellurite-filled silica photonic crystal fibers

Markus A. Schmidt, Nicolai Granzow, Ning Da, Mingying Peng, Lothar Wondraczek, Philip St. J. Russell

We report all-solid bandgap-guiding fibers formed by pumping molten tellurite glass into silica-air photonic crystal fiber at high pressure. The spectral positions of the guidance bands agree well with multipole simulations and bandgap calculations. The micrometer-diameter tellurite strands are found to contain microheterogeneities (most probably originating from devitrification), which increase the fiber attenuation, although no evidence of crystallization is seen in the bulk tellurite glass. The technique offers a potential route to employing difficult-to-handle glasses, or glasses unsuitable for fiber drawing, in fiber-based amplifiers, modulators, filters, and nonlinear devices. (C) 2009 Optical Society of America

Transverse Angular Momentum and Geometric Spin Hall Effect of Light

Andrea Aiello, Norbert Lindlein, Christoph Marquardt, Gerd Leuchs

We present a novel fundamental phenomenon occurring when a polarized beam of light is observed from a reference frame tilted with respect to the direction of propagation of the beam. This effect has a purely geometric nature and amounts to a polarization-dependent shift or split of the beam intensity distribution evaluated as the time-averaged flux of the Poynting vector across the plane of observation. We demonstrate that such a shift is unavoidable whenever the beam possesses a nonzero transverse angular momentum. This latter result has general validity and applies to arbitrary systems such as, e.g., electronic and atomic beams.

Development of new free-fall absolute gravimeters

Ch Rothleitner, S. Svitlov, H. Merimeche, H. Hu, L. J. Wang

The design and first results of two free-fall absolute gravimeters are reported: a stationary gravimeter is designed and can be used as a reference system and a portable gravimeter is aimed at field measurements. The determination of the acceleration due to gravity is done interferometrically in both instruments. The whole fringe signal is digitized by a high-speed analogue-to-digital converter, which is locked to a rubidium frequency standard. This fringe recording and processing is novel as compared with commercial free-fall gravimeters, which use an electronic zero-crossing discrimination. Advantages such as the application of a zero-phase-shifting digital filter to the digitized data are depicted. The portable gravimeter's mechanics deviate from the conventional type. Springs are used to accelerate and decelerate the carriage supporting the falling object. A detailed uncertainty budget is given for both gravimeters. The combined standard uncertainty for the portable and for the stationary gravimeter is estimated at 38.8 mu Gal and 16.6 mu Gal, respectively. The corresponding statistical uncertainties are 1.6 mu Gal (over one day of measurement) and 0.6 mu Gal (over one month of measurement). The different designs and dimensions of the new free-fall gravimeters can help to reveal unknown or so far underestimated systematic effects. The assessments of the uncertainties due to seismic noise and shock vibrations, and electronic phase shifts give validity to this assumption.

Spatial modes in waveguided parametric down-conversion

Andreas Christ, Kaisa Laiho, Andreas Eckstein, Thomas Lauckner, Peter J. Mosley, Christine Silberhorn

The propagation of several spatial modes has a significant impact on the structure of the emission from parametric down-conversion in a nonlinear waveguide. This manifests itself not only in the spatial correlations of the photon pairs but also, due to new phase-matching conditions, in the output spectrum, radically altering the degree of entanglement within each pair. Here we investigate both theoretically and experimentally the results of higher-order spatial-mode propagation in nonlinear waveguides. We derive conditions for the creation of pairs in these modes and present observations of higher-order mode propagation in both the spatial and spectral domains. Furthermore, we observe correlations between the different degrees of freedom and finally we discuss strategies for mitigating any detrimental effects and optimizing pair production in the fundamental mode.

Audiovisual Non-Verbal Dynamic Faces Elicit Converging fMRI and ERP Responses

Julie Brefczynski-Lewis, Svenja Lowitszch, Michael Parsons, Susan Lemieux, Aina Puce

In an everyday social interaction we automatically integrate another's facial movements and vocalizations, be they linguistic or otherwise. This requires audiovisual integration of a continual barrage of sensory input-a phenomenon previously well-studied with human audiovisual speech, but not with non-verbal vocalizations. Using both fMRI and ERPs, we assessed neural activity to viewing and listening to an animated female face producing non-verbal, human vocalizations (i.e. coughing, sneezing) under audio-only (AUD), visual-only (VIS) and audiovisual (AV) stimulus conditions, alternating with Rest (R). Underadditive effects occurred in regions dominant for sensory processing, which showed AV activation greater than the dominant modality alone. Right posterior temporal and parietal regions showed an AV maximum in which AV activation was greater than either modality alone, but not greater than the sum of the unisensory conditions. Other frontal and parietal regions showed Common-activation in which AV activation was the same as one or both unisensory conditions. ERP data showed an early superadditive effect (AV > AUD + VIS, no rest), mid-range underadditive effects for auditory N140 and face-sensitive N170, and late AV maximum and common-activation effects. Based on convergence between fMRI and ERP data, we propose a mechanism where a multisensory stimulus may be signaled or facilitated as early as 60 ms and facilitated in sensory-specific regions by increasing processing speed (at N170) and efficiency (decreasing amplitude in auditory and face-sensitive cortical activation and ERPs). Finally, higher-order processes are also altered, but in a more complex fashion.

Spectrum conditions for symmetric extendible states

Geir Ove Myhr, Norbert Luetkenhaus

We analyze bipartite quantum states that admit a symmetric extension. Any such state can be decomposed into a convex combination of states that allow a pure symmetric extension. A necessary condition for a state to admit a pure symmetric extension is that the spectra of the local and global density matrices are equal. This condition is also sufficient for two qubits but not for any larger systems. Using this condition, we present a conjectured necessary and sufficient condition for a two-qubit state to admit symmetric extension, which we prove in some special cases. The results from symmetric extension carry over to degradable and antidegradable channels and we use this to prove that all degradable channels with qubit output have a qubit environment.

Duality between spatial and angular shift in optical reflection

A. Aiello, M. Merano, J. P. Woerdman

We report a unified representation of the spatial and angular Goos-Hanchen and Imbert-Fedorov shifts that occur when a light beam reflects from a plane interface. We thus reveal the dual nature of spatial and angular shifts in optical beam reflection. In the Goos-Hanchen case we show theoretically and experimentally that this unification naturally arises in the context of reflection from a lossy surface (e. g., a metal)

Detector decoy quantum key distribution

Tobias Moroder, Marcos Curty, Norbert Luetkenhaus

Photon number resolving detectors can enhance the performance of many practical quantum cryptographic setups. In this paper, we employ a simple method to estimate the statistics provided by such a photon number resolving detector using only a threshold detector together with a variable attenuator. This idea is similar in spirit to that of the decoy state technique, and is especially suited to those scenarios where only a few parameters of the photon number statistics of the incoming signals have to be estimated. As an illustration of the potential applicability of the method in quantum communication protocols, we use it to prove security of an entanglement-based quantum key distribution scheme with an untrusted source without the need for a squash model and by solely using this extra idea. In this sense, this detector decoy method can be seen as a different conceptual approach to adapt a single-photon security proof to its physical, full optical implementation. We show that in this scenario, the legitimate users can now even discard the double click events from the raw key data without compromising the security of the scheme, and we present simulations on the performance of the BB84 and the 6-state quantum key distribution protocols.

Atmospheric transfer of optical and radio frequency clock signals

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

The phase instability induced during the transfer of radio frequency and optical clock signals through the turbulent atmosphere was measured in a rooftop experiment. Radio frequency intensity modulation of a laser to transmit signals over 100 m results in an Allan deviation of 1.31 x 10(-10) at 1 s. Optical transfer is more accurate at 1.68 x 10(-13) at 1 s. As a consequence, fiber links are more suitable for the transfer of optical frequencies over very long distances while free space transmission might find applications in short distances of less than 1 km. (C) 2009 Optical Society of America

Direct Measurement of the Spatial-Spectral Structure of Waveguided Parametric Down-Conversion

Peter J. Mosley, Andreas Christ, Andreas Eckstein, Christine Silberhorn

We present a study of the propagation of higher-order spatial modes in a waveguided parametric down-conversion photon-pair source. Observing the multimode photon-pair spectrum from a periodically poled KTiOPO(4) waveguide allowed us to isolate individual spatial modes through their distinctive spectral properties. We have measured directly the spatial distribution of each mode of the photon pairs, confirming the findings of our waveguide model, and demonstrated by coincidence measurements that the total parity of the modes is conserved in the nonlinear interaction. Furthermore, we show that we can combine the advantages of a waveguide source with the potential to generate spatially entangled photon pairs as in bulk-crystal down-converters.

Quantum error correction beyond qubits

Takao Aoki, Go Takahashi, Tadashi Kajiya, Jun-ichi Yoshikawa, Samuel L. Braunstein, Peter van Loock, Akira Furusawa

Quantum computation and communication rely on the ability to manipulate quantum states robustly and with high fidelity. To protect fragile quantum-superposition states from corruption through so-called decoherence noise, some form of error correction is needed. Therefore, the discovery of quantum error correction(1,2) (QEC) was a key step to turn the field of quantum information from an academic curiosity into a developing technology. Here, we present an experimental implementation of a QEC code for quantum information encoded in continuous variables, based on entanglement among nine optical beams(3). This nine-wave-packet adaptation of Shor's original nine-qubit scheme(1) enables, at least in principle, full quantum error correction against an arbitrary single-beam error.

phi(FLIM)-F-2: a technique for alias-free frequency domain fluorescence lifetime imaging

Alan D. Elder, Clemens F. Kaminski, Jonathan H. Frank

A new approach to alias-free wide-field fluorescence lifetime imaging in the frequency domain is demonstrated using a supercontinuum source for fluorescence excitation and a phase-modulated image intensifier for detection. This technique is referred to as phi-squared fluorescence lifetime imaging (phi(FLIM)-F-2). The phase modulation and square-wave gating of the image intensifier eliminate aliasing by the effective suppression of higher harmonics. The ability to use picosecond excitation pulses without aliasing expands the range of excitation sources available for frequency-domain fluorescence lifetime imaging (fd-FLIM) and improves the modulation depth of conventional homodyne fd-FLIM measurements, which use sinusoidal intensity modulation of the excitation source. The phi(FLIM)-F-2 results are analyzed using AB-plots, which facilitate the identification of mono-exponential and multi-exponential fluorescence decays and provide measurements of the fluorophore fractions in two component mixtures. The rapid acquisition speed of the technique enables lifetime measurements in dynamic systems, such as temporally evolving samples and samples that are sensitive to photo-bleaching. Rapid phi(FLIM)-F-2 measurements are demonstrated by imaging the dynamic mixing of two different dye solutions at 5.5 Hz. The tunability of supercontinuum radiation enables excitation wavelength resolved FLIM measurements, which facilitates analysis of samples containing multiple fluorophores with different absorption spectra. (C) 2009 Optical Society of America

Quadrature measurements of a bright squeezed state via sideband swapping

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

The measurement of an arbitrary quadrature of a bright quantum state of light is a commonly requested action in many quantum information protocols, but it is experimentally challenging with previously proposed schemes. We suggest that the quadrature be measured at a specific sideband frequency of a bright quantum state by transferring the sideband modes under interrogation to a vacuum state and subsequently measuring the quadrature via homodyne detection. The scheme is implemented experimentally, and it is successfully tested with a bright squeezed state of light. (C) 2009 Optical Society of America

Precise balancing of viscous and radiation forces on a particle in liquid-filled photonic bandgap fiber

T. G. Euser, M. K. Garbos, J. S. Y. Chen, P. St. J. Russell

A great challenge in microfluidics is the precise control of laser radiation forces acting on single particles or cells, while allowing monitoring of their optical and chemical properties. We show that, in the liquid-filled hollow core of a single-mode photonic crystal fiber, a micrometer-sized particle can be held stably against a fluidic counterflow using radiation pressure and can be moved to and fro (over tens of centimeters) by ramping the laser power up and down. Accurate studies of the microfluidic drag forces become possible, because the particle is trapped in the center of the single guided optical mode, resulting in highly reproducible radiation forces. The counterflowing liquid can be loaded with sequences of chemicals in precisely controlled concentrations and doses, making possible studies of single particles, vesicles, or cells. (C) 2009 Optical Society of America

Interferometric quasi-absolute tests for aspherics using a radial shear position

Klaus Mantel, Eduard Geist, Irina Harder, Norbert Lindlein, Gerd Leuchs

Increasing accuracy requirements in aspheric metrology make the development of absolute testing procedures for aspheric surfaces important. One strategy is transferring the standard practice three-position test for spheres to aspherics. The three-position test, however, involves a cat's eye position and therefore has certain drawbacks. We propose an absolute testing method for rotationally symmetric aspherics where the cat's eye position is replaced with a radially sheared position. Together with rotational movements of the specimen, the surface deviations can be obtained in an absolute manner. To demonstrate the validity of the procedure, we present a measurement result for a sphere and compare it with a result obtained by the standard three-position test. (C) 2009 Optical Society of America

Non-Poissonian statistics from Poissonian light sources with application to passive decoy state quantum key distribution

Marcos Curty, Tobias Moroder, Xiongfeng Ma, Norbert Luetkenhaus

We propose a method to prepare different non-Poissonian signal pulses from sources of Poissonian photon number distribution, using only linear optical elements and threshold photon detectors. This method allows a simple passive preparation of decoy states for quantum key distribution. We show that the resulting key rates are comparable with the performance of active choices of intensities of Poissonian signals. (C) 2009 Optical Society of America

Nonlinear Phase Noise Reduction in a DPSK Transmission System Using Cascaded Nonlinear Amplifying Loop Mirrors

C. Stephan, K. Sponsel, G. Onishchukov, B. Schmauss, G. Leuchs

The performance of a nonlinear amplifying loop mirror as a phase-preserving amplitude 2R regenerator in a differential phase-shift-keying transmission system with nonlinear phase noise as dominant limiting effect has been investigated in a recirculating fiber-loop setup. The experimental results show that cascaded regenerators can efficiently prevent the accumulation of nonlinear phase noise in such systems. It was possible to significantly increase the transmission quality; alternatively, a considerable increase of fiber launch power could be achieved for the same bit-error ratio. As a limiting effect, the amplified Rayleigh backscattering in the highly nonlinear fiber is identified when the regenerator is passed multiple times.

Feasibility of free space quantum key distribution with coherent polarization states

D. Elser, T. Bartley, B. Heim, Ch Wittmann, D. Sych, G. Leuchs

We demonstrate for the first time the feasibility of free space quantum key distribution with continuous variables under real atmospheric conditions. More specifically, we transmit coherent polarization states over a 100m free space channel on the roof of our institute's building. In our scheme, signal and local oscillator (LO) are combined in a single spatial mode, which auto-compensates atmospheric fluctuations and results in an excellent interference. Furthermore, the LO acts as a spatial and spectral filter, thus allowing unrestrained daylight operation.

Fiber-modes and fiber-anisotropy characterization using low-coherence interferometry

Y. Z. Ma, Y. Sych, G. Onishchukov, S. Ramachandran, U. Peschel, B. Schmauss, G. Leuchs

An optical low-coherence interferometry technique has been used to simultaneously resolve the mode profile and to measure the intermodal dispersion of guided modes of a few-mode fiber. Measurements are performed using short samples of fiber (about 50 cm). There is no need for a complex mode-conversion technique to reach a high interference visibility. Four LP mode groups of the few-mode fiber are resolved. Experimental results and numerical simulations show that the ellipticity of the fiber core leads to a distinct splitting of the degenerate high-order modes in group index. For the first time, to the best of our knowledge, it has been demonstrated that degenerate LP11 modes are much more sensitive to core shape variations than the fundamental modes and that intermodal dispersion of high-order degenerate modes can be used for characterizing the anisotropy of an optical waveguide.

Microwave whispering-gallery resonator for efficient optical up-conversion

D. V. Strekalov, H. G. L. Schwefel, A. A. Savchenkov, A. B. Matsko, L. J. Wang, N. Yu

Conversion of microwave radiation into the optical range has been predicted to reach unity quantum efficiency in whispering-gallery resonators made from an optically nonlinear crystal and supporting microwave and optical modes simultaneously. In this work, we theoretically explore and experimentally demonstrate a resonator geometry that can provide the required phase matching for such a conversion at any desired frequency in the sub-THz range. We show that such a ring-shaped resonator not only allows for the phase matching but also maximizes the overlap of the interacting fields. As a result, unity-efficient conversion is expected in a resonator with feasible parameters.

Generation and Direct Detection of Broadband Mesoscopic Polarization-Squeezed Vacuum

Timur Iskhakov, Maria V. Chekhova, Gerd Leuchs

Using a traveling-wave optical parametric amplifier with two orthogonally oriented type-I BBO crystals pumped by picosecond pulses, we generate vertically and horizontally polarized squeezed vacuum states within a broad frequency-angular range. Depending on the phase between these states, fluctuations in one or another Stokes parameter are suppressed below the shot-noise limit. Because of the large number of photon pairs produced, no local oscillator is required, and 3 dB squeezing is observed by means of direct detection.

Perfect excitation of a matter qubit by a single photon in free space

M. Stobinska, G. Alber, G. Leuchs

We propose a scheme for perfect excitation of a single two-level atom by a single photon in free space. The photon state has to match the time reversed photon state originating from spontaneous decay of a two-level system. Here, we discuss its experimental preparation. The state is characterized by a particular asymmetric exponentially shaped temporal profile. Any deviations from this ideal state limit the maximum absorption. Although perfect excitation requires an infinite amount of time, we demonstrate that there is a class of initial one-photon quantum states which can achieve almost perfect absorption even for a finite interaction time. Our results pave the way for realizing perfect coupling between flying and stationary qubits in free space thus opening a possibility for building scalable quantum networks. Copyright (C) EPLA, 2009

Direct probing of the Wigner function by time-multiplexed detection of photon statistics

K. Laiho, M. Avenhaus, K. N. Cassemiro, Ch Silberhorn

We investigate the capabilities of loss-tolerant quantum state characterization using a photon-number resolving, time-multiplexed detector (TMD). We employ the idea of probing the Wigner function point-by-point in phase space via photon parity measurements and displacement operations, replacing the conventional homodyne tomography. Our emphasis lies on reconstructing the Wigner function of non-Gaussian Fock states with highly negative values in a scheme that is based on a realistic experimental set-up. In order to establish the concept of loss-tolerance for state characterization, we show how losses can be decoupled from the impact of other experimental imperfections, i.e. the non-unity transmittance of the displacement beamsplitter and non-ideal mode overlap. We relate the experimentally accessible parameters to effective ones that are needed for an optimized state reconstruction. The feasibility of our approach is tested by Monte Carlo simulations, which provide bounds resulting from statistical errors that are due to limited data sets. Our results clearly show that high losses can be accepted for a defined parameter range, and moreover, that-in contrast to homodyne detection-mode mismatch results in a distinct signature, which can be evaluated by analysing the photon number oscillations of the displaced Fock states.

Influence of air-filling fraction on forward Raman-like scattering by transversely trapped acoustic resonances in photonic crystal fibers

A. Brenn, G. S. Wiederhecker, M. S. Kang, H. Hundertmark, N. Joly, P. St. J. Russell

Raman-like forward scattering by acoustic phonons transversely trapped in birefringent silica-air photonic crystal fibers is studied. As the air-filling fraction increases, core-confined acoustic resonances become increasingly apparent at higher frequencies (> 1.1 GHz), while the number of cladding-confined acoustic modes involved in scattering falls. Two main types of scattering are observed: intramodal (scattering to new frequencies within the same optical mode) and intermodal (frequency-shifted scattering to a different optical mode). It is shown that the twofold symmetric microstructure in a birefringent fiber causes strongly polarization-dependent intramodal scattering. Good agreement is obtained between the experimental measurements and numerical solutions of both the acoustic and electromagnetic wave equations by using a full-vectorial finite-element approach. Phononic bandgaps are found to play a significant role at higher air-filling fractions, leading to the appearance of additional bands in the scattering spectrum. (C) 2009 Optical Society of America

Manipulation of coherent Stokes light by transient stimulated Raman scattering in gas filled hollow-core PCF

A. V. Chugreev, A. Nazarkin, A. Abdolvand, J. Nold, A. Podlipensky, P. St. J. Russell

Transient stimulated Raman scattering is investigated in methane-filled hollow-core photonic crystal fiber. Using frequency-chirped ps-pulses at 1.06 mu m as pump and tunable CW-radiation as Stokes seed, the vibrational excitation of the CH4 molecules can be controlled on the sub T-2 time-scale. In this way the generated Stokes pulse can be phase-locked to the pump pulse and its spectrum manipulated. (c) 2009 Optical Society of America

Phase-Preserving Amplitude Regeneration in DPSK Transmission Systems Using a Nonlinear Amplifying Loop Mirror

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

A phase-preserving 2R regenerator based on a nonlinear amplifying loop mirror was implemented in a RZ-DPSK transmission system. Its performance has been investigated in numerical simulations and experimentally. The results show that amplitude regeneration using a NALM can efficiently prevent accumulation of nonlinear phase noise in a 10 Gb/s DPSK transmission system. In the experiments, significant improvements of eye opening and of BER as well as a 3 dB increase in fiber launch power have been demonstrated. Simulations at 10 Gb/s and 100 Gb/s indicated that the enhancement of the transmission quality is smaller at 100 Gb/s. The reason is that at 100 Gb/s nonlinear intra-channel effects rather than pure nonlinear phase noise are the main limiting factor and the NALM can only reduce the accumulation of amplitude noise in that case.

Heralded entanglement of arbitrary degree in remote qubits

U. Schilling, C. Thiel, E. Solano, T. Bastin, J. von Zanthier

Incoherent scattering of photons off two remote atoms with a Lambda-level structure is used as a basic Young-type interferometer to herald long-lived entanglement of an arbitrary degree. The degree of entanglement, as measured by the concurrence, is found to be tunable by two easily accessible experimental parameters. Fixing one of them to certain values unveils an analog to the Malus' law. An estimate of the variation in the degree of entanglement due to uncertainties in an experimental realization is given.

CO2 laser writing of long-period fiber grating in photonic crystal fiber under tension

H. W. Lee, K. S. Chiang

We demonstrate that the efficiency of CO2 laser writing of longperiod fiber gratings in a solid-core photonic crystal fiber (PCF) can be enhanced greatly by applying tension to the fiber during the writing process through the mechanism of frozen-in viscoelasticity. Using this mechanism, we are able to write strong gratings in PCFs with a dosage of CO2 laser radiation low enough not to cause any significant fiber structure deformation. (C) 2009 Optical Society of America

Broadband sensitive pump-probe setup for ultrafast optical switching of photonic nanostructures and semiconductors

Tijmen G. Euser, Philip J. Harding, Willem L. Vos

We describe an ultrafast time resolved pump-probe spectroscopy setup aimed at studying the switching of nanophotonic structures. Both femtosecond pump and probe pulses can be independently tuned over broad frequency range between 3850 and 21 050 cm(-1). A broad pump scan range allows a large optical penetration depth, while a broad probe scan range is crucial to study strongly photonic crystals. A new data acquisition method allows for sensitive pump-probe measurements, and corrects for fluctuations in probe intensity and pump stray light. We observe a tenfold improvement of the precision of the setup compared to laser fluctuations, allowing a measurement accuracy of better than Delta R=0.07% in a 1 s measurement time. Demonstrations of the improved technique are presented for a bulk Si wafer, a three-dimensional Si inverse opal photonic bandgap crystal, and z-scan measurements of the two-photon absorption coefficient of Si, GaAs, and the three-photon absorption coefficient of GaP in the infrared wavelength range.

Goos-Hanchen shift for a rough metallic mirror

M. Merano, J. B. Goette, A. Aiello, M. P. van Exter, J. P. Woerdman

We investigate experimentally the dependence of the Goos-Hanchen shift on the surface properties of an air-metal interface. The shift depends on the microscopic roughness of the metal surface but it is insensitive to the large-scale variations associated with surface non-flatness. Both an effective medium model of roughness and the Rayleigh-Rice theory of scattering are used to interpret the observed phenomenon. (C) 2009 Optical Society of America

Coherent Superposition of Terahertz Beams from a Phased Linear Photomixer Array

Sebastian Bauerschmidt, Sascha Preu, Stefan Malzer, Gottfried H. Doehler, Lijun Wang, Hong Lu, Arthur C. Gossard

We report on a 4 element linear array of free standing, tunable THz n-i-pn-i-p superlattice photomixers. The individual THz beams are coherently combined at a target plane distance of 4.2 m. The measurements of the interference pattern confirm the theoretically expected quadratic intensity increase of the center beam with the number of sources. At the same time, the beam waist is reduced in one dimension by a factor of 6 compared to a single beam. This technique allows for high resolution, THz stand off imaging.

Characterization of the absolute frequency stability of an individual reference cavity

T. Liu, Y. N. Zhao, V. Elman, A. Stejskal, L. J. Wang

We demonstrated for the first time (to our knowledge) the characterization of absolute frequency stability of three reference cavities by cross beating three laser beams that are independently locked to these reference cavities. This method shows the individual feature of each reference cavity, while conventional beat note measurement between two cavities can provide only an upper bound. This method allows for numerous applications such as optimizing the performance of the reference cavity for optical clockwork. (C) 2009 Optical Society of America

4OD-mediated solitonic radiations in HC-PCF cladding

F. Biancalana, F. Benabid, P. S. Light, F. Couny, A. Luiten, P. J. Roberts, Jiahui Peng, A. V. Sokolov

We observe for the first time the simultaneous emission of two resonant radiation frequencies by optical solitons in a small waveguiding glass-feature within the cladding of a HC-PCF. We attributes this phenomenon to the unusually large 4th-order GVD coefficient of the waveguide. (C)2009 Optical Society of America

Cascaded Phase-Preserving Amplitude Regeneration in a DPSK Transmission System

Ch Stephan, K. Sponsel, G. Onishchukov, B. Schmauss, G. Leuchs

Experimental results obtained on 10 Gb/s RZ-DPSK transmission in a recirculating fiber-loop setup show that nonlinear amplifying loop mirrors can efficiently enhance system performance. Main limiting factor is amplified Rayleigh backscattering in highly nonlinear fiber.

Optimization of Dispersion-Imbalanced Loop Mirror for Phase-Preserving Amplitude Regeneration

T. Roethlingshoefer, K. Sponsel, C. Stephan, G. Onishchukov, B. Schmauss, G. Leuchs

The adaptation and optimization of a dispersion-imbalance loop mirror for phase-preserving amplitude 2R regeneration is shown by numerical simulations. Its performance has been evaluated for DPSK transmission. A 4.8dB Q-factor improvement has been demonstrated.

Temperature response of turbulent premixed flames to inlet velocity oscillations

B. Ayoola, G. Hartung, C. A. Armitage, J. Hult, R. S. Cant, C. F. Kaminski

Flame-turbulence interactions are at the heart of modern combustion research as they have a major influence on efficiency, stability of operation and pollutant emissions. The problem remains a formidable challenge, and predictive modelling and the implementation of active control measures both rely on further fundamental measurements. Model burners with simple geometry offer an opportunity for the isolation and detailed study of phenomena that take place in real-world combustors, in an environment conducive to the application of advanced laser diagnostic tools. Lean premixed combustion conditions are currently of greatest interest since these are able to provide low NO (x) and improved increased fuel economy, which in turn leads to lower CO(2) emissions. This paper presents an experimental investigation of the response of a bluff-body-stabilised flame to periodic inlet fluctuations under lean premixed turbulent conditions. Inlet velocity fluctuations were imposed acoustically using loudspeakers. Spatially resolved heat release rate imaging measurements, using simultaneous planar laser-induced fluorescence (PLIF) of OH and CH(2)O, have been performed to explore the periodic heat release rate response to various acoustic forcing amplitudes and frequencies. For the first time we use this method to evaluate flame transfer functions and we compare these results with chemiluminescence measurements. Qualitative thermometry based on two-line OH PLIF was also used to compare the periodic temperature distribution around the flame with the periodic fluctuation of local heat release rate during acoustic forcing cycles.

Bistability and Stationary Gap Solitons in Quasiperiodic Photonic Crystals Based on Thue-Morse Sequence

Victor Grigoriev, Fabio Biancalana

The nonlinear properties of quasiperiodic photonic crystals based on Thue-Morse sequence are investigated. The intrinsic asymmetry of these 1D structures for odd generation numbers results in bistability thresholds which are sensitive to propagation direction. Along with resonances of perfect transmission, this feature allows to obtain strongly nonreciprocal propagation and to create an all-optical diode (ADD). The efficiency of two schemes is compared: passive and active when an additional short-term pump signal is applied. The existence of stationary gap solitons for quasiperiodic photonic crystals is shown numerically, and their difference from the Bragg case is emphasized.

Propagation of a decentered astigmatic partially coherent beam in a turbulent atmosphere

Yangjian Cai, Qiang Lin

Propagation properties of a decentered general astigmatic partially coherent beam (i.e., decentered twisted anisotropic Gaussian Schell-model beam) in a turbulent atmosphere are investigated. Analytical formulas for the cross-spectral density and decentered parameter of a decentered astigmatic partially coherent beam propagating in a turbulent atmosphere are derived. Irradiance properties of a decentered astigmatic partially coherent beam in a turbulent atmosphere are studied graphically, and are found to be quite different from its properties in free space. (C) 2007 Elsevier GmbH. All rights reserved.

Producing high fidelity single photons with optimal brightness via waveguided parametric down-conversion

K. Laiho, K. N. Cassemiro, Ch. Silberhorn

Parametric down-conversion (PDC) offers the possibility to control the fabrication of non-Gaussian states such as Fock states. However, in conventional PDC sources energy and momentum conservation introduce strict frequency and photon number correlations, which impact the fidelity of the prepared state. In our work we optimize the preparation of single-photon Fock states from the emission of waveguided PDC via spectral filtering. We study the effect of correlations via photon number resolving detection and quantum interference. Our measurements show how the reduction of mixedness due to filtering can be evaluated. Interfering the prepared photon with a coherent state we establish an experimentally measured fidelity of the produced target state of 78%. (C) 2009 Optical Society of America

Optomechanical stochastic resonance in a macroscopic torsion oscillator

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

Linear mechanical oscillators have been applied to measure very small forces, mostly with the help of noise suppression. In contrast, adding noise to nonlinear oscillators can improve the measurement conditions. Here, this effect of stochastic resonance is demonstrated in a macroscopic torsion oscillator, for an optomechanical nonlinear potential. The signal output is enhanced for a subthreshold electronic signal. This nonlinear oscillator serves as a model system for the enhancement of signal-to-noise ratio in high precision optomechanical experiments.

Telecom-Wavelength Compatible THz n-i-pn-i-p Super lattice Photomixers for Spectroscopical Applications

Sascha Preu, Stefan Malzer, Gottfried H. Doehler, Lijun Wang, Hong Lu, Arthur C. Gossard

We report on the progress of room-temperature operating, continuous-wave, tunable n-i-pn-i-p superlattice THz photomixers, compatible with 1.55 mu m telecom laser systems. An output power of 0.2 mu W at 1 THz has been achieved at a photocurrent of 9.4 mA, using a broadband antenna. The spectral power is at a level where high resolution spectroscopy becomes attractive. This is demonstrated by measuring the absorption spectrum of water vapor between 0.4 and 1.6 THz.

Brewster cross polarization

A. Aiello, M. Merano, J. P. Woerdman

We theoretically derive the polarization-resolved intensity distribution of a TM-polarized fundamental Gaussian beam reflected by an air-glass plane interface at Brewster incidence. The reflected beam has both a dominant (TM) and a cross-polarized (TM) component, carried by a TEM(10) and a TEM(01) Hermite-Gaussian spatial mode, respectively. Remarkably, we find that the TE-mode power scales quadratically with the angular spread of the incident beam and is comparable to the TM-mode power. Experimental confirmations of the theoretical results are also presented. (C) 2009 Optical Society of America

Whispering-gallery-mode-resonator-stabilized narrow-linewidth fiber loop laser

B. Sprenger, H. G. L. Schwefel, L. J. Wang

We demonstrate a narrow-line fiber loop laser using erbium-doped fiber as the gain material, stabilized by using a microsphere as a transmissive frequency selective element. Stable lasing with a linewidth of 170 kHz is observed, limited by the experimental spectral resolution. A linear increase in output power and a redshift of the lasing mode were also observed with increasing pump power. Its potential applications are discussed. (C) 2009 Optical Society of America

Use of Fiber Nonlinearities for Signal Improvement in Optical Transmission Systems

Bernhard Schmauss, Michael Holtmannspoetter, Christian Stephan, Klaus Sponsel, Georgy Onishchukov, Gerd Leuchs

Optical data signals suffer from signal distortion and noise accumulation during transmission over long distances. In this paper we report on the usage of nonlinear fiber effects for improvement of the signal quality in two examples. On the one hand side we describe our work on the regeneration of phase encoded signals using nonlinear optical loop mirror setups, On the other hand side Raman effect based optical attenuators for the suppression of power transient caused by changes in channel numbers are discussed.

Dynamics and control of the early stage of supercontinuum generation in submicron-core optical fibers

Truong X. Tran, Fabio Biancalana

We describe the precise mechanism behind the rapid and large spectral broadening in the early stages of supercontinuum generation in submicron-core glass waveguides. The first spectral and temporal breathing period of higher-order solitons activates the solitonic resonant radiation by means of energy bursts. We demonstrate that this "activation length" is largely independent of the Raman effect but can be efficiently controlled by changing the input pulse chirp. Energy transfer between the input pulse and its resonant radiations has a marked steplike character, and can reach a large efficiency. This will lead to the design of devices based on millimeter-size fibers, which are able to emit a clean, coherent, and controllable spectrum.

Fiber-assisted single-photon spectrograph

Malte Avenhaus, Andreas Eckstein, Peter J. Mosley, Christine Silberhorn

We demonstrate the implementation of a fiber-integrated spectrograph utilizing chromatic group velocity dispersion (GVD) in a single-mode fiber. By means of GVD we stretch an ultrafast pulse in time in order to spectrally resolve single photons in the time domain, detected by single-photon counting modules with very accurate temporal resolution. As a result, the spectrum of a very weak pulse is recovered from a precise time measurement with high signal-to-noise ratio. We demonstrate the potential of our technique by applying our scheme to analyzing the joint spectral intensity distribution of a parametric downconversion source at a telecommunication wavelength. (C) 2009 Optical Society of America

Colloquium: The Einstein-Podolsky-Rosen paradox: From concepts to applications

M. D. Reid, P. D. Drummond, W. P. Bowen, E. G. Cavalcanti, P. K. Lam, H. A. Bachor, U. L. Andersen, G. Leuchs

This Colloquium examines the field of the Einstein, Podolsky, and Rosen (EPR) gedanken experiment, from the original paper of Einstein, Podolsky, and Rosen, through to modern theoretical proposals of how to realize both the continuous-variable and discrete versions of the EPR paradox. The relationship with entanglement and Bell's theorem are analyzed, and the progress to date towards experimental confirmation of the EPR paradox is summarized, with a detailed treatment of the continuous-variable paradox in laser-based experiments. Practical techniques covered include continuous-wave parametric amplifier and optical fiber quantum soliton experiments. Current proposals for extending EPR experiments to massive-particle systems are discussed, including spin squeezing, atomic position entanglement, and quadrature entanglement in ultracold atoms. Finally, applications of this technology to quantum key distribution, quantum teleportation, and entanglement swapping are examined.

Symmetric extension in two-way quantum key distribution

Geir Ove Myhr, Joseph M. Renes, Andrew C. Doherty, Norbert Luetkenhaus

We introduce symmetric extensions of bipartite quantum states as a tool for analyzing protocols that distill secret key from quantum correlations. Whether the correlations are coming from a prepare-and-measure quantum key distribution scheme or from an entanglement-based scheme, the protocol has to produce effective states without a symmetric extension in order to succeed. By formulating the symmetric extension problem as a semidefinite program, we solve the problem for Bell-diagonal states. Applying this result to the six-state and Bennett-Brassard 1984 schemes, we show that for the entangled states that cannot be distilled by current key distillation procedures, the failure can be understood in terms of a failure to break a symmetric extension.

An accurate envelope equation for light propagation in photonic nanowires: new nonlinear effects

Truong X. Tran, Fabio Biancalana

We derive a set of new unidirectional evolution equations for photonic nanowires, i.e. waveguides with sub-wavelength core diameter. Contrary to previous approaches, our formulation simultaneously takes into account both the vector nature of the electromagnetic field and the full variations of the effective modal profiles with wavelength. This leads to the discovery of new, previously unexplored nonlinear effects which have the potential to affect soliton propagation considerably. We specialize our theoretical considerations to the case of perfectly circular silica strands in air, and we support our analysis with detailed numerical simulations. (C) 2009 Optical Society of America

Recent developments in photonic crystal fibres

N. Y. Joly, A. Chugreev, T. G. Euser, H. Hundertmark, A. Nazarkin, A. Podlipensky, M. Scharrer, M. A. Schmidt, P. St. J. Russell

In photonic crystal fibres (PCF), light propagates in either a solid or hollow-core surrounded by a periodic microstructured cladding, which greatly affects its optical properties. We will report recent results for both types of PCF.

Generation of total angular momentum eigenstates in remote qubits

A. Maser, U. Schilling, T. Bastin, E. Solano, C. Thiel, J. von Zanthier

We propose a scheme enabling the universal coupling of angular momentum of N remote noninteracting qubits using linear optical tools only. Our system consists of N single-photon emitters in a Lambda configuration that are entangled among their long-lived ground-state qubits through suitably designed measurements of the emitted photons. In this manner, we present an experimentally feasible algorithm that is able to generate any of the 2(N) symmetric and nonsymmetric total angular momentum eigenstates spanning the Hilbert space of the N-qubit compound.

A complete basis of generalized Bell states

Denis Sych, Gerd Leuchs

A generalization of the Bell states and Pauli matrices to dimensions which are powers of 2 is considered. A basis of maximally entangled multidimensional bipartite states (MEMBS) is chosen very similar to the standard Bell states and constructed of only symmetric and antisymmetric states. This special basis of MEMBS preserves all basic properties of the standard Bell states. We present a recursive and non-recursive method for the construction of MEMBS and discuss their properties. The antisymmetric MEMBS possess the property of rotationally invariant exclusive correlations which is a generalization of the rotational invariance of the antisymmetric singlet Bell state.

High-repetition-rate combustion thermometry with two-line atomic fluorescence excited by diode lasers

Robin S. M. Chrystie, Iain S. Burns, Johan Hult, Clemens F. Kaminski

We report on kilohertz-repetition-rate flame temperature measurements performed using blue diode lasers. Two-line atomic fluorescence was performed by using diode lasers emitting at around 410 and 451 nm to probe seeded atomic indium. At a repetition rate of 3.5 kHz our technique offers a precision of 1.5% at 2000 K in laminar methane/air flames. The spatial resolution is better than 150 mu m, while the setup is compact and easy to operate, at much lower cost than alternative techniques. By modeling the spectral overlap between the locked laser and the probed indium lines we avoid the need for any calibration of the measurements. We demonstrate the capability of the technique for time-resolved measurements in an acoustically perturbed flame. The technique is applicable in flames with a wide range of compositions including sooting flames. (C) 2009 Optical Society of America

Quantum computing with continuous-variable clusters

Mile Gu, Christian Weedbrook, Nicolas C. Menicucci, Timothy C. Ralph, Peter van Loock

Continuous-variable cluster states offer a potentially promising method of implementing a quantum computer. This paper extends and further refines theoretical foundations and protocols for experimental implementation. We give a cluster-state implementation of the cubic phase gate through photon detection, which, together with homodyne detection, facilitates universal quantum computation. In addition, we characterize the offline squeezed resources required to generate an arbitrary graph state through passive linear optics. Most significantly, we prove that there are universal states for which the offline squeezing per mode does not increase with the size of the cluster. Simple representations of continuous-variable graph states are introduced to analyze graph state transformations under measurement and the existence of universal continuous-variable resource states.

Light-Matter Interactions in Photonic Crystal Fibres

P. St J. Russell

The microstructure in photonic crystal fibre permits low-loss guidance of light both in solid material and in vacuum. The field profiles and group velocity dispersion of the guided modes can be engineered over wide ranges of parameters. The resulting improved control over light-matter interactions is permitting studies of, for example, intense nonlinear phonon-photon interactions, efficient generation of broad-band supercontinuum light, precisely controlled propulsion and guidance of small particles, plasmonic effects in metallic nanowire arrays, and hugely enhanced nonlinear gas-laser interactions.

Octave-spanning supercontinuum generated in SF6-glass PCF by a 1060 nm mode-locked fibre laser delivering 20 pJ per pulse

H. Hundertmark, S. Rammler, T. Wilken, R. Holzwarth, T. W. Haensch, P. St. J. Russell

We report the generation of an octave-spanning supercontinuum in SF6-glass photonic crystal fiber using a diode-pumped passively modelocked fs Yb-fiber laser oscillating at 1060 nm. The pulses (energy up to 500 pJ and duration 60 fs) were launched into a 4 cm length of PCF (core diameter 1.7 mu m and zero-dispersion wavelength similar to 1060 nm). Less than 20 pJ of launched pulse energy was sufficient to generate a supercontinuum from 600 nm to 1450 nm, which represents the lowest energy so far reported for generation of an octave-spanning supercontinuum from a 1 mu m pump. Since the laser pulse energy scales inversely with the repetition rate, highly compact and efficient sources based on SF6-glass PCF are likely to be especially useful for efficient spectral broadening at high repetition rates (several GHz), such as those needed for the precise calibration of astronomical spectrographs, where a frequency comb spacing > 10 GHz is required for the best performance. (C) 2009 Optical Society of America

Generation of Emission Centers for Broadband NIR Luminescence in Bismuthate Glass by Femtosecond Laser Irradiation

Mingying Peng, Quanzhong Zhao, Jianrong Qiu, Lothar Wondraczek

Bismuth-doped glasses have recently received significant interest as potential material for ultrabroadband optical amplification in the telecommunication spectral bands, as well as as gain material for fiber lasers. However, the nature of the active centers that are responsible for the observed near-infrared (NIR) luminescence is still highly debated. In order to probe the mechanism that leads to NIR emission in bismuth-containing glasses, femtosecond (fs) laser irradiation was used. It is shown that local absorption properties in the visible spectral range can be altered in initially transparent bismuthate glasses after fs laser irradiation. Induced absorption centers exhibit the well-known broadband optical emission peaking at similar to 1250 nm when excited with a 785 nm diode laser. Absorption and emission intensities increase with increasing average pulse energy. These observations are interpreted as a photo-induced reduction reaction of Bi(3+) to Bi(+) species, while the previously discussed formation of Bi-clusters by ion diffusion is excluded due to the very short interaction time that results from the use of fs laser. Bi(+) species are, therefore, proposed as the major origin of NIR emission from Bi-doped glasses.

Experimental Observation of Bloch Oscillations in the Spectral Domain

Christoph Bersch, Georgy Onishchukov, Ulf Peschel

We have experimentally demonstrated for the first time spectral Bloch oscillations using the interaction between a probe signal and a traveling-wave periodic potential in an optical fiber.

Generalized tensor ABCD law for an elliptical Gaussian beam passing through an astigmatic optical system in turbulent atmosphere

Yangjian Cai, Q. Lin, H. T. Eyyuboglu, Y. Baykal

The propagation of an elliptical Gaussian beam (EGB) through an astigmatic ABCD optical system in a turbulent atmosphere is investigated. An analytical formula for the average intensity of an EGB and a generalized tensor ABCD law for the generalized complex curvature tensor are derived. As an application example, we derived an analytical formula for the average intensity of an elliptical flat-topped beam propagating through an astigmatic ABCD optical system in a turbulent atmosphere. As a numerical example, the focusing properties of an EGB focused by a thin lens in a turbulent atmosphere are studied. It is found that the focused beam at the focal plane becomes a circular Gaussian beam when the atmospheric turbulence is strong enough, and the beam width of the circular Gaussian beam is determined by atmospheric turbulence strength, focal length of the thin lens, and wavelength of the initial beam but is independent of the initial beam widths (i.e., initial intensity distribution).

Improvement of DPSK Transmission by Phase-Preserving Amplitude Regeneration Using Cascaded Nonlinear Amplifying Loop Mirrors

Ch. Stephan, K. Sponsel, G. Onishchukov, B. Schmauss, G. Leuchs

Experimental results obtained on 10 Gb/s RZ-DPSK transmission in a recirculating fiber-loop setup have shown that nonlinear amplifying loop mirrors (NALM) can efficiently enhance system performance: cascaded phase-preserving amplitude regeneration can efficiently prevent the accumulation of amplitude and nonlinear phase noise components in such systems. It was possible to significantly augment the transmitted signal quality; alternatively a considerable increase of fiber launch power could be achieved. Amplified Rayleigh backscattering in the highly nonlinear fiber in the NALM has been identified as main limiting effect when the regenerator is passed multiple times.

Nonparaxial polarizers

Andrea Aiello, Christoph Marquardt, Gerd Leuchs

We develop a theoretical description for polarizers that goes beyond the paraxial approximation. By combining existing theories for fields with nonplanar wavefronts, we are able to derive a simple power series expansion expressing the electric field of a light beam after a polarizer as a linear function of the field and its spatial derivatives evaluated before the polarizer. The first few terms of such expansion are explicitly given, and their physical meaning is discussed. (C) 2009 Optical Society of America

Extra phase noise from thermal fluctuations in nonlinear optical crystals

J. E. S. Cesar, A. S. Coelho, K. N. Cassemiro, A. S. Villar, M. Lassen, P. Nussenzveig, M. Martinelli

We show theoretically and experimentally that scattered light by thermal phonons inside a second-order nonlinear crystal is the source of additional phase noise observed in optical parametric oscillators. This additional phase noise reduces the quantum correlations and has hitherto hindered the direct production of multipartite entanglement in a single nonlinear optical system. We cooled the nonlinear crystal and observed a reduction in the extra noise. Our treatment of this noise can be successfully applied to different systems in the literature.

An improved method for calculating resonances of multiple dielectric disks arbitrarily positioned in the plane

Harald G. L. Schwefel, Christopher G. Poulton

We present a numerically improved multipole formulation for the calculation of resonances of multiple disks located at arbitrary positions in a 2-d plane, and suitable for the accurate computation of the resonances of large numbers of disks and of high-wavenumber eigenstates. Using a simple reformulation of the field expansions and boundary conditions, we are able to transform the multipole formalism into a linear eigenvalue problem, for which fast and accurate methods are available. Observing that the motion of the eigenvalues in the complex plane is analytic with respect to a two parameter family, we present a numerical algorithm to compute a range of multiple-disk resonances and field distributions using only two diagonalizations. This method can be applied to photonic molecules, photonic crystals, photonic crystal fibers, and random lasers. (C) 2009 Optical Society of America

Absolute frequency measurement of the molecular iodine hyperfine components near 560 nm with a solid-state laser source

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

We report absolute frequency measurements of the molecular iodine R(34) 20-0 a(1), a(10), and a(15) hyperfine transitions, and the P(144) 23-0 a(1) hyperfine transition at 560 nm with a frequency comb. The light source is based on an all-solid-state frequency quadrupled laser system. A frequency stability of 4 x 10(-12) is achieved over a 100 s integration time when the light source is frequency stabilized to the R(34) 20-0 a(1) line. The pressure and power broadening dependences of the R(34) 20-0 a(10) line are also investigated. (C) 2009 Optical Society of America

Pure single photon generation by type-I PDC with backward-wave amplification

A. Christ, A. Eckstein, P. J. Mosley, C. Silberhorn

We explore a promising method of generating pure heralded single photons. Our approach is based on parametric downconversion in a periodically-poled waveguide. However, unlike conventional downconversion sources, the photon pairs are counter-propagating: one travels with the pump beam in the forward direction while the other is backpropagating towards the laser source. Our calculations reveal that these downconverted two-photon states carry minimal spectral correlations within each photon-pair. This approach offers the possibility to employ a new range of downconversion processes and materials like PPLN (previously considered unsuitable due to its unfavorable phasematching properties) to produce heralded pure single photons over a broad frequency range. (c) 2009 Optical Society of America

Cavity Enhanced Spectroscopy of High-Temperature H2O in the Near-Infrared Using a Supercontinuum Light Source

Rosalynne S. Watt, Toni Laurila, Clemens F. Kaminski, Johan Hult

In this paper we demonstrate how broadband cavity enhanced absorption spectroscopy (CEAS) with supercontinuum (SC) radiation in the near-infrared spectral range can be used as a sensitive, multiplexed, and simple tool to probe gas-phase species in high-temperature environments. Near-infrared SC radiation is generated by pumping a standard single-mode fiber with a picosecond fiber laser. Standard low reflectivity mirrors are used for the cavity and an optical spectrum analyzer is used for the detection of gas-phase species in combustion. The method is demonstrated by measuring flame generated H2O in the 1500 to 1550 non region and room-temperature CO2 between 1520 non and 1660 nm. The broadband nature of the technique permits hundreds of rotational features to be recorded, giving good potential to unravel complex, convoluted spectra. We discuss practical issues concerning the implementation of the technique and present a straightforward method for calibration of the CEAS system via a cavity ringdown measurement. Despite the large spectral variation of SC radiation from pulse to pulse, it is shown that SC sources can offer good stability for CEAS where a large number of SC pulses are typically averaged.

Self-organized regular arrays of carbon nanocones induced by ultrashort laser pulses and their field emission properties

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

Periodic, self-organized carbon nanocone structures with a spatial period of up to 170 nm (less than 1/5 laser wavelength) are induced by exposing a graphite surface to a single beam 800 nm ultrashort pulsed laser. When a linearly polarized laser beam is used, the nanocones are aligned perpendicularly to the direction of polarization. This paper also demonstrates the fabrication of large-area carbon nanocone structures. The resulting carbon nanocones show a field emission performance with a turn-on electric field of as low as 3.2 V/mu m.

Experimental verification of high spectral entanglement for pulsed waveguided spontaneous parametric down-conversion

Malte Avenhaus, Maria V. Chekhova, Leonid A. Krivitsky, Gerd Leuchs, Christine Silberhorn

We study the spectral properties of spontaneous parametric down-conversion (SPDC) in a periodically poled waveguided structure of potassium-titanyl-phosphate (KTP) crystal pumped by ultrashort pulses. Our theoretical analysis reveals a strongly entangled and asymmetric structure of the two-photon spectral amplitude for type-II SPDC. We confirm these predictions experimentally by measuring single-photon spectra, on one hand, and the dependence of Hong-Ou-Mandel interference visibility on the width of spectral filtering, on the other hand.

Triplet-like correlation symmetry of continuous variable entangled states

Gerd Leuchs, Ruifang Dong, Denis Sych

We report on a continuous variable analogue of the triplet two-qubit Bell states. We discuss the symmetry properties of entangled states of either kind, and theoretically and experimentally demonstrate a remarkable similarity of two-mode continuous variable entangled states with triplet Bell states with respect to their correlation patterns. Understanding the symmetry properties helps finding decoherence-free subspaces.

Tightly trapped acoustic phonons in photonic crystal fibres as highly nonlinear artificial Raman oscillators

M. S. Kang, A. Nazarkin, A. Brenn, P. St. J. Russell

Interactions between light and hypersonic waves can be enhanced by tight field confinement, as shown in periodically structured materials(1), microcavities(2), micromechanical resonators(3) and photonic crystal fibres(4-6) (PCFs). There are many examples of weak sound-light interactions, for example, guided acoustic-wave Brillouin scattering in conventional optical fibres(7). This forward-scattering effect results from the interaction of core-guided light with acoustic resonances of the entire fibre cross-section, and is viewed as a noise source in quantum-optics experiments(8). Here, we report the observation of strongly nonlinear forward scattering of laser light by gigahertz acoustic vibrations, tightly trapped together in the small core of a silica-air PCF. Bouncing to and fro across the core at close to 90 degrees to the fibre axis, the acoustic waves form optical-phonon-like modes with a flat dispersion curve and a distinct cutoff frequency Omega(a). This ensures automatic phase-matching to the guided optical mode so that, on pumping with a dual-frequency laser source tuned to Omega(a), multiple optical side bands are generated, spaced by Omega(a). The number of strong side bands in this Raman-like process increases with pump power. The results point to a new class of designable nonlinear optical device with applications in, for example, pulse synthesis, frequency comb generation for telecommunications and fibre laser mode-locking.

Three-Color Entanglement

A. S. Coelho, F. A. S. Barbosa, K. N. Cassemiro, A. S. Villar, M. Martinelli, P. Nussenzveig

Entanglement is an essential quantum resource for the acceleration of information processing as well as for sophisticated quantum communication protocols. Quantum information networks are expected to convey information from one place to another by using entangled light beams. We demonstrated the generation of entanglement among three bright beams of light, all of different wavelengths (532.251, 1062.102, and 1066.915 nanometers). We also observed disentanglement for finite channel losses, the continuous variable counterpart to entanglement sudden death.

Upper bounds for the secure key rate of the decoy-state quantum key distribution

Marcos Curty, Tobias Moroder, Xiongfeng Ma, Hoi-Kwong Lo, Norbert Luetkenhaus

The use of decoy states in quantum key distribution (QKD) has provided a method for substantially increasing the secret key rate and distance that can be covered by QKD protocols with practical signals. The security analysis of these schemes, however, leaves open the possibility that the development of better proof techniques or better classical postprocessing methods might further improve their performance in realistic scenarios. In this paper, we derive upper bounds on the secure key rate for decoy-state QKD. These bounds are based basically only on the classical correlations established by the legitimate users during the quantum communication phase of the protocol. The only assumption about the possible postprocessing methods is that double click events are randomly assigned to single click events. Further, we consider only secure key rates based on the uncalibrated device scenario which assigns imperfections such as detection inefficiency to the eavesdropper. Our analysis relies on two preconditions for secure two-way and one-way QKD. 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. Both criteria have been previously applied to evaluate single-photon implementations of QKD. Here we use them to investigate a realistic source of weak coherent pulses. The resulting upper bounds can be formulated as a convex optimization problem known as a semidefinite program which can be efficiently solved. For the standard four-state QKD protocol, they are quite close to known lower bounds, thus showing that there are clear limits to the further improvement of classical postprocessing techniques in decoy-state QKD.

Toward the production of micropolarizers by irradiation of composite glasses with silver nanoparticles

Andrei Stalmashonak, Gerhard Seifert, Ahmet Akin Unal, Ulrich Skrzypczak, Alexander Podlipensky, Amin Abdolvand, Heinrich Graener

We investigate the possibility of preparation of laser-induced dichroism in composite glasses with a high concentration of silver nanoparticles. A detailed analysis based on the Maxwell-Garnett theory and experimental results shows that particles at different volume fractions react differently to the same laser irradiation parameters. Based on these findings, we demonstrate that a well-defined sequence of multiple irradiation and intermediate annealing can maximize the particles' aspect ratio and avoid unwanted partial destruction. The proposed irradiation technique permits production of micropolarizing structures with high polarization contrast in the visible and near-IR regions. (C) 2009 Optical Society of America

Flame front tracking in turbulent lean premixed flames using stereo PIV and time-sequenced planar LIF of OH

G. Hartung, J. Hult, R. Balachandran, M. R. Mackley, C. F. Kaminski

This paper describes the simultaneous application of time-sequenced laser-induced fluorescence imaging of OH radicals and stereoscopic particle image velocimetry for measurements of the flame front dynamics in lean and premixed LP turbulent flames. The studied flames could be acoustically driven, to simulate phenomena important in LP combustion technologies. In combination with novel image post processing techniques we show how the data obtained can be used to track the flame front contour in a plane defined by the illuminating laser sheets. We consider effects of chemistry and convective fluid motion on the dynamics of the observed displacements and analyse the influence of turbulence and acoustic forcing on the observed contour velocity, a quantity we term as s (d) (2D) . We show that this quantity is a valuable and sensitive indicator of flame turbulence interactions, as (a) it is measurable with existing experimental methodologies, and (b) because computational data, e.g. from large eddy simulations, can be post processed in an identical fashion. s (d) (2D) is related (to a two-dimensional projection) of the three-dimensional flame displacement speed s (d) , but artifacts due to out of plane convective motion of the flame surface and the uncertainty in the angle of the flame surface normal have to be carefully considered. Monte Carlo simulations were performed to estimate such effects for several distributions of flame front angle distributions, and it is shown conclusively that s (d) (2D) is a sensitive indicator of a quantity related to s (d) in the flames we study. s (d) (2D) was shown to increase linearly both with turbulent intensity and with the amplitude of acousting forcing for the range of conditions studied.

The security of practical quantum key distribution

Valerio Scarani, Helle Bechmann-Pasquinucci, Nicolas J. Cerf, Miloslav Dusek, Norbert Luetkenhaus, Momtchil Peev

Quantum key distribution (QKD) is the first quantum information task to reach the level of mature technology, already fit for commercialization. It aims at the creation of a secret key between authorized partners connected by a quantum channel and a classical authenticated channel. The security of the key can in principle be guaranteed without putting any restriction on an eavesdropper's power. This article provides a concise up-to-date review of QKD, biased toward the practical side. Essential theoretical tools that have been developed to assess the security of the main experimental platforms are presented (discrete-variable, continuous-variable, and distributed-phase-reference protocols).

A method to unmix multiple fluorophores in microscopy images with minimal a priori information

S. Schlachter, S. Schwedler, A. Esposito, G. S. Kaminski Schierle, G. D. Moggridge, C. F. Kaminski

The ability to quantify the fluorescence signals from multiply labeled biological samples is highly desirable in the life sciences but often difficult, because of spectral overlap between fluorescent species and the presence of autofluorescence. Several so called unmixing algorithms have been developed to address this problem. Here, we present a novel algorithm that combines measurements of lifetime and spectrum to achieve unmixing without a priori information on the spectral properties of the fluorophore labels. The only assumption made is that the lifetimes of the fluorophores differ. Our method combines global analysis for a measurement of lifetime distributions with singular value decomposition to recover individual fluorescence spectra. We demonstrate the technique on simulated datasets and subsequently by an experiment on a biological sample. The method is computationally efficient and straightforward to implement. Applications range from histopathology of complex and multiply labelled samples to functional imaging in live cells. (C) 2009 Optical Society of America

Continuous Variable Entanglement and Squeezing of Orbital Angular Momentum States

M. Lassen, G. Leuchs, U. L. Andersen

We report the first experimental characterization of the first-order continuous variable orbital angular momentum states. Using a spatially nondegenerate optical parametric oscillator (OPO) we produce quadrature entanglement between the two first-order Laguerre-Gauss modes. The family of orbital angular momentum modes is mapped on an orbital Poincare sphere, where the mode's position on the sphere is spanned by the three orbital parameters. Using a nondegenerate OPO we produce squeezing of these parameters, and as an illustration, we reconstruct the "cigar-shaped" uncertainty volume on the orbital Poincare sphere.

Stylus ion trap for enhanced access and sensing

Robert Maiwald, Dietrich Leibfried, Joe Britton, James C. Bergquist, Gerd Leuchs, David J. Wineland

Small, controllable, highly accessible quantum systems can serve as probes at the single-quantum level to study a number of physical effects, for example in quantum optics or for electric- and magnetic-field sensing. The applicability of trapped atomic ions as probes is highly dependent on the measurement situation at hand and thus calls for specialized traps. Previous approaches for ion traps with enhanced optical access included traps consisting of a single ring electrode(1,2) or two opposing endcap electrodes(2,3). Other possibilities are planar trap geometries, which have been investigated for Penning traps(4,5) and radiofrequency trap arrays(6-8). By not having the electrodes lie in a common plane, the optical access can be substantially increased. Here, we report the fabrication and experimental characterization of a novel radiofrequency ion trap geometry. It has a relatively simple structure and provides largely unrestricted optical and physical access to the ion, of up to 96% of the total 4 pi solid angle in one of the three traps tested. The trap might find applications in quantum optics and field sensing. As a force sensor, we estimate sensitivity to forces smaller than 1 yN Hz(-1/2).

A vibration-insensitive optical cavity and absolute determination of its ultrahigh stability

Y. N. Zhao, J. Zhang, A. Stejskal, T. Liu, V. Elman, Z. H. Lu, L. J. Wang

We use the three-cornered-hat method to evaluate the absolute frequency stabilities of three different ultrastable reference cavities, one of which has a vibration-insensitive design that does not even require vibration isolation. An Nd: YAG laser and a diode laser are implemented as light sources. We observe similar to 1 Hz beat note linewidths between all three cavities. The measurement demonstrates that the vibration-insensitive cavity has a good frequency stability over the entire measurement time from 100 mu s to 200 s. An absolute, correlation-removed Allan deviation of 1.4 x 10(-15) at 1 s of this cavity is obtained, giving a frequency uncertainty of only 0.44 Hz. (C) 2009 Optical Society of America

Reduction of Nonlinear Phase Noise in DPSK Transmission Using a NALM

C. Stephan, K. Sponsel, G. Onishchukov, B. Schmauss, G. Leuchs

Performance of a NALM as phase-preserving amplitude 2R-regenerator in a DPSK transmission system has been investigated experimentally and numerically. A 3dB improvement of eye opening or alternatively a 3dB increase of fibre-launched power are demonstrated.

Scintillation properties of a rectangular dark hollow beam

Yangjian Cai, Yan Zhang

The scintillation properties of a rectangular dark hollow beam (DHB) in a weak turbulent atmosphere are investigated. Explicit expression for the on-axis scintillation index of a rectangular DHB is derived. It is found that the scintillation index value of a rectangular DHB can be smaller than that of Gaussian, elliptical Gaussian and rectangular flat-topped beams in a weak turbulent atmosphere under certain conditions. Our results will be useful in long-distance free-space optical communications. The scintillation properties of a rectangular DHB are closely controlled by its initial beam parameters.

Optimizing anti-Stokes Raman scattering in gas-filled hollow-core photonic crystal fibers

A. Nazarkin, A. Abdolvand, P. St. J. Russell

Anti-Stokes Raman scattering in gas-filled hollow-core photonic crystal fibers is discussed. It is shown that the efficient anti-Stokes generation observed under conditions of significant wave mismatch is caused by phase locking of the interacting fields. This leads to the establishment of a phase difference that is independent of the optical path. An optimization technique, based on the adjustment of the wave mismatch along a gas-filled hollow fiber using pressure control, is proposed. Anti-Stokes conversion efficiencies close to the theoretical maximum of 50% are predicted.

Suppression of Nonlinear Phase Noise in a DPSK Transmission Using a Nonlinear Amplifying Loop Mirror

C. Stephan, K. Sponsel, G. Onishchukov, B. Schmauss, G. Leuchs

Experimental and numerical investigations of phase-preserving amplitude regeneration in a DPSK transmission with a NALM have shown that a 3dB improvement of the Q-factor or alternatively a 3dB increase of fiber-launched power could be achieved. (C) 2008 Optical Society of America

Correlation measurement 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 which is nearly free of electronic noise. With two different sources of squeezed light, we show that the sign of the covariance coefficient, revealed from the time-resolved correlation data, is witnessing the presence of squeezing in the system. Furthermore, we estimate the degree of squeezing using the correlation method and compare it to the standard homodyne measurement scheme. We show that the role of electronic detector noise is minimized using the correlation approach as opposed to homodyning where it often becomes a crucial issue.

Microcavity polaritonlike dispersion doublet in resonant Bragg gratings

Fabio Biancalana, Leonidas Mouchliadis, Celestino Creatore, Simon Osborne, Wolfgang Langbein

Periodic structures resonantly coupled to excitonic media allow the existence of extra intragap modes ("Braggoritons") due to the coupling between Bragg photon modes and bulk excitons. This induces unique dispersive features, which can be tailored by properly designing the photonic band gap around the exciton resonance. We report that Braggoritons realized with semiconductor gratings have the ability to mimic the dispersion of quantum-well microcavity polaritons. This gives rise to peculiar nonlinear phenomena, such as slow-light-enhanced nonlinear propagation and an efficient parametric scattering at two "magic frequencies."

Experimental observation of spectral Bloch oscillations

Christoph Bersch, Georgy Onishchukov, Ulf Peschel

We report on the first, to our knowledge, experimental observation of spectral Bloch oscillations in an optical fiber employing the interaction between a probe signal and a traveling-wave periodic potential. The spectrum of weak probe pulses is shown to oscillate on account of their group-velocity mismatch to the periodic field. The behavior of a cw probe spectrum reveals the actual discrete nature of the effect. Recurrences of the spectrum after one and two Bloch periods are demonstrated. (C) 2009 Optical Society of America

Solitary Pulse Generation by Backward Raman Scattering in H-2-Filled Photonic Crystal Fibers

A. Abdolvand, A. Nazarkin, A. V. Chugreev, C. F. Kaminski, P. St. J. Russell

Using a hydrogen-filled hollow-core photonic crystal fiber as a nonlinear optical gas cell, we study amplification of ns-laser pulses by backward rotational Raman scattering. We find that the amplification process has two characteristic stages. Initially, the pulse energy grows and its duration shortens due to gain saturation at the trailing edge of the pulse. This phase is followed by formation of a symmetric pulse with a duration significantly shorter than the phase relaxation time of the Raman transition. Stabilization of the Stokes pulse profile to a solitonlike hyperbolic secant shape occurs as a result of nonlinear amplification at its front edge and nonlinear absorption at its trailing edge (caused by energy conversion back to the pump field), leading to a reshaped pulse envelope that travels at superluminal velocity.

Demonstration of a universal one-way quantum quadratic phase gate

Yoshichika Miwa, Jun-ichi Yoshikawa, Peter van Loock, Akira Furusawa

We demonstrate a quadratic phase gate for one-way quantum computation in the continuous-variable regime. This canonical gate, together with phase-space displacements and Fourier rotations, completes the set of universal gates for realizing any single-mode Gaussian transformation such as arbitrary squeezing. As opposed to previous implementations of measurement-based squeezers, the current gate is fully controlled by the local oscillator phase of the homodyne detector. Verifying this controllability, we give an experimental demonstration of the principles of one-way quantum computation over continuous variables. Moreover, we can observe sub-shot-noise quadrature variances in the output states, confirming that nonclassical states are created through cluster computation.