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
Nonparaxial polarizers
Andrea Aiello,
Christoph Marquardt,
Gerd Leuchs
OPTICS LETTERS
34
(20)
3160-3162
(2009)
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
Excitation of plasmonic gap waveguides by nanoantennas
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
Triplet-like correlation symmetry of continuous variable entangled
states
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.
Continuous Variable Entanglement and Squeezing of Orbital Angular
Momentum States
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.
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.
An improved method for calculating resonances of multiple dielectric
disks arbitrarily positioned in the plane
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
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.
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.
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.
Optomechanical stochastic resonance in a macroscopic torsion oscillator
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.
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.
The information of ambiguity
Ludmila Praxmeyer,
Stig Stenholm,
Nikolay V. Vitanov
JOURNAL OF MODERN OPTICS
56
(10)
PII 912447899
1205-1219
(2009)
| Journal
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.
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
REVIEWS OF MODERN PHYSICS
81
(4)
1727-1751
(2009)
| Journal
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.
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.
Perfect excitation of a matter qubit by a single photon in free space
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
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.
Experimental observation of spectral Bloch oscillations
Christoph Bersch,
Georgy Onishchukov,
Ulf Peschel
OPTICS LETTERS
34
(15)
2372-2374
(2009)
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
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.
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.
Interferometric quasi-absolute tests for aspherics using a radial shear
position
Klaus Mantel,
Eduard Geist,
Irina Harder,
Norbert Lindlein,
Gerd Leuchs
OPTICS LETTERS
34
(20)
3178-3180
(2009)
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
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.
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).
Quadrature measurements of a bright squeezed state via sideband swapping
Jessica Schneider,
Oliver Gloeckl,
Gerd Leuchs,
Ulrik L. Andersen
OPTICS LETTERS
34
(8)
1186-1188
(2009)
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
Generation and Direct Detection of Broadband Mesoscopic
Polarization-Squeezed Vacuum
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.
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.
Brewster cross polarization
A. Aiello,
M. Merano,
J. P. Woerdman
OPTICS LETTERS
34
(8)
1207-1209
(2009)
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
Duality between spatial and angular shift in optical reflection
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)
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.
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.
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
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