Electron-plasmon scattering in chiral one-dimensional systems with nonlinear dispersion

M. Heyl, S. Kehrein, F. Marquardt, C. Neuenhahn

We investigate systems of spinless one-dimensional chiral fermions realized, e. g., in the arms of electronic Mach-Zehnder interferometers, at high energies. Taking into account the curvature of the fermionic spectrum and a finite interaction range, we find a new scattering mechanism where high-energy electrons scatter off plasmons (density excitations). This leads to an exponential decay of the single-particle Green's function even at zero temperature with an energy-dependent rate. As a consequence of this electron-plasmon scattering channel, we observe the coherent excitation of a plasmon wave in the wake of a high-energy electron resulting in the buildup of a monochromatic sinusoidal density pattern.

Resonant quantum gates in circuit quantum electrodynamics

G. Haack, F. Helmer, M. Mariantoni, F. Marquardt, E. Solano

We propose the implementation of fast resonant gates in circuit quantum electrodynamics for quantum information processing. We show how a suitable utilization of three-level superconducting qubits inside a resonator constitutes a key tool to perform diverse two-qubit resonant gates, improving the operation speed when compared to slower dispersive techniques. To illustrate the benefit of resonant two-qubit gates in circuit quantum electrodynamics, we consider the implementation of a two-dimensional cluster state in an array of N x N superconducting qubits by using resonant controlled-phase and one-qubit gates, where the generation time grows linearly with N. For N = 3, and taking into account decoherence mechanisms, a fidelity over 60% for the generation of this cluster state is obtained.

Introduction to quantum noise, measurement, and amplification

A. A. Clerk, M. H. Devoret, S. M. Girvin, Florian Marquardt, R. J. Schoelkopf

The topic of quantum noise has become extremely timely due to the rise of quantum information physics and the resulting interchange of ideas between the condensed matter and atomic, molecular, optical-quantum optics communities. This review gives a pedagogical introduction to the physics of quantum noise and its connections to quantum measurement and quantum amplification. After introducing quantum noise spectra and methods for their detection, the basics of weak continuous measurements are described. Particular attention is given to the treatment of the standard quantum limit on linear amplifiers and position detectors within a general linear-response framework. This approach is shown how it relates to the standard Haus-Caves quantum limit for a bosonic amplifier known in quantum optics and its application to the case of electrical circuits is illustrated, including mesoscopic detectors and resonant cavity detectors.

ac conductance through an interacting quantum dot

Bjoern Kubala, Florian Marquardt

We investigate the linear ac conductance for tunneling through an arbitrary interacting quantum dot in the presence of a finite dc bias. In analogy to the well-known Meir-Wingreen formula for the dc case, we are able to derive a general formula for the ac conductance. It can be expressed entirely in terms of local correlations on the quantum dot in the form of a Keldysh block diagram with four external legs. We illustrate the use of this formula as a starting point for diagrammatic calculations by considering the ac conductance of the noninteracting resonant-level model and deriving the result for the lowest order of electron-phonon coupling. We show how known results are recovered in the appropriate limits.

Single-atom cavity QED and optomicromechanics

M. Wallquist, K. Hammerer, P. Zoller, C. Genes, M. Ludwig, F. Marquardt, P. Treutlein, J. Ye, H. J. Kimble

In a recent publication [ K. Hammerer, M. Wallquist, C. Genes, M. Ludwig, F. Marquardt, P. Treutlein, P. Zoller, J. Ye, and H. J. Kimble, Phys. Rev. Lett. 103, 063005 ( 2009)] we have shown the possibility to achieve strong coupling of the quantized motion of a micron-sized mechanical system to the motion of a single trapped atom. In the proposed setup the coherent coupling between a SiN membrane and a single atom is mediated by the field of a high finesse cavity and can be much larger than the relevant decoherence rates. This makes the well-developed tools of cavity quantum electrodynamics with single atoms available in the realm of cavity optomechanics. In this article we elaborate on this scheme and provide detailed derivations and technical comments. Moreover, we give numerical as well as analytical results for a number of possible applications for transfer of squeezed or Fock states from atom to membrane as well as entanglement generation, taking full account of dissipation. In the limit of strong-coupling the preparation and verification of nonclassical states of a mesoscopic mechanical system is within reach.

Optimal control of circuit quantum electrodynamics in one and two dimensions

R. Fisher, F. Helmer, S. J. Glaser, F. Marquardt, T. Schulte-Herbrueggen

Optimal control can be used to significantly improve multi-qubit gates in quantum information processing hardware architectures based on superconducting circuit quantum electrodynamics. We apply this approach not only to dispersive gates of two qubits inside a cavity, but, more generally, to architectures based on two-dimensional (2D) arrays of cavities and qubits. For high-fidelity gate operations, simultaneous evolutions of controls and couplings in the two coupling dimensions of cavity grids are shown to be significantly faster than conventional sequential implementations. Even under experimentally realistic conditions speedups by a factor of three can be gained. The methods immediately scale to large grids and indirect gates between arbitrary pairs of qubits on the grid. They are anticipated to be paradigmatic for 2D arrays and lattices of controllable qubits.

Accessing Higher Order Correlations in Quantum Optical States by Time Multiplexing

M. Avenhaus, K. Laiho, M. V. Chekhova, C. Silberhorn

We experimentally measured higher order normalized correlation functions (NCF) of pulsed light with a time-multiplexing detector. We demonstrate excellent performance of our device by verifying unity valued NCF up to the eighth order for coherent light and factorial dependence of the NCF for pseudothermal light. We applied our measurement technique to a type-II parametric down-conversion source to investigate mutual two-mode correlation properties and ascertain nonclassicality.

Emergence of Geometrical Optical Nonlinearities in Photonic Crystal Fiber Nanowires

Fabio Biancalana, Truong X. Tran, Sebastian Stark, Markus A. Schmidt, Philip St J. Russell

We demonstrate analytically and numerically that a subwavelength-core dielectric photonic nanowire embedded in a properly designed photonic crystal fiber cladding shows evidence of a previously unknown kind of nonlinearity (the magnitude of which is strongly dependent on the waveguide parameters) which acts on solitons so as to considerably reduce their Raman self-frequency shift. An explanation of the phenomenon in terms of indirect pulse negative chirping and broadening is given by using the moment method. Our conclusions are supported by detailed numerical simulations.

Prospect for detecting squeezed states of light created by a single atom in free space

Magdalena Stobinska, Markus Sondermann, Gerd Leuchs

We discuss the possibilities of studying in detail the dynamics of spontaneous emission of a single photon by a single atom and measuring the transient degree of squeezing by means of full solid angle fluorescence detection. (C) 2009 Elsevier B.V. All rights reserved.

Widely tunable optical delay generator

K. Jamshidi, A. Wiatrek, C. Bersch, G. Onishchukov, G. Leuchs, T. Schneider

We propose and demonstrate a method for quasi storage of light based on periodic spectral filtering realized in the time domain by amplitude modulation using frequency-to-time conversion. The delay can be tuned in a wide range by changing the frequency of an electrical modulation signal. In our experiments, the delay of single 2.5 ps pulses varied by 66 pulse widths. The technique works equally well for more complex optical data packets. Contrary to known approaches, the method has a very large spectral bandwidth and can be implemented by either fiber or integrated solutions using existing technologies. Because of the large bandwidth, fractional delays up to several tens of thousands of pulse widths can be achieved potentially for subpicosecond pulses, which is a tremendous value regarding the implementation simplicity. (C) 2010 Optical Society of America

Bio-sensing using recessed gold-filled capillary amperometric electrodes

A. Kacanovska, Z. Rong, M. Schmidt, P. St. J. Russell, P. Vadgama

A novel recessed electrode is reported for amperometric detection of hydrogen peroxide and via glucose oxidase for the detection of glucose. The electrode utilised electrodeposited platinum over a gold wire surface, which proved to be an effective peroxide-detecting surface. Compared with a traditional exposed electrode surface, the recessed tip facilitated an extended linear range for glucose from 4 to over 14 mM. Bio-fouling, as assessed by exposure to bovine serum albumin, was also significantly reduced. Though response time at the recess was increased, it was within an acceptable range for physiological monitoring. Moreover, the recess enabled precise measurement of the hydrogen peroxide diffusion coefficient; this was based on a bipartite expression for the transient amperometric current at the recessed structure following a step change in ambient hydrogen peroxide concentration. An important aspect of the diffusion measurement was the curve fitting routine used to map on to the theoretical response curve.

Experimental demonstration of squeezed-state quantum averaging

Mikael Lassen, Lars Skovgaard Madsen, Metin Sabuncu, Radim Filip, Ulrik L. Andersen

We propose and experimentally demonstrate a universal quantum averaging process implementing the harmonic mean of quadrature variances. The averaged variances are prepared probabilistically by means of linear optical interference and measurement-induced conditioning. We verify that the implemented harmonic mean yields a lower value than the corresponding value obtained for the standard arithmetic-mean strategy. The effect of quantum averaging is experimentally tested for squeezed and thermal states as well as for uncorrelated and partially correlated noise sources. The harmonic-mean protocol can be used to efficiently stabilize a set of squeezed-light sources with statistically fluctuating noise levels.

Noise-powered probabilistic concentration of phase information

Mario A. Usuga, Christian R. Mueller, Christoffer Wittmann, Petr Marek, Radim Filip, Christoph Marquardt, Gerd Leuchs, Ulrik L. Andersen

Phase-insensitive optical amplification of an unknown quantum state is known to be a fundamentally noisy operation that inevitably adds noise to the amplified state(1-5). However, this fundamental noise penalty in amplification can be circumvented by resorting to a probabilistic scheme as recently proposed and demonstrated in refs 6-8. These amplifiers are based on highly non-classical resources in a complex interferometer. Here we demonstrate a probabilistic quantum amplifier beating the fundamental quantum limit using a thermal-noise source and a photon-number-subtraction scheme(9). The experiment shows, surprisingly, that the addition of incoherent noise leads to a noiselessly amplified output state with a phase uncertainty below the uncertainty of the state before amplification. This amplifier might become a valuable quantum tool in future quantum metrological schemes and quantum communication protocols.

Quantum optical coherence can survive photon losses using a continuous-variable quantum erasure-correcting code

Mikael Lassen, Metin Sabuncu, Alexander Huck, Julien Niset, Gerd Leuchs, Nicolas J. Cerf, Ulrik L. Andersen

A fundamental requirement for enabling fault-tolerant quantum information processing is an efficient quantum error-correcting code that robustly protects the involved fragile quantum states from their environment(1-10). Just as classical error-correcting codes are indispensible in today's information technologies, it is believed that quantum error-correcting code will play a similarly crucial role in tomorrow's quantum information systems. Here, we report on the experimental demonstration of a quantum erasure-correcting code that overcomes the devastating effect of photon losses. Our quantum code is based on linear optics, and it protects a four-mode entangled mesoscopic state of light against erasures. We investigate two approaches for circumventing in-line losses, and demonstrate that both approaches exhibit transmission fidelities beyond what is possible by classical means. Because in-line attenuation is generally the strongest limitation to quantum communication, such an erasure-correcting code provides a new tool for establishing quantum optical coherence over longer distances.

Thermal blinding of gated detectors in quantum cryptography

Lars Lydersen, Carlos Wiechers, Christoffer Wittmann, Dominique Elser, Johannes Skaar, Vadim Makarov

It has previously been shown that the gated detectors of two commercially available quantum key distribution (QKD) systems are blindable and controllable by an eavesdropper using continuous-wave illumination and short bright trigger pulses, manipulating voltages in the circuit [Nat. Photonics 4, 686 (2010)]. This allows for an attack eavesdropping the full raw and secret key without increasing the quantum bit error rate (QBER). Here we show how thermal effects in detectors under bright illumination can lead to the same outcome. We demonstrate that the detectors in a commercial QKD system Clavis2 can be blinded by heating the avalanche photo diodes (APDs) using bright illumination, so-called thermal blinding. Further, the detectors can be triggered using short bright pulses once they are blind. For systems with pauses between packet transmission such as the plug-and-play systems, thermal inertia enables Eve to apply the bright blinding illumination before eavesdropping, making her more difficult to catch. (C) 2010 Optical Society of America

Influence of Surface Tension and Inner Pressure on the Process of Fibre Drawing

Giovanni Luzi, Philipp Epple, Michael Scharrer, Ken Fujimoto, Cornelia Rauh, Antonio Delgado

The present contribution deals with thermofluidynamical features occurring during the drawing of capillaries for microstructured optical fibres. Here, the process stability depends strongly on flow and thermal processes taking place as a preform is heated and drawn in the furnace. This is the case particularly for hollow fibres for which the existence of the inner hole directly depends on material parameters such as the surface tension and the rheological properties and on process parameter such as hole internal pressure and the process temperature. A fluid-mechanics model suggested in the literature [8] that makes use of asymptotic analysis based on small aspect ratio of the micro capillaries, has been revisited and improved recently and the leading-order equations have been then examined in some asymptotic limits by Luzi et al. [7]. Starting from the novel class of solutions of the simplified equations of motion the present paper focuses on the effect of both surface tension and internal hole pressure since those are of essential importance during drawing. Thus, comparisons with experimental data are performed, in order to validate the analytical model developed in [7], which will be briefly presented here. The theoretical model gives very accurate predictions both when the internal hole is pressurized or when no pressure is applied, as long as the temperature does not reach too high values.

Coherent state quantum key distribution with multi letter phase-shift keying

Denis Sych, Gerd Leuchs

We present a protocol for quantum key distribution using discrete modulation of coherent states of light. Information is encoded in the variable phase of coherent states which can be chosen from a regular discrete set ranging from binary to continuous modulation, similar to phase-shift keying in classical communication. Information is decoded by simultaneous homodyne measurement of both quadratures and requires no active choice of basis. The protocol utilizes either direct or reverse reconciliation, both with and without postselection. We analyze the security of the protocol and show how to enhance it by the optimal choice of all variable parameters of the quantum signal.

High index-contrast all-solid photonic crystal fibers by pressure-assisted melt infiltration of silica matrices

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

All-solid photonic crystal fibers (PCFs) are created by pressure-assisted filling of low-melting-point chalcogenide and tellurite glasses into silica matrix fibers with channel diameters as small as 200 nm. Overcoming to a large extent the problem of viscosity and, thus, process incompatibility of silica and non-silicate optical glasses, the technique provides a unique way of producing waveguiding devices with high core-cladding index-contrast, high optical non-linearity and a transmission range that extends into the mid infrared. In this paper, as a prerequisite for waveguide production, the rheologic properties and controlled flow of highly-viscous liquids under geometrically confined conditions are considered, and deviations from Newtonian behavior are discussed. Because the filling process requires only very small quantities of filling material that do not come into contact with the environment, and because ultra-high cooling rates can be achieved, the technique enables the use of difficult-to-handle or reactive optical glasses. (C) 2010 Elsevier B.V. All rights reserved.

Continuous-variable entanglement distillation of non-Gaussian mixed states

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

Many different quantum-information communication protocols such as teleportation, dense coding, and entanglement-based quantum key distribution are based on the faithful transmission of entanglement between distant location in an optical network. The distribution of entanglement in such a network is, however, hampered by loss and noise that is inherent in all practical quantum channels. Thus, to enable faithful transmission one must resort to the protocol of entanglement distillation. In this paper we present a detailed theoretical analysis and an experimental realization of continuous variable entanglement distillation in a channel that is inflicted by different kinds of non-Gaussian noise. The continuous variable entangled states are generated by exploiting the third order nonlinearity in optical fibers, and the states are sent through a free-space laboratory channel in which the losses are altered to simulate a free-space atmospheric channel with varying losses. We use linear optical components, homodyne measurements, and classical communication to distill the entanglement, and we find that by using this method the entanglement can be probabilistically increased for some specific non-Gaussian noise channels.

Comparison of three digital fringe signal processing methods in a ballistic free-fall absolute gravimeter

S. Svitlov, P. Maslyk, Ch Rothleitner, H. Hu, L. J. Wang

This paper reports results of comparison of three digital fringe signal processing methods implemented in the same free-fall absolute gravimeter. A two-sample zero-crossing method, a windowed second-difference method and a method of non-linear least-squares adjustment on the undersampled fringe signal are compared in numerical simulations, hardware tests and actual measurements with the MPG-2 absolute gravimeter, developed at the Max Planck Institute for the Science of Light, Germany. The two-sample zero-crossing method realizes data location schemes that are both equally spaced in distance and equally spaced in time (EST) along the free-fall trajectory. The windowed second-difference method and the method of non-linear least-squares adjustment with complex heterodyne demodulation operate with the EST data. Results of the comparison verify an agreement of the three methods within one part in 10(9) of the measured gravity value, provided a common data location scheme is considered.

Optical Properties of Individual Silicon Nanowires for Photonic Devices

Gerald Broenstrup, Norbert Jahr, Christian Leiterer, Andrea Csaki, Wolfgang Fritzsche, Silke Christiansen

Silicon is a high refractive index material. Consequently, silicon nanowires (SiNWs) with diameters on the order of the wavelengths of visible light show strong resonant field enhancement of the incident light, so this type of nanomaterial is a good candidate for all kinds of photonic devices. Surprisingly enough, a thorough experimental and theoretical analysis of both the polarization dependence of the absorption and the scattering behavior of individual SiNWs under defined illumination has not been presented yet. Here, the present paper will contribute by showing optical properties such as scattering and absorption of individual SiNWs experimentally in an optical microscope using bright- and dark-field illumination modes as well as in analytical Mie calculations. Experimental and calculation results are in good agreement, and both reveal a strong correlation of the optical properties of individual SINWs to their diameters. This finding supports the notion that SiNWs can be used in photonic applications such as for photovoltaics or optical sensors.

Robustness of bipartite Gaussian entangled beams propagating in lossy channels

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

Subtle quantum properties offer exciting new prospects in optical communications. For example, quantum entanglement enables the secure exchange of cryptographic keys(1) and the distribution of quantum information by teleportation(2,3). Entangled bright beams of light are increasingly appealing for such tasks, because they enable the use of well-established classical communications techniques(4). However, quantum resources are fragile and are subject to decoherence by interaction with the environment. The unavoidable losses in the communication channel can lead to a complete destruction of entanglement(5-8), limiting the application of these states to quantum-communication protocols. We investigate the conditions under which this phenomenon takes place for the simplest case of two light beams, and analyse characteristics of states which are robust against losses. Our study sheds new light on the intriguing properties of quantum entanglement and how they may be harnessed for future applications.

Understanding Raman-shifting multipeak states in photonic crystal fibers: two convergent approaches

Alexander Hause, Truong X. Tran, Fabio Biancalana, Alexander Podlipensky, Philip St J. Russell, Fedor Mitschke

In this Letter we give theoretical explanations for the recent observations of the excitation of Raman-shifting pulse pairs in solid-core photonic crystal fibers. The formation of these pairs is surprisingly common in the deep anomalous dispersion regime of a large variety of highly nonlinear optical fibers, away from zero group-velocity dispersion points. We have developed two different theoretical models, which agree very well in their conclusions. A qualitative and a quantitative explanation of pair formation is provided, and the existence of multipeak states is predicted. (C) 2010 Optical Society of America

The quantum vacuum at the foundations of classical electrodynamics

G. Leuchs, A. S. Villar, L. L. Sanchez-Soto

In the classical theory of electromagnetism, the permittivity epsilon (0) and the permeability mu (0) of free space are constants whose magnitudes do not seem to possess any deeper physical meaning. By replacing the free space of classical physics with the quantum notion of the vacuum, we speculate that the values of the aforementioned constants could arise from the polarization and magnetization of virtual pairs in vacuum. A classical dispersion model with parameters determined by quantum and particle physics is employed to estimate their values. We find the correct orders of magnitude. Additionally, our simple assumptions yield an independent estimate for the number of charged elementary particles based on the known values of epsilon (0) and mu (0) and for the volume of a virtual pair. Such an interpretation would provide an intriguing connection between the celebrated theory of classical electromagnetism and the quantum theory in the weak-field limit.

Witnessing effective entanglement over a 2km fiber channel

Christoffer Wittmann, Josef Fuerst, Carlos Wiechers, Dominique Elser, Hauke Haeseler, Norbert Luetkenhaus, Gerd Leuchs

We present a fiber-based continuous-variable quantum key distribution system. In the scheme, a quantum signal of two non-orthogonal weak optical coherent states is sent through a fiber-based quantum channel. The receiver simultaneously measures conjugate quadratures of the light using two homodyne detectors. From the measured Q-function of the transmitted signal, we estimate the attenuation and the excess noise caused by the channel. The estimated excess noise originating from the channel and the channel attenuation including the quantum efficiency of the detection setup is investigated with respect to the detection of effective entanglement. The local oscillator is considered in the verification. We witness effective entanglement with a channel length of up to 2km. (C) 2010 Optical Society of America

Imaging Pharmaceutical Tablets with Optical Coherence Tomography

Jakob M. A. Mauritz, Richard S. Morrisby, Roger S. Hutton, Coulton H. Legge, Clemens F. Kaminski

Optical coherence tomography (OCT) is a recently developed optical technique that produces depth profiles of three-dimensional objects. It is a nondestructive interferometric method responding to refractive index variation in the sample under study and can. reach a penetration depth of a few millimetres. OCT employs near-infrared (NIR) light and therefore provides a link between NIR spectroscopy and Terahertz (THz) measurements that are often used to characterise tablets. In this article we assess the potential of OCT as a reliable and practical tool in the analysis of pharmaceutical tablets and coatings. A variety of tablets were tested with different shapes, formulations and coatings. We consider the origins of contrast in the obtained images and demonstrate that it correlates strongly with the expected tablet structure. The influence of absorption and scattering are considered for the wavelength ranges used. The results show that OCT is a promising diagnostic tool with an important role to play in the tablet and coating technologies. The high measurement speed of OCT and its relative ease of implementation make it also an attractive candidate technology for in-line quality control during manufacturing. (C) 2009 Wiley-Liss, Inc. and the American Pharmacists Association J Pharm Sci 99:385-391, 2010

Free-Space Squeezing Assists Perfectly Matched Layers in Simulations on a Tight Domain

Dzmitry M. Shyroki, Aliaksandra M. Ivinskaya, Andrei V. Lavrinenko

To minimize computer memory consumption in the finite-difference modeling, one tends to place computational domain boundaries as close to the simulated object as possible. Unfortunately, this leads to inaccurate solution in the case when evanescent electromagnetic field is expected to spread far outside the object, as in simulations of eigenmodes or scattering at a wavelength comparable to or larger than the object itself. Here, we show how, in addition to applying the perfectly matched layers (PMLs), outer free space can be squeezed to avoid cutting the evanescent field tails by the PMLs or computational domain borders. Adding the squeeze-transform layers to the standard PMLs requires no changes to the finite-difference algorithms.

Naturally Phase-Matched Second-Harmonic Generation in a Whispering-Gallery-Mode Resonator

J. U. Fuerst, D. V. Strekalov, D. Elser, M. Lassen, U. L. Andersen, C. Marquardt, G. Leuchs

We demonstrate for the first time natural phase matching for optical frequency doubling in a high-Q whispering-gallery-mode resonator made of lithium niobate. A conversion efficiency of 9% is achieved at 30 mu W in-coupled continuous wave pump power. The observed saturation pump power of 3.2 mW is almost 2 orders of magnitude lower than the state-of-the-art value. This suggests an application of our frequency doubler as a source of nonclassical light requiring only a low-power pump, which easily can be quantum noise limited. Our theoretical analysis of the three-wave mixing in a whispering-gallery-mode resonator provides the relative conversion efficiencies for frequency doubling in various modes.

Bistability and stationary gap solitons in quasiperiodic photonic crystals based on Thue-Morse sequence

V. V. Grigoriev, F. Biancalana

The nonlinear properties of quasiperiodic photonic crystals based on the Thue-Morse sequence are investigated. The intrinsic asymmetry of these one-dimensional 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. The efficiency of two schemes is compared: passive and active when an additional short pump signal is applied to the system. The existence of stationary gap solitons in quasiperiodic photonic crystals is shown numerically, and their difference from the Bragg case is emphasized. (C) 2010 Elsevier B.V. All rights reserved.

Measurement of group-velocity dispersion of Bloch modes in photonic-crystal-fiber rocking filters

G. K. L. Wong, L. Zang, M. S. Kang, P. St. J. Russell

We use low-coherence interferometry to measure the group-velocity dispersion (GVD) of the fast and slow Bloch modes of structural rocking filters, produced by twisting a highly birefringent photonic crystal fiber to and fro while scanning a focused CO(2) laser beam along it. The GVD curves in the vicinity of the resonant wavelength differ dramatically from those of the unperturbed fiber, suggesting that rocking filters could be used in the optimization of, e.g., four-wave mixing and supercontinuum generation. Excellent agreement is obtained between theory and experiment. (C) 2010 Optical Society of America

Angular momenta and spin-orbit interaction of nonparaxial light in free space

Konstantin Y. Bliokh, Miguel A. Alonso, Elena A. Ostrovskaya, Andrea Aiello

We give an exact self-consistent operator description of the spin and orbital angular momenta, position, and spin-orbit interactions of nonparaxial light in free space. Both quantum-operator formalism and classical energy-flow approach are presented. We apply the general theory to symmetric and asymmetric Bessel beams exhibiting spin-and orbital-dependent intensity profiles. The exact wave solutions are clearly interpreted in terms of the Berry phases, quantization of caustics, and Hall effects of light, which can be readily observed experimentally.

Interacting waves on chains of split-ring resonators in the presence of retardation

V. Lomanets, O. Zhuromskyy, G. Onishchukov, O. Sydoruk, E. Tatartschuk, E. Shamonina, G. Leuchs, U. Peschel

Wave propagation is studied experimentally in a one-dimensional periodic chain of magnetically coupled split-ring resonators with a spacing of about one tenth of the resonant wavelength. Retardation leads to a strong interaction between magnetoinductive and free-space waves. Two kinds of guided modes are observed: a slow backward wave which propagates far outside the light cone, and a fast forward wave close to the light cone. The two merge in a region of zero group velocity. The results are relevant for all one- and two-dimensional periodic systems interacting with waves of the surrounding space. (C) 2010 American Institute of Physics. [doi: 10.1063/1.3462314]

Continuous-variable quantum information processing

Ulrik L. Andersen, Gerd Leuchs, Christine Silberhorn

Observables of quantum systems can possess either a discrete or a continuous spectrum. For example, upon measurements of the photon number of a light state, discrete outcomes will result whereas measurements of the light's quadrature amplitudes result in continuous outcomes. If one uses the continuous degree of freedom of a quantum system for encoding, processing or detecting information, one enters the field of continuous-variable (CV) quantum information processing. In this paper we review the basic principles of CV quantum information processing with main focus on recent developments in the field. We will be addressing the three main stages of a quantum information system; the preparation stage where quantum information is encoded into CVs of coherent states and single-photon states, the processing stage where CV information is manipulated to carry out a specified protocol and a detection stage where CV information is measured using homodyne detection or photon counting. [GRAPHICS] (C) 2010 by WILEY-VCH Verlag GmbH & Co. KGaA. Weinheim

Multiple hydrodynamical shocks induced by the Raman effect in photonic crystal fibers

C. Conti, S. Stark, P. St. J. Russell, F. Biancalana

We theoretically predict the occurrence of multiple hydrodynamical-like shock phenomena in the propagation of ultrashort intense pulses in a suitably engineered photonic crystal fiber. The shocks are due to the Raman effect, which acts as a nonlocal term favoring their generation in the focusing regime. It is shown that the problem is mapped to shock formation in the presence of a slope and a gravity-like potential. The signature of multiple shocks in cross-correlation frequency-resolved optical gating (XFROG) signals is unveiled.

A bridge between the single-photon and squeezed-vacuum states

Nitin Jain, S. R. Huisman, Erwan Bimbard, A. I. Lvovsky

The two modes of the Einstein-Podolsky-Rosen quadrature entangled state generated by parametric down-conversion interfere on a beam splitter of variable splitting ratio. Detection of a photon in one of the beam splitter output channels heralds preparation of a signal state in the other, which is characterized using homodyne tomography. By controlling the beam splitting ratio, the signal state can be chosen anywhere between the single-photon and squeezed state. (C) 2010 Optical Society of America

Demonstration of a quasi-scalar angular Goos-Hanchen effect

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

We show experimentally that the angular Goos-Hanchen (GH) effect can be easily observed, also without employing its resonant enhancement at Brewster incidence. An s-polarized beam was used to decouple the polarization from the propagation dynamics of the beam. We found that, in this case, the angular GH effect can be strongly enhanced by increasing the angular aperture of the Gaussian beam. Our experiments suggest a route toward observing the angular GH effect for true scalar waves, such as acoustic waves and quantum matter waves. (C) 2010 Optical Society of America

Hacking commercial quantum cryptography systems by tailored bright illumination

Lars Lydersen, Carlos Wiechers, Christoffer Wittmann, Dominique Elser, Johannes Skaar, Vadim Makarov

The peculiar properties of quantum mechanics allow two remote parties to communicate a private, secret key, which is protected from eavesdropping by the laws of physics(1-4). So-called quantum key distribution (QKD) implementations always rely on detectors to measure the relevant quantum property of single photons(5). Here we demonstrate experimentally that the detectors in two commercially available QKD systems can be fully remote-controlled using specially tailored bright illumination. This makes it possible to tracelessly acquire the full secret key; we propose an eavesdropping apparatus built from off-the-shelf components. The loophole is likely to be present in most QKD systems using avalanche photodiodes to detect single photons. We believe that our findings are crucial for strengthening the security of practical QKD, by identifying and patching technological deficiencies.

Electron Microscopy Mechanical Testing of Silicon Nanowires Using Electrostatically Actuated Tensile Stages

Dongfeng Zhang, Jean-Marc Breguet, Reymond Clavel, Vladimir Sivakov, Silke Christiansen, Johann Michler

Two types of electrostatically actuated tensile stages for in situ electron microscopy mechanical testing of 1-D nanostructures were designed, microfabricated, and tested. Testing was carried out for mechanical characterization of silicon nanowires (SiNWs). The bulk micromachined stages consist of a combdrive actuator and either a differential capacitive sensor or a clamped-clamped beam force sensor. High-aspect-ratio structures (height/gap = 20) were designed to increase the driving force of the geometrically optimized actuator and the sensitivity of the capacitive sensor. The actuator stiffness is kept low to enable high tensile force to be exerted in the specimen rather than in the suspensions of the comb drive. Individual SiNWs were mounted on the devices by in situ scanning electron microscopy nanomanipulation, and their tensile properties were determined to demonstrate the device capability. The phosphorus-doped SiNWs, which were grown in a bottom-up manner by the vapor-liquid-solid process, show an average Young's modulus of (170.0 +/- 2.4) GPa and a tensile strength of at least 4.2 GPa. Top-down electroless chemically etched SiNWs, with their long axis along the [100] direction, show a fracture strength of 5.4 GPa.

Demonstration of Coherent-State Discrimination Using a Displacement-Controlled Photon-Number-Resolving Detector

Christoffer Wittmann, Ulrik L. Andersen, Masahiro Takeoka, Denis Sych, Gerd Leuchs

We experimentally demonstrate a new measurement scheme for the discrimination of two coherent states. The measurement scheme is based on a displacement operation followed by a photon-number-resolving detector, and we show that it outperforms the standard homodyne detector which we, in addition, prove to be optimal within all Gaussian operations including conditional dynamics. We also show that the non-Gaussian detector is superior to the homodyne detector in a continuous variable quantum key distribution scheme.

Approaching the full octave: Noncollinear optical parametric chirped pulse amplification with two-color pumping

D. Herrmann, C. Homann, R. Tautz, M. Scharrer, P. St J. Russell, F. Krausz, L. Veisz, E. Riedle

We present a new method to broaden the amplification range in optical parametric amplification toward the bandwidth needed for single cycle femtosecond pulses. Two-color pumping of independent stages is used to sequentially amplify the long and short wavelength parts of the ultrabroadband seed pulses. The concept is tested in two related experiments. With multi-mJ pumping pulses with a nearly octave spanning spectrum and an uncompressed energy of 3 mJ are generated at low repetition rate. The spectral phase varies slowly and continuously in the overlap region as shown with 100 kHz repetition rate. This should allow the compression to the Fourier limit of below 5 fs in the high energy system. (C) 2010 Optical Society of America

Assessing the Polarization of a Quantum Field from Stokes Fluctuations

A. B. Klimov, G. Bjoerk, J. Soederholm, L. S. Madsen, M. Lassen, U. L. Andersen, J. Heersink, R. Dong, Ch. Marquardt, et al.

We propose an operational degree of polarization in terms of the variance of the Stokes vector minimized over all the directions of the Poincare sphere. We examine the properties of this second-order definition and carry out its experimental determination. Quantum states with the same standard (first-order) degree of polarization are correctly discriminated by this new measure. We argue that a comprehensive quantum characterization of polarization properties requires a whole hierarchy of higher-order degrees.

Ordered arrays of epitaxial silicon nanowires produced by nanosphere lithography and chemical vapor deposition

Damiana Lerose, Mikhael Bechelany, Laetitia Philippe, Johann Michler, Silke Christiansen

Gold dot arrays on (1 1 1) Si substrates obtained through nanosphere lithography (NSL) combined with sputtering and annealing in Ar at 1000 degrees C are used to catalyze vapor liquid solid (VLS) epitaxial growth of silicon nanowires (Si NWs) using chemical vapor deposition (CVD) with SiH(4) in Ar. The NWs grow primarily epitaxially on the underlying (1 1 1) Si wafer following the four independent < 1 1 1 > directions. The diameter distribution of the wires reflects the diameter distribution of the catalyst gold dot arrays and is therefore predictable. The wire length depends on the size of the gold catalyst for the same CVD parameters. The wire position is foreseeable within the limits of the pattern geometrical quality, but one-to-one growth of NWs to gold dots is not always observed, probably due to (very locally) the remaining presence of silicon oxide. Overall, this inexpensive patterning method for obtaining high-quality crystalline VLS Si NWs by CVD fulfills the requirements of many device applications, where patterning control, quality and reproducibility of the nanostructures are crucial. (C) 2010 Elsevier B.V. All rights reserved.

Measurement of nonlinear rheology of cross-linked biopolymer gels

Chase P. Broedersz, Karen E. Kasza, Louise M. Jawerth, Stefan Muenster, David A. Weitz, Frederick C. MacKintosh

One of the hallmarks of biopolymer gels is their nonlinear viscoelastic response to stress, making the measurement of the mechanics of these gels very challenging. Various rheological protocols have been proposed for this; however, a thorough understanding of the techniques and their range of applicability as well as a careful comparison between these methods are still lacking. Using both strain ramp and differential prestress protocols, we investigate the nonlinear response of a variety of systems ranging from extracellular fibrin gels to intracellular F-actin solutions and F-actin cross-linked with permanent and physiological transient linkers. We find that the prestress and strain ramp results agree well for permanently cross-linked networks over two decades of strain rates, while the protocols only agree at high strain rates for more transient networks. Surprisingly, the nonlinear response measured with the prestress protocol is insensitive to creep; although a large applied steady stress can lead to significant flow, this has no significant effect on either the linear or nonlinear response of the system. A simple model is presented to provide insight into these observations.

Direct Observation of Self-Similarity in Evolution of Transient Stimulated Raman Scattering in Gas-Filled Photonic Crystal Fibers

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

A unique characteristic of transient stimulated Raman scattering, in which the spatiotemporal evolution of the fields and the molecular excitation follow a universal self-similarity law, is observed in gas-filled photonic crystal fibers. As the input laser power is increased, the coupled system "optical fields + molecular excitation" goes through the same phases of time evolution but at a higher rate. Using the self-similarity law we are able to completely reconstruct the evolution of the pump and Stokes fields from one measurement.

Two-color bright squeezed vacuum

Ivan N. Agafonov, Maria V. Chekhova, Gerd Leuchs

In a strongly pumped nondegenerate traveling-wave optical parametric amplifier, we produce a two-color squeezed vacuum with up to millions of photons per pulse. Our approach to registering this macroscopic quantum state is direct detection of a large number of transverse and longitudinal modes, which is achieved by making the detection time and area much larger than the coherence time and area, respectively. Using this approach, we obtain a record value of twin-beam squeezing for direct detection of bright squeezed vacuum. This makes direct detection of macroscopic squeezed vacuum a practical tool for quantum information applications.

Silver Coated Platinum Core-Shell Nanostructures on Etched Si Nanowires: Atomic Layer Deposition (ALD) Processing and Application in SERS

Vladimir A. Sivakov, Katja Hoeflich, Michael Becker, Andreas Berger, Thomas Stelzner, Kai-Erik Elers, Viljami Pore, Mikko Ritala, Silke H. Christiansen

A new method to prepare plasmonically active noble metal nanostructures on large surface area silicon nanowires (SiNWs) mediated by atomic layer deposition (ALD) technology has successfully been demonstrated for applications of surface-enhanced Raman spectroscopy (SERS)-based sensing. As host material for the plasmonically active nanostructures we use dense single-crystalline SiNWs with diameters of less than 100 nm as obtained by a wet chemical etching method based on silver nitrate and hydrofluoric acid solutions. The SERS active metal nanoparticles/islands are made from silver (Ag) shells as deposited by autometallography on the core nanoislands made from platinum (Pt) that can easily be deposited by ALD in the form of nanoislands covering the SiNW surfaces in a controlled way. The density of the plasmonically inactive Pt islands as well as the thickness of noble metal Ag shell are two key factors determining the magnitude of the SERS signal enhancement and sensitivity of detection. The optimized Ag coated Pt islands on SiNWs exhibit great potential for ultrasensitive molecular sensing in terms of high SERS signal enhancement ability, good stability and reproducibility. The plasmonic activity of the core-shell Pt//Ag system that will be experimentally realized in this paper as an example was demonstrated in numerical finite element simulations as well as experimentally in Raman measurements of SERS activity of a highly diluted model dye molecule. The morphology and structure of the core-shell Pt//Ag nanoparticles on SiNW surfaces were investigated by scanning- and transmission electron microscopy. Optimized core-shell nanoparticle geometries for maximum Raman signal enhancement is discussed essentially based on the finite element modeling.

Catalytic and chaperone-like functions in an intrinsically disordered protein associated with desiccation tolerance

Sohini Chakrabortee, Filip Meersman, Gabriele S. Kaminski Schierle, Carlos W. Bertoncini, Brian McGee, Clemens F. Kaminski, Alan Tunnacliffe

Intrinsically disordered proteins (IDPs) lack well-defined structure but are widely represented in eukaryotic proteomes. Although the functions of most IDPs are not understood, some have been shown to have molecular recognition and/or regulatory roles where their disordered nature might be advantageous. Anhydrin is an uncharacterized IDP induced by dehydration in an anhydrobiotic nematode, Aphelenchus avenae. We show here that anhydrin is a moonlighting protein with two novel, independent functions relating to desiccation tolerance. First, it has a chaperone-like activity that can reduce desiccation-induced enzyme aggregation and inactivation in vitro. When expressed in a human cell line, anhydrin localizes to the nucleus and reduces the propensity of a polyalanine expansion protein associated with oculopharyngeal muscular dystrophy to form aggregates. This in vivo activity is distinguished by a loose association of anhydrin with its client protein, consistent with a role as a molecular shield. In addition, anhydrin exhibits a second function as an endonuclease whose substrates include supercoiled, linear, and chromatin linker DNA. This nuclease activity could be involved in either repair of desiccation-induced DNA damage incurred during anhydrobiosis or in apoptotic or necrotic processes, for example, but it is particularly unexpected for anhydrin because IDP functions defined to date anticorrelate with enzyme activity. Enzymes usually require precise three-dimensional positioning of residues at the active site, but our results suggest this need not be the case. Anhydrin therefore extends the range of IDP functional categories to include catalysis and highlights the potential for the discovery of new functions in disordered proteomes.

On the Analogy between a Single Atom and an Optical Resonator

S. Heugel, A. S. Villar, M. Sondermann, U. Peschel, G. Leuchs

A single atom in free space can have a strong influence on a light beam and a single photon can have a strong effect on a single atom in free space. Regarding this interaction, two conceptually different questions can be asked: can a single atom fully absorb a single photon and can a single atom fully reflect a light beam. The conditions for achieving the full effect in either case are different. Here we discuss related questions in the context of an optical resonator. When shaping a laser pulse properly it will be fully absorbed by an optical resonator, i.e., no light will be reflected and all the pulse energy will accumulate inside the resonator before it starts leaking out. We show in detail that in this case the temporal pulse shape has to match the time-reversed pulse obtained by the cavity's free decay. On the other hand a resonator, made of highly reflecting mirrors which normally reflect a large portion of any incident light, may fully transmit the light, as long as the light is narrow band and resonant with the cavity. The analogy is the single atom-normally letting most of the light pass-which under special conditions may fully reflect the incident light beam. Using this analogy we are able to study the effects of practical experimental limitations in the atom-photon coupling, such as finite pulses, bandwidths, and solid angle coverage, and to use the optical resonator as a test bed for the implementation of the quantum experiment.

Distillation of mixed-state continuous-variable entanglement by photon subtraction

Sheng Li Zhang, Peter van Loock

We present a detailed theoretical analysis for the distillation of one copy of a mixed two-mode continuous-variable entangled state using beam splitters and coherent photon-detection techniques, including conventional on-off detectors and photon-number-resolving detectors. The initial Gaussian mixed-entangled states are generated by transmitting a two-mode squeezed state through a lossy bosonic channel, corresponding to the primary source of errors in current approaches to optical quantum communication. We provide explicit formulas to calculate the entanglement in terms of logarithmic negativity before and after distillation, including losses in the channel and the photon detection, and show that one-copy distillation is still possible even for losses near the typical fiber channel attenuation length. A lower bound for the transmission coefficient of the photon-subtraction beam splitter is derived, representing the minimal value that still allows to enhance the entanglement.

Quantum key distribution with multi letter alphabets

D. Sych, G. Leuchs

We present a new protocol for continuous variable quantum key distribution (CV QKD). 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 QKD protocol, especially in the case when losses in the communication channel are low.

Direct measurement of micromotion speed in a linear quadrupole trap

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

We demonstrate a simple method to directly measure the micromotion speed and amplitude of ions far away from the nodal line of the linear quadrupole trap using the cross-correlation technique. For the ions very close to the trap nodal line, the micromotion speed and amplitude of ions can also be deduced through linear fitting. This work gives us a direct picture to the ions' micromotion modes at different displacements in the linear trap. With this work, an absolute measurement of the second-order Doppler effect in the research of atomic clocks based on large number of ions becomes possible. (C) 2010 American Institute of Physics. [doi: 10.1063/1.3457904]

Entanglement properties of optical coherent states under amplitude damping

Ricardo Wickert, Nadja Kolb Bernardes, Peter van Loock

Through concurrence, we characterize the entanglement properties of optical coherent-state qubits subject to an amplitude damping channel. We investigate the distillation capabilities of known error-correcting codes and obtain upper bounds on the entanglement depending on the nonorthogonality of the coherent states and the channel damping parameter. This work provides a full quantitative analysis of these photon-loss codes which are naturally reminiscent of the standard qubit codes against Pauli errors.

Phase-shifting point-diffraction interferometry with common-path and in-line configuration for microscopy

Peng Gao, Irina Harder, Vanusch Nercissian, Klaus Mantel, Baoli Yao

A new common-path and in-line point-diffraction interferometer for quantitative phase microscopy is proposed. The interferometer is constructed by introducing a grating pair into the point-diffraction interferometer, thus forming a common-path and in-line configuration for object and reference waves. Achromatic phase shifting is implemented by linearly moving one of the two gratings in its grating vector direction. The feasibility of the proposed configuration is demonstrated by theoretical analysis and experiments. (C) 2010 Optical Society of America

Stress and doping uniformity of laser crystallized amorphous silicon in thin film silicon solar cells

R. M. B. Agaiby, M. Becker, S. B. Thapa, U. Urmoneit, A. Berger, A. Gawlik, G. Sarau, S. H. Christiansen

Simultaneous and locally resolved determination of the mechanical stress variation and the free hole concentration using Raman spectroscopy is demonstrated in laser crystallized amorphous silicon layers. Such layers are often used for the fabrication of thin film solar cells, e. g., on borosilicate glass substrates. The combined effects of stress and doping on the Raman signal can be separated based on the use of three wavelengths in the visible. The results show that the free hole concentration in the samples investigated varies between 1 x 10(18) and 1.3 x 10(19) cm(-3). Stress as well as the free hole concentration vary substantially within the sample. The stress level varies between 575 and 850 MPa (+/- 12 MPa). Cross-sectional transmission electron microscopy images show the presence of extended lattice defects such as dislocations and grain boundaries in the crystallized Si layer which could account for the lateral stress variations detected by Raman spectroscopy. The impact of film inhomogeneity in terms of stress and doping on the performance of a solar cell will be discussed. (C) 2010 American Institute of Physics. [doi: 10.1063/1.3319654]

Entanglement verification with realistic measurement devices via squashing operations

Tobias Moroder, Otfried Guehne, Normand Beaudry, Marco Piani, Norbert Luetkenhaus

Many protocols and experiments in quantum information science are described in terms of simple measurements on qubits. However in a real implementation the exact description is more difficult and more complicated observables are used. The question arises whether a claim of entanglement in the simplified description still holds, if the difference between the realistic and simplified models is taken into account. We show that a positive entanglement statement remains valid if a certain positive linear map connecting the two descriptions-a so-called squashing operation-exists; then lower bounds on the amount of entanglement are also possible. We apply our results to polarization measurements of photons using only threshold detectors, and derive procedures under which multiphoton events can be neglected.

Accessing the purity of a single photon by the width of the Hong-Ou-Mandel interference

Katiuscia N. Cassemiro, Kaisa Laiho, Christine Silberhorn

We demonstrate a method for determining the spectral purity of single photons. The technique is based on the Hong-Ou-Mandel ( HOM) interference between a single-photon state and a suitably prepared coherent field. We show that the temporal width of the HOM dip is related not only to the reciprocal of the spectral width but also to the underlying quantum coherence. Therefore, by measuring the width of both the HOM dip and the spectrum, one can directly quantify the degree of spectral purity. The distinct advantage of our proposal is that it obviates the need for perfect mode matching, since it does not rely on the visibility of the interference. Our method is particularly useful for characterizing the purity of heralded single-photon states.

Entanglement of mechanical oscillators coupled to a nonequilibrium environment

Max Ludwig, K. Hammerer, Florian Marquardt

Recent experiments aim at cooling nanomechanical resonators to the ground state by coupling them to nonequilibrium environments in order to observe quantum effects such as entanglement. This raises the general question of how such environments affect entanglement. Here we show that there is an optimal dissipation strength for which the entanglement between two coupled oscillators is maximized. Our results are established with the help of a general framework of exact quantum Langevin equations valid for arbitrary bath spectra, in and out of equilibrium. We point out why the commonly employed Lindblad approach fails to give even a qualitatively correct picture.

Quantum Measurement of Phonon Shot Noise

A. A. Clerk, Florian Marquardt, J. G. E. Harris

We provide a full quantum mechanical analysis of a weak energy measurement of a driven mechanical resonator. We demonstrate that measurements too weak to resolve individual mechanical Fock states can nonetheless be used to detect the nonclassical energy fluctuations of the driven mechanical resonator, i.e., "phonon shot noise". We also show that the third moment of the oscillator's energy fluctuations provides a far more sensitive probe of quantum effects than the second moment, and that measuring the third moment via the phase shift of light in an optomechanical setup directly yields the type of operator ordering postulated in the theory of full-counting statistics.

Universal linear Bogoliubov transformations through one-way quantum computation

Ryuji Ukai, Jun-ichi Yoshikawa, Noriaki Iwata, Peter van Loock, Akira Furusawa

We show explicitly how to realize an arbitrary linear unitary Bogoliubov (LUBO) transformation on a multimode quantum state through homodyne-based one-way quantum computation. Any LUBO transformation can be approximated by means of a fixed, finite-sized, sufficiently squeezed Gaussian cluster state that allows for the implementation of beam splitters (in form of three-mode connection gates) and general one-mode LUBO transformations. In particular, we demonstrate that a linear four-mode cluster state is a sufficient resource for an arbitrary one-mode LUBO transformation. Arbitrary-input quantum states including non-Gaussian states could be efficiently attached to the cluster through quantum teleportation.

A generator for unique quantum random numbers based on vacuum states

Christian Gabriel, Christoffer Wittmann, Denis Sych, Ruifang Dong, Wolfgang Mauerer, Ulrik L. Andersen, Christoph Marquardt, Gerd Leuchs

Random numbers are a valuable component in diverse applications that range from simulations(1) over gambling to cryptography(2,3). The quest for true randomness in these applications has engendered a large variety of different proposals for producing random numbers based on the foundational unpredictability of quantum mechanics(4-11). However, most approaches do not consider that a potential adversary could have knowledge about the generated numbers, so the numbers are not verifiably random and unique(12-15). Here we present a simple experimental setup based on homodyne measurements that uses the purity of a continuous-variable quantum vacuum state to generate unique random numbers. We use the intrinsic randomness in measuring the quadratures of a mode in the lowest energy vacuum state, which cannot be correlated to any other state. The simplicity of our source, combined with its verifiably unique randomness, are important attributes for achieving high-reliability, high-speed and low-cost quantum random number generators.

Following interfacial kinetics in real time using broadband evanescent wave cavity-enhanced absorption spectroscopy: a comparison of light-emitting diodes and supercontinuum sources

Lineke van der Sneppen, Gus Hancock, Clemens Kaminski, Toni Laurila, Stuart R. Mackenzie, Simon R. T. Neil, Robert Peverall, Grant A. D. Ritchie, Mathias Schnippering, et al.

A white light-emitting diode (LED) with emission between 420 and 700 nm and a supercontinuum (SC) source with emission between 450 and 2500 nm have been compared for use in evanescent wave broadband cavity-enhanced absorption spectroscopy (EW-BB-CEAS). The method is calibrated using a dye with known absorbance. While the LED is more economic as an excitation source, the SC source is superior both in terms of baseline noise (noise equivalent absorbances lower than 10(-5) compared to 10(-4) absorbance units (a.u.)) and accuracy of the measurement; these baseline noise levels are comparable to evanescent wave cavity ringdown spectroscopy (EW-CRDS) studies while the accessible spectral region of EW-BB-CEAS is much larger (420-750 nm in this study, compared to several tens of nanometres for EW-CRDS). The improvements afforded by the use of an SC source in combination with a high sensitivity detector are demonstrated in the broadband detection of electrogenerated Ir(IV) complexes in a thin-layer electrochemical cell arrangement. Excellent signal to noise is achieved with 10 ms signal accumulation times at a repetition rate of 600 Hz, easily fast enough to follow, in real time, solution kinetics and interfacial processes.

Quantum benchmarks for the storage or transmission of quantum light from minimal resources

Hauke Haeseler, Norbert Luetkenhaus

We investigate several recently published benchmark criteria for storage or transmission of continuous-variable quantum information. A comparison reveals that criteria based on a Gaussian distribution of coherent states are most resilient to noise. We then address the issue of experimental resources and derive an equally strong benchmark, solely based on three coherent states and homodyne detection. This benchmark is further simplified in the presence of naturally occurring random phases, which remove the need for active input-state modulation.

CaF2 whispering-gallery-mode-resonator stabilized-narrow-linewidth laser

B. Sprenger, H. G. L. Schwefel, Z. H. Lu, S. Svitlov, L. J. Wang

A fiber laser is stabilized by introducing a calcium fluoride (CaF2) whispering-gallery-mode resonator as a filtering element in a ring cavity. It is set up using a semiconductor optical amplifier as a gain medium. The resonator is critically coupled through prisms, and used as a filtering element to suppress the laser linewidth. A three-cornered-hat method is used and shows a stability of 10(-11) after 10 mu s. Using the self-heterodyne beat technique, the linewidth is determined to be 13 kHz. This implies an enhancement factor of 10(3) with respect to the passive cavity linewidth. (C) 2010 Optical Society of America

Improvements of the MPG-2 transportable absolute ballistic gravimeter

H. Hu, S. Svitlov, C. Rothleitner, J. Schaefer, J. Zhang, L. J. Wang

The MPG-2 (Max-Planck-Gravimeter) is a transportable absolute gravimeter built on a classical free-fall scheme to measure the local gravity value. With significant improvements and further investigations in recent years, the standard deviation of the mean for a typical measurement over 12 h to 24 h is 1.0 mu Gal to 3.0 mu Gal (1 mu Gal = 10(-8) ms(-2)), and the combined standard uncertainty is estimated to be less than 10 mu Gal. The major improvements include the new interferometer design and alignment, longer drop length, reduced recoil effects and demagnetization of the falling body. The revised uncertainty budget and new measurement results of MPG-2 are reported. The results of observations at the reference gravity station Bad Homburg confirmed the revised uncertainty budget.

Spatiotemporal evolution of femtosecond laser pulses guided in air-clad fused-silica nanoweb

C. Kreuzer, A. Podlipensky, P. St. J. Russell

We investigate nonlinear propagation and self-focusing of femtosecond Ti:sapphire laser pulses in an 800-nm-thick silica nanoweb fiber. Different dispersion regimes are accessible by launching TE- or TM-polarized light. Excitation in the anomalous dispersion regime (TM) results in pulse splitting and spectral broadening, which lead to supercontinuum generation, whereas, for normal dispersion (TE, excited close to a zero dispersion wavelength), self-phase modulation causes spectral broadening, which leads at higher power to beam collapse and the creation of a damage track. (C) 2010 Optical Society of America

Efficient III-V tunneling diodes with ErAs recombination centers

S. Preu, S. Malzer, G. H. Doehler, H. Lu, A. C. Gossard, L. J. Wang

We report on recombination diodes containing monolayer depositions of ErAs between highly doped n- and p-layers in the (Al)GaAs and In(Al)GaAs material system. The ErAs material provides metallic states across the band gap of the host semiconductor that act as efficient recombination centers. Both electrons and holes can tunnel into the ErAs, resulting in extremely high tunneling current densities in the tens of kA cm(-2) range. A device resistance area product of 1-2 x 10(-5) Omega cm(2) at low bias (+/- 0.2 V) has been measured. Measurements on In(Al)GaAs and (Al)GaAs diodes will be compared to a simple theoretical model. Low band gap and high (p-) doping levels are identified as key parameters for achieving highest recombination currents. Compared to ErAs-free diodes, ErAs-enhanced recombination diodes provide orders of magnitude higher current densities at both moderate and low forward and reverse bias. This is attributed to the smaller and narrower tunnel barriers from the n- and p- layers into the ErAs compared to tunneling from the n- to the p-side through the whole depletion region.

Photochemistry in Photonic Crystal Fiber Nanoreactors

Jocelyn S. Y. Chen, Tijmen G. Euser, Nicola J. Farrer, Peter J. Sadler, Michael Scharrer, Philip St. J. Russell

We report the use of a liquid-filled hollow-core photonic crystal fiber (PCF) as a highly controlled photochemical reactor. Hollow-core PCFs have several major advantages over conventional sample cells: the sample volume per optical path length is very small (2.8 nL cm(-1) in the fiber used), long optical path lengths are possible as a result of very low intrinsic waveguide loss, and furthermore the light travels in a diffractionless single mode with a constant transverse intensity profile. As a proof of principle, the (very low) quantum yield of the photochemical conversion of vitamin Bp, cyanocobalamin (CNCbl) to hydroxocobalamin ([H(2)OCbl](+)) in aqueous solution was measured for several pH values from 2.5 to 7.5. The dynamics of the actively induced reaction were monitored in real-time by broadband absorption spectroscopy. The PCF nanoreactor required ten thousand times less sample volume compared to conventional techniques. Furthermore. the enhanced sensitivity and optical pump intensity implied that even systems with very small quantum yields can be measured very quickly in our experiments one thousand times faster than in a conventional cuvette.

Broadband NIR photoluminescence from Bi-doped Ba2P2O7 crystals: Insights into the nature of NIR-emitting Bismuth centers

Mingying Peng, Benjamin Sprenger, Markus A. Schmidt, Harald G. L. Schwefel, Lothar Wondraczek

We report on a novel type of Bi-doped crystal that exhibits ultrabroadband photoluminescence in the near infrared (NIR). Emission centers can be generated and degenerated reversibly by annealing the material in CO atmosphere and air, respectively, indicating that emission is related to the presence of Bi-species in low valence states. Correlating static and dynamic excitation and emission data with the size and charge of available lattice sites suggests that two types of Bi-0-species, each located on one of the two available Ba2+ lattice sites, are responsible for NIR photoemission. This is further confirmed by the absence of NIR emission in polycrystalline Ca2P2O7:Bi and Sr2P2O7:Bi. Excitation is assigned to transitions between the doubly degenerated ground state S-4(3/2) and the degenerated excited levels D-2(3/2), D-2(5/2) and P-2(1/2), respectively. NIR emission is attributed to D-2(3/2) -> S-4(3/2). The NIR emission center can coexist with Bi2+ species. Then, also Bi2+ is accommodated on one of the two Ba2+-sites. Energy transfer between Bi2+ ions occurs within a critical distance of 25.9 angstrom. (C) 2010 Optical Society of America

Highly Noninstantaneous Solitons in Liquid-Core Photonic Crystal Fibers

Claudio Conti, Markus A. Schmidt, Philip St J. Russell, Fabio Biancalana

The nonlinear propagation of pulses in liquid-filled photonic crystal fibers is considered. Because of the slow reorientational nonlinearity of some molecular liquids, the nonlinear modes propagating inside such structures can be approximated, for pulse durations much shorter than the molecular relaxation time, by temporally highly nonlocal solitons, analytical solutions of a linear Schrodinger equation. The physical relevance of these novel solitons is discussed.

Discrimination of binary coherent states using a homodyne detector and a photon number resolving detector

Christoffer Wittmann, Ulrik L. Andersen, Masahiro Takeoka, Denis Sych, Gerd Leuchs

We investigate quantum measurement strategies capable of discriminating two coherent states probabilistically with significantly smaller error probabilities than can be obtained using nonprobabilistic state discrimination. We apply a postselection strategy to the measurement data of a homodyne detector as well as a photon number resolving detector in order to lower the error probability. We compare the two different receivers with an optimal intermediate measurement scheme where the error rate is minimized for a fixed rate of inconclusive results. The photon number resolving (PNR) receiver is experimentally demonstrated and compared to an experimental realization of a homodyne receiver with postselection. In the comparison, it becomes clear that the performance of the PNR receiver surpasses the performance of the homodyne receiver, which we prove to be optimal within any Gaussian operations and conditional dynamics.

Avoiding the blinding attack in QKD reply

Lars Lydersen, Carlos Wiechers, Christoffer Wittmann, Dominique Elser, Johannes Skaar, Vadim Makarov

Extraordinary transmission through a single coaxial aperture in a thin metal film

P. Banzer, J. Kindler (nee Mueller), S. Quabis, U. Peschel, G. Leuchs

We investigate experimentally the transmission properties of single sub-wavelength coaxial apertures in thin metal films (t = 110 nm). Enhanced transmission through a single sub-wavelength coaxial aperture illuminated with a strongly focused radially polarized light beam is reported. In our experiments we achieved up to four times enhanced transmission through a single coaxial aperture as compared to a (hollow) circular aperture with the same outer diameter. We attribute this enhancement of transmission to the excitation of a TEM-mode for illumination with radially polarized light inside the single coaxial aperture. A strong polarization contrast is observed between the transmission for radially and azimuthally polarized illumination. Furthermore, the observed transmission through a single coaxial aperture can be strongly reduced if surface plasmons are excited. The experimental results are in good agreement with finite difference time domain (FDTD) simulations. (C)2010 Optical Society of America

Tuning the structural properties of femtosecond-laser-induced nanogratings

Lourdes Patricia R. Ramirez, Matthias Heinrich, Soeren Richter, Felix Dreisow, Robert Keil, Alexander V. Korovin, Ulf Peschel, Stefan Nolte, Andreas Tuennermann

We present the results of our investigations on the formation process of nanogratings in fused silica and the influence of fabrication parameters, thereby identifying ways to systematically control the grating properties. Nanogratings, self-organized nanostructures with subwavelength periodicity, are formed in certain parameter ranges during femtosecond-laser processing of transparent materials, resulting in characteristic birefringent modifications. They provide the opportunity for the fabrication of arbitrary three-dimensional birefringent elements with position-dependent retardation. Based on our findings, we were able to fabricate birefringent elements with various precise retardations in otherwise isotropic fused silica.

Scaling and complex avalanche dynamics in the Abelian sandpile model

Amir Abdolvand, Afshin Montakhab

We study the two-dimensional Abelian Sandpile Model on a square lattice of linear size L. We introduce the notion of avalanche's fine structure and compare the behavior of avalanches and waves of toppling. We show that according to the degree of complexity in the fine structure of avalanches, which is a direct consequence of the intricate superposition of the boundaries of successive waves, avalanches fall into two different categories. We propose scaling ansatz for these avalanche types and verify them numerically. We find that while the first type of avalanches (alpha) has a simple scaling behavior, the second complex type (beta) is characterized by an avalanche-size dependent scaling exponent. In particular, we define an exponent gamma to characterize the conditional probability distribution functions for these types of avalanches and show that gamma (alpha) = 0.42, while 0.7 a parts per thousand currency sign gamma (beta) a parts per thousand currency sign 1.0 depending on the avalanche size. This distinction provides a framework within which one can understand the lack of a consistent scaling behavior in this model, and directly addresses the long-standing puzzle of finite-size scaling in the Abelian sandpile model.

Transverse angular momentum of photons

Andrea Aiello, Christoph Marquardt, Gerd Leuchs

We develop the quantum theory of transverse angular momentum of light beams. The theory applies to paraxial and quasiparaxial photon beams in vacuum and reproduces the known results for classical beams when applied to coherent states of the field. Both the Poynting vector, alias the linear momentum, and the angular-momentum quantum operators of a light beam are calculated including contributions from first-order transverse derivatives. This permits a correct description of the energy flow in the beam and the natural emergence of both the spin and the angular momentum of the photons. We show that for collimated beams of light, orbital angular-momentum operators do not satisfy the standard commutation rules. Finally, we discuss the application of our theory to some concrete cases.

Ultraviolet-enhanced supercontinuum generation in tapered photonic crystal fiber

S. P. Stark, A. Podlipensky, N. Y. Joly, P. St. J. Russell

We investigate numerically and experimentally the propagation of visible sub-50 fs pulses in a tapered small core photonic crystal fiber. The fiber has anomalous dispersion between two closely spaced zero dispersion wavelengths at 509 and 640 nm, and the excitation wavelength was varied within this range. We find that the spectral evolution in the low power regime is dominated by higher-order soliton fission, soliton self-frequency shift, and dispersive wave generation. At higher powers, extremely wide spectral broadening of the input pulse occurs within the first few millimeters of fiber. The wavelength conversion into the blue and red spectral ranges is studied as a function of the input power and excitation wavelength. Conversions into the spectral range 300-470 nm at efficiencies as high as 40% are observed when pumping at 523 nm. (C) 2010 Optical Society of America

Bistability, multistability and non-reciprocal light propagation in Thue-Morse multilayered structures

Victor Grigoriev, Fabio Biancalana

The nonlinear properties of quasi-periodic photonic crystals based on the Thue-Morse sequence are investigated. The intrinsic spatial asymmetry of these one-dimensional structures for odd generation numbers results in bistability thresholds, which are sensitive to the propagation direction. Along with resonances of perfect transmission, this feature allows us to achieve strongly non-reciprocal propagation and to create an all-optical diode. The salient qualitative features of such optical diode action are readily explained through a simple coupled resonator model. The efficiency of a passive scheme that does not necessitate an additional short pump signal is compared to an active scheme where such a signal is required.

Teleportation is necessary for faithful quantum state transfer through noisy channels of maximal rank

Raffaele Romano, Peter van Loock

Quantum teleportation enables deterministic and faithful transmission of quantum states, provided a maximally entangled state is preshared between sender and receiver, and a one-way classical channel is available. Here, we prove that these resources are not only sufficient, but also necessary, for deterministically and faithfully sending quantum states through any fixed noisy channel of maximal rank, when a single use of the cannel is admitted. In other words, for this family of channels, there are no other protocols, based on different (and possibly cheaper) sets of resources, capable of replacing quantum teleportation.

Photon shuttle: Landau-Zener-Stuckelberg dynamics in an optomechanical system

Georg Heinrich, J. G. E. Harris, Florian Marquardt

The motion of micro- and nanomechanical resonators can be coupled to electromagnetic fields. Such optomechanical setups allow one to explore the interaction of light and matter in a new regime at the boundary between quantum and classical physics. We propose an approach to investigate nonequilibrium photon dynamics driven by mechanical motion in a recently developed setup with a membrane between two mirrors, where photons can be shuttled between the two halves of the cavity. For modest driving strength we predict the possibility of observing an Autler-Townes splitting indicative of Rabi dynamics. For large drive, we show that this system displays Landau-Zener-Stueckelberg dynamics originally known from atomic two-state systems.

Passive decoy-state quantum key distribution with practical light sources

Marcos Curty, Xiongfeng Ma, Bing Qi, Tobias Moroder

Decoy states have been proven to be a very useful method for significantly enhancing the performance of quantum key distribution systems with practical light sources. Although active modulation of the intensity of the laser pulses is an effective way of preparing decoy states in principle, in practice passive preparation might be desirable in some scenarios. Typical passive schemes involve parametric down-conversion. More recently, it has been shown that phase-randomized weak coherent pulses (WCP) can also be used for the same purpose [M. Curty et al., Opt. Lett. 34, 3238 (2009).] This proposal requires only linear optics together with a simple threshold photon detector, which shows the practical feasibility of the method. Most importantly, the resulting secret key rate is comparable to the one delivered by an active decoy-state setup with an infinite number of decoy settings. In this article we extend these results, now showing specifically the analysis for other practical scenarios with different light sources and photodetectors. In particular, we consider sources emitting thermal states, phase-randomized WCP, and strong coherent light in combination with several types of photodetectors, like, for instance, threshold photon detectors, photon number resolving detectors, and classical photodetectors. Our analysis includes as well the effect that detection inefficiencies and noise in the form of dark counts shown by current threshold detectors might have on the final secret key rate. Moreover, we provide estimations on the effects that statistical fluctuations due to a finite data size can have in practical implementations.

Sub-Hertz frequency stabilization of a commercial diode laser

Y. N. Zhao, J. Zhang, J. Stuhler, G. Schuricht, F. Lison, Z. H. Lu, L. J. Wang

We report ultra-stable locking of a commercially available extended cavity diode laser to a vibration-insensitive, high finesse Fabry-Perot cavity. A servo bandwidth of 2 MHz is demonstrated. The individual frequency stability of the diode laser after locking is independently measured with a three-cornered-hat method. The resulting Allan deviation reaches a level of 3 x 10(-15) at 1 s, even without vibration isolation of the reference cavity. (C) 2010 Elsevier B.V. All rights reserved

Phase-Coherent Frequency Comparison of Optical Clocks Using a Telecommunication Fiber Link

Harald Schnatz, Osama Terra, Katharina Predehl, Thorsten Feldmann, Thomas Legero, Burghard Lipphardt, Uwe Sterr, Gesine Grosche, Ronald Holzwarth, et al.

We have explored the performance of 2 "dark fibers" of a commercial telecommunication fiber link for a remote comparison of optical clocks. These fibers establish a network in Germany that will eventually link optical frequency standards at PTB with those at the Institute of Quantum Optics (IQ) at the Leibniz University of Hanover, and the Max Planck Institutes in Erlangen (MPL) and Garching (MPQ). We demonstrate for the first time that within several minutes a phase coherent comparison of clock lasers at the few 10(-15) level can also be accomplished when the lasers are more than 100 km apart. Based on the performance of the fiber link to the IQ, we estimate the expected stability for the link from PTB to MPQ via MPL that bridges a distance of approximately 900 km.

Microstructure and lattice bending in polycrystalline laser-crystallized silicon thin films for photovoltaic applications

X. Maeder, C. Niederberger, S. Christiansen, A. Bochmann, G. Andrae, A. Gawlik, F. Falk, J. Michler

Grain size, grain boundary population, orientation distribution and lattice defects of polycrystalline silicon thin films are investigated by electron backscatter diffraction (EBSD). The silicon thin films are produced by a combination of diode laser melt-mediated crystallization of an amorphous silicon seed layer and epitaxial thickening of the seed layer by solid phase epitaxy (SPE). The combined laser-SPE process delivers grains exceeding several 10 mu m of width and far larger than 100 mu m in length. Strong lattice rotations between 10 and 50 degrees from one side of the grain to the other are observed within the larger grains of the film. The misorientation axes are well aligned with the direction of movement of the laser. The intragranular misorientation is associated both with geometrically necessary dislocations and low angle boundaries, which can serve as recombination centres for electron-hole pairs. Since the lateral grain size is up to two orders of magnitude larger than the film thickness, the high dislocation density could become an important factor reducing the solar cell performance. (C) 2010 Elsevier B.V. All rights reserved.

Environment-assisted quantum-information correction for continuous variables

Metin Sabuncu, Radim Filip, Gerd Leuchs, Ulrik L. Andersen

Quantum-information protocols are inevitably affected by decoherence which is associated with the leakage of quantum information into an environment. In this article we address the possibility of recovering the quantum information from an environmental measurement. We investigate continuous-variable quantum information, and we propose a simple environmental measurement that under certain circumstances fully restores the quantum information of the signal state although the state is not reconstructed with unit fidelity. We implement the protocol for which information is encoded into conjugate quadratures of coherent states of light and the noise added under the decoherence process is of Gaussian nature. The correction protocol is tested using both a deterministic as well as a probabilistic strategy. The potential use of the protocol in a continuous-variable quantum-key distribution scheme as a means to combat excess noise is also investigated.

Near-unit-fidelity entanglement distribution scheme using Gaussian communication

Ludmila Praxmeyer, Peter van Loock

We show how to distribute with percentage success probabilities almost perfectly entangled qubit memory pairs over repeater channel segments of the order of the optical attenuation distance. In addition to some weak, dispersive light-matter interactions, only Gaussian state transmissions and measurements are needed for this scheme. Our protocol outperforms the existing coherent-state-based schemes for entanglement distribution, even those using error-free non-Gaussian measurements. This is achieved through two innovations: First, optical squeezed states are utilized instead of coherent states. Second, the amplitudes of the bright signal pulses are reamplified at each repeater station. This latter variation is a strategy reminiscent of classical repeaters and would be impossible in single-photon-based schemes.

Low-Threshold Optical Parametric Oscillations in a Whispering Gallery Mode Resonator

J. U. Fuerst, D. V. Strekalov, D. Elser, A. Aiello, U. L. Andersen, Ch. Marquardt, G. Leuchs

In whispering gallery mode (WGM) resonator light is guided by continuous total internal reflection along a curved surface. Fabricating such resonators from an optically nonlinear material one takes advantage of their exceptionally high quality factors and small mode volumes to achieve extremely efficient optical frequency conversion. Our analysis of the phase-matching conditions for optical parametric down-conversion (PDC) in a spherical WGM resonator shows their direct relation to the sum rules for photons' angular momenta and predicts a very low parametric oscillation threshold. We realized such an optical parametric oscillator (OPO) based on naturally phase-matched PDC in lithium niobate. We demonstrated a single-mode, strongly nondegenerate OPO with a threshold of 6: 7 mu W and linewidth under 10 MHz. This work demonstrates the remarkable capabilities of WGM-based OPOs.

The Phenomenology of Ion Implantation-Induced Blistering and Thin-Layer Splitting in Compound Semiconductors

R. Singh, S. H. Christiansen, O. Moutanabbir, U. Goesele

Hydrogen and/or helium implantation-induced surface blistering and layer splitting in compound semiconductors such as InP, GaAs, GaN, AlN, and ZnO are discussed. The blistering phenomenon depends on many parameters such as the semiconductor material, ion fluence, ion energy, and implantation temperature. The optimum values of these parameters for compound semiconductors are presented. The blistering and splitting processes in silicon have been studied in detail, motivated by the fabrication of the widely used silicon-on-insulator wafers. Hence, a comparison of the blistering process in Si and compound semiconductors is also presented. This comparative study is technologically relevant since ion implantation-induced layer splitting combined with direct wafer bonding in principle allows the transfer of any type of semiconductor layer onto any foreign substrate of choice-the technique is known as the ion-cut or Smart-Cut (TM) method. For the aforementioned compound semiconductors, investigations regarding layer transfer using the ion-cut method are still in their infancy. We report feasibility studies of layer transfer by the ion-cut method for some of the most important and widely used compound semiconductors. The importance of characteristic values for successful wafer bonding such as wafer bow and surface flatness as well as roughness are discussed, and difficulties in achieving some of these values are pointed out.

Comment on 'Evaluation of the local value of the Earth gravity field in the context of the new definition of the kilogram'

S. M. Svitlov

A recent paper (Baumann et al 2009 Metrologia 46 178-86) presents a method to evaluate the free-fall acceleration at a desired point in space, as required for the watt balance experiment. The claimed uncertainty of their absolute gravity measurements is supported by two bilateral comparisons using two absolute gravimeters of the same type. This comment discusses the case where absolute gravity measurements are traceable to a key comparison reference value. Such an approach produces a more complete uncertainty budget and reduces the risk of the results of different watt balance experiments not being compatible.

A note on quantum error correction with continuous variables

Peter van Loock

We demonstrate that continuous-variable quantum error correction based on Gaussian ancilla states and Gaussian operations (for encoding, syndrome extraction, and recovery) can be very useful to suppress the effect of non-Gaussian error channels. For a certain class of stochastic error models, reminiscent of those typically considered in the qubit case, quantum error correction codes designed for single-channel errors may enhance the transfer fidelities even when errors occur in every channel employed for transmitting the encoded state. In fact, in this case, the error-correcting capability of the continuous-variable scheme turns out to be higher than that of its discrete-variable analogues.

Plasmon resonances on gold nanowires directly drawn in a step-index fiber

H. K. Tyagi, H. W. Lee, P. Uebel, M. A. Schmidt, N. Joly, M. Scharrer, P. St. J. Russell

We report the successful production of high-quality gold wires, with diameters down to 260 nm, by direct fiber drawing from a gold-filled fused-silica cane. The stack-and-draw technique makes it straightforward to incorporate a conventional step-index core, adjacent to the gold wire, in the cane. In the drawn fiber, strong coupling of light from the glass core to SPP resonances on the gold wire is observed at specific well-defined wavelengths. Such embedded wires have many potential applications, for example, as nanoscale electrodes, in nonlinear optical plasmonics, and as near-field scanning optical microscope tips. (C) 2010 Optical Society of America

Flexible beam shaping system using fly's eye condenser

Norbert Lindlein, Andreas Bich, Martin Eisner, Irina Harder, Maik Lano, Reinhard Voelkel, Ken Weible, Maik Zimmermann

Normally, fly's eye condensers are used to homogenize light. However, in the case of fully coherent light, a fly's eye condenser, in connection with some simple optical elements, such as a diffractive axicon, a grating, and a telescope, can be used as a quite flexible beam shaping system, forming arrays of rings, parts of rings, or other structures with varying diameters. We present the principle, some simulation results, and some first experimental results. (C) 2010 Optical Society of America

Bridging visible and telecom wavelengths with a single-mode broadband photon pair source

C. Soeller, B. Brecht, P. J. Mosley, L. Y. Zang, A. Podlipensky, N. Y. Joly, P. St. J. Russell, C. Silberhorn

We present a spectrally decorrelated photon pair source bridging the visible and telecom wavelength regions. Tailored design and fabrication of a solid-core photonic crystal fiber (PCF) lead to the emission of signal and idler photons into only a single spectral and spatial mode. Thus no narrowband filtering is necessary and the heralded generation of pure photon number states in ultrafast wave packets at telecom wavelengths becomes possible.

A Blind Spot in Confocal Reflection Microscopy: The Dependence of Fiber Brightness on Fiber Orientation in Imaging Biopolymer Networks

Louise M. Jawerth, Stefan Muenster, David A. Vader, Ben Fabry, David A. Weitz

We investigate the dependence of fiber brightness on three-dimensional fiber orientation when imaging biopolymer networks with confocal reflection microscopy (CRM) and confocal fluorescence microscopy (CFM). We compare image data of fluorescently labeled type I collagen networks concurrently acquired using each imaging modality. For CRM, fiber brightness decreases for more vertically oriented fibers, leaving fibers above similar to 50 degrees from the imaging plane entirely undetected. As a result, the three-dimensional network structure appears aligned with the imaging plane. In contrast, CFM data exhibit little variation of fiber brightness with fiber angle, thus revealing an isotropic collagen network. Consequently, we find that CFM detects almost twice as many fibers as are visible with CRM, thereby yielding more complete structural information for three-dimensional fiber networks. We offer a simple explanation that predicts the detected fiber brightness as a function of fiber orientation in CRM.

Dispersion of photonic Bloch modes in periodically twisted birefringent media

Leyun Zang, Myeong Soo Kang, Miroslav Kolesik, Michael Scharrer, Philip Russell

We investigate the polarization evolution and dispersive properties of the eigenmodes of birefringent media with arbitrarily twisted axes of birefringence. Analytical and numerical methods based on a transfer matrix approach are developed and used to study specifically helically twisted structures and the Bloch modes of periodically twisted media, as represented in particular by structural "rocking" filters inscribed in highly birefringent photonic crystal fibers. The presence of periodically twisted birefringence axes causes the group velocity dispersion curves to separate strongly from each other in the vicinity of the anti-crossing wavelength, where the inter-polarization beat-length equals an integer multiple of the rocking period. The maximum separation between these curves and the bandwidth of the splitting depend on the amplitude of the rocking angle. We also show that suitably designed adiabatic transitions, formed by chirping the rocking period, allow a broadband conversion between a linearly polarized fiber eigenmode and a single Bloch mode of a uniform rocking filter. The widely controllable dispersive properties provided by rocking filters may be useful for manipulating the phase-matching conditions in nonlinear optical processes such as four-wave mixing, supercontinuum generation, and the generation of resonant radiation from solitons. (C) 2010 Optical Society of America

Atmospheric channel characteristics for quantum communication with continuous polarization variables

B. Heim, D. Elser, T. Bartley, M. Sabuncu, C. Wittmann, D. Sych, C. Marquardt, G. Leuchs

We investigate the properties of an atmospheric channel for free space quantum communication with continuous polarization variables. In our prepare-and-measure setup, coherent polarization states are transmitted through an atmospheric quantum channel of 100 m length on the flat roof of our institute's building. The signal states are measured by homodyne detection with the help of a local oscillator (LO) which propagates in the same spatial mode as the signal, orthogonally polarized to it. Thus the interference of signal and LO is excellent and atmospheric fluctuations are auto-compensated. The LO also acts as a spatial and spectral filter, which allows for unrestrained daylight operation. Important characteristics for our system are atmospheric channel influences that could cause polarization, intensity and position excess noise. Therefore we study these influences in detail. Our results indicate that the channel is suitable for our quantum communication system in most weather conditions.

Realization of Vertical and Zigzag Single Crystalline Silicon Nanowire Architectures

V. A. Sivakov, G. Broenstrup, B. Pecz, A. Berger, G. Z. Radnoczi, M. Krause, S. H. Christiansen

Silicon nanowire (SiNW) ensembles, with vertical and zigzag architectures have been realized using wet chemical etching of bulk silicon wafers (p-Si(l 11) and p-Si(100)) with it mask of silver nanoparticles that are deposited by wet electroless deposition. The etching of SiNWs is based oil Subsequent treatments in chemical Solutions Such is 0.02 M aqueous Solutions of silver nitrate (AgNO(3)) followed by 5 M hydrofluoric acid and 30% hydrogen peroxide (H(2)O(2)). The etching of the Si wafers is mediated by the reduction of silver oil the Silicon Surface and in parallel by the oxidation of Si thereby forming SiO(2) which is dissolved ill the HF Surroundings. The morphology of the starting silver (Ag) layer/Ag nanoparticles that form during processing oil the Si wafer surfaces strongly influences the morphology of the SiNW ensembles and homogeneity of the etch profile. Our observations Suggest that the Ag layer/Ag nanoparticles not only catalyze the wet chemical etching of silicon but also strongly catalyze the decomposition of H(2)O(2) so that the temperature of the etching Solution substantially increases (strong exothermic reaction) and thus the etching velocity of bulk material. The morphology and microstructure of single crystalline SiNWs with respect to their crystallographic orientation was investigated by scanning (SEM) and transmission electron (TEM) microscopies and by electron backscatter diffraction (EBSD) in ill SEM. Straight SiNWs Lis well as zigzag SiNWs can be realized depending oil processing peculiarities. The optical characteristics such as absorption, transmission, and reflectance of the different silicon 1D architectures were investigated in an integrating sphere. Strong absorption and less reflection of visible and near-infrared light by the SiNW ensembles Suggest that Such material call he applied in the fields of opto-electronics, photonics and photovoltaics.

Photons Walking the Line: A Quantum Walk with Adjustable Coin Operations

A. Schreiber, K. N. Cassemiro, V. Potocek, A. Gabris, P. J. Mosley, E. Andersson, I. Jex, Ch. Silberhorn

We present the first robust implementation of a coined quantum walk over five steps using only passive optical elements. By employing a fiber network loop we keep the amount of required resources constant as the walker's position Hilbert space is increased. We observed a non-Gaussian distribution of the walker's final position, thus characterizing a faster spread of the photon wave packet in comparison to the classical random walk. The walk is realized for many different coin settings and initial states, opening the way for the implementation of a quantum-walk-based search algorithm.

Precise balancing of viscous and radiation forces on a particle in liquid-filled photonic-bandgap fiber (vol 34, pg 3674, 2009)

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

Complex source beam: A tool to describe highly focused vector beams analytically

S. Orlov, U. Peschel

The scalar-complex-source model is used to develop an accurate description of highly focused radially, azimuthally, linearly, and circularly polarized monochromatic vector beams. We investigate the power and full beam widths at half maximum of vigorous Maxwell equation solutions. The analytical expressions are employed to compare the vector complex source beams with the real beams produced by various high-numerical-aperture (NA) focusing systems. We find a parameter set for which the spatial extents of the analytical beams are the same as those of experimentally realized ones. We ensure the same shape of the considered beams by investigating an overlap of the complex source beams with high-NA beams. We demonstrate that the analytical expressions are good approximations for realistic highly focused beams.

Probing the Negative Wigner Function of a Pulsed Single Photon Point by Point

Kaisa Laiho, Katiuscia N. Cassemiro, David Gross, Christine Silberhorn

We investigate quantum properties of pulsed light fields point by point in phase space. We probe the negative region of the Wigner function of a single photon generated by the means of waveguided parametric down conversion. This capability is achieved by employing loss-tolerant photon-number resolving detection, allowing us to directly observe the oscillations of the photon statistics in dependence of applied displacements in phase space. Our scheme is highly mode sensitive and can reveal the single-mode character of the signal state.

Solitons in curved space of constant curvature

Sascha Batz, Ulf Peschel

We consider spatial solitons as, for example, self-confined optical beams in spaces of constant curvature, which are a natural generalization of flat space. Due to the symmetries of these spaces we are able to define respective dynamical parameters, for example, velocity and position. For positively curved space we find stable multiple-hump solitons as a continuation from the linear modes. In the case of negatively curved space we show that no localized solution exists and a bright soliton will always decay through a nonlinear tunneling process.

How orbital angular momentum affects beam shifts in optical reflection

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

It is well known that reflection of a Gaussian light beam (TEM(00)) by a planar dielectric interface leads to four beam shifts when compared to the geometrical-optics prediction. These are the spatial Goos-Hanchen (GH) shift, the angular GH shift, the spatial Imbert-Fedorov (IF) shift, and the angular IF shift. We report here, theoretically and experimentally, that endowing the beam with orbital angular momentum leads to coupling of these four shifts; this is described by a 4 x 4 mixing

Effect of fabrication errors on the diffraction pattern produced by sawtooth gratings

Francisco Jose Torcal-Milla, Irina Harder, Norbert Lindlein

In this work we investigate, analytically and numerically, the effect on the diffracted field produced by typical fabrication errors in sawtooth gratings. The analysis is carried out for the near and far field, showing the effects on the intensity and on the diffraction orders efficiency. When the grating profile is not perfect but presents a curved profile or overdevelopment error, some different diffraction orders appear, changing the intensity and the efficiency of each order. In addition, when roughness is present, a decreasing of efficiency is produced, but without generating different diffraction orders than the first one. We show the analytical dependence of these modifications in terms of the profile of the grating, corroborating the results with numerical methods. (C) 2010 Optical Society of America

Roughness of silicon nanowire sidewalls and room temperature photoluminescence

Vladimir A. Sivakov, Felix Voigt, Andreas Berger, Gottfried Bauer, Silke H. Christiansen

Strong room temperature visible (red-orange) photoluminescence (PL) has been observed in silicon nanowires (SiNWs) that were realized by wet chemical etching of heavily (arsenic, As: 10(20) cm(-3)) and lowly doped (boron, B: 10(15) cm(-3)) single crystalline silicon (Si) wafers. Optical characterization of these SiNWs by PL combined with structural characterization by transmission and scanning electron microscopy strongly suggest that the visible PL at room temperature results from the rough SiNW sidewall structure that is composed of nanoscale features in which quantum confinement effects may occur.

Polarization sensitive lateral photoconductivity in GaAs/AlGaAs quantum well based structures on low-temperature grown GaAs(001)

Ashish Arora, Sandip Ghosh, B. M. Arora, Stefan Malzer, Gottfried Doehler

Polarization-resolved lateral-photoconductivity measurements are reported on device structures made of GaAs/Al(0.3)Ga(0.7)As quantum wells sandwiched between low-temperature grown GaAs(001) layers. The mesa device structures have long length (3 mm parallel to y) and narrow width (10 and 20 mu m parallel to x) in the (001) plane. For light incident along [001], the ground state light-hole exciton transition is much stronger for light polarization E parallel to x, compared to E parallel to y. The heavy-hole exciton transition shows a weaker polarization anisotropy of opposite sign, being stronger for E parallel to y. Through calculations based on the Bir-Pikus Hamiltonian, the observed in-plane optical polarization anisotropy is shown to arise from valence band mixing induced by anisotropic strain in the plane of quantum wells. (C) 2010 American Institute of Physics. [doi: 10.1063/1.3479501]

All-Optical Control of Gigahertz Acoustic Resonances by Forward Stimulated Interpolarization Scattering in a Photonic Crystal Fiber

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

We report the observation of a novel nonlinear optoacoustic phenomenon, that we name forward stimulated interpolarization scattering. When two frequency-offset laser signals are colaunched into orthogonally polarized guided modes of a birefringent small-core (1.8 mu m diameter) photonic crystal fiber, a pattern of axially moving polarization fringes is produced, with a velocity and spacing that depends on the frequency offset. At values of frequency offset in the few-GHz range, the pattern of moving fringes can perfectly match the phase velocity and axial wavelength (3.9 mm) of the torsional-radial acoustic mode tightly guided in the core. An intense optoacoustic interaction ensues, leading to efficient nonlinear exchange of power from the higher frequency (pump) mode to the orthogonally polarized lower frequency (Stokes) mode. A full-vectorial theory is developed to explain the observations.

Theory of Raman multipeak states in solid-core photonic crystal fibers

Truong X. Tran, Alexander Podlipensky, Philip St. J. Russell, Fabio Biancalana

We provide a full theoretical understanding of the recent observations of excitation of Raman two-peak states in solid-core photonic crystal fibers. Based on a "gravity-like" potential approach we derive simple equations for the "magic" peak power ratio and the temporal separation between pulses forming these two-peak states. We develop a model to calculate the magic input power of the input pulse around which the phenomenon can be observed. We also predict the existence of exotic multipeak states that strongly violate the perturbative pulse splitting law, and we study their stability and excitation conditions. (C) 2010 Optical Society of America

Multi-pass Shack-Hartmann planeness test: monitoring thermal stress

J. Schwider, G. Leuchs

The multi-pass solution for surface measurements with the help of a Shack-Hartmann sensor (SHS) on the basis of a Fizeau cavity enables fast access to surface deviation data due to the high speed of the SHS and easy referencing of the measured data through difference measurements. The multi-pass solution described in a previous publication [J. Schwider, Opt. Express 16, 362 (2008)], provides highly sensitive measurements of small displacements caused by thermal non-equilibrium states of the test set up. Here, we want to demonstrate how a pulsed thermal load changes the surface geometry. In addition the temporal response for different plate materials is monitored through a fast wave front measurement with very high sensitivity. The thermal load close to a delta-function in time will be applied from the back-side of a plane plate by heating a small Peltier element with a heat impulse of known order of magnitude. The development of the surface deviation on the time axis can be monitored by storing a set of successive deviation pictures. (c) 2010 Optical Society of America

A novel method for polarization squeezing with Photonic Crystal Fibers

Josip Milanovic, Mikael Lassen, Ulrik L. Andersen, Gerd Leuchs

Photonic Crystal Fibers can be tailored to increase the effective Kerr nonlinearity, while producing smaller amounts of excess noise compared to standard silicon fibers. Using these features of Photonic Crystal Fibers we create polarization squeezed states with increased purity compared to standard fiber squeezing experiments. Explicit we produce squeezed states in counter propagating pulses along the same fiber axis to achieve near identical dispersion properties. This enables the production of polarization squeezing through interference in a polarization type Sagnac interferometer. We observe Stokes parameter squeezing of -3.9 +/- 0.3dB and anti-squeezing of 16.2 +/- 0.3dB. (C) 2010 Optical Society of America

Quantum-optical state engineering up to the two-photon level

Erwan Bimbard, Nitin Jain, Andrew MacRae, A. I. Lvovsky

The ability to prepare arbitrary quantum states within a certain Hilbert space is the holy grail of quantum information technology. It is particularly important for light, as this is the only physical system that can communicate quantum information over long distances. We propose and experimentally verify a scheme to produce arbitrary single-mode states of a travelling light field up to the two-photon level. The desired state is remotely prepared in the signal channel of spontaneous parametric down-conversion by means of conditional measurements on the idler channel. The measurement consists of bringing the idler field into interference with two ancilla coherent states, followed by two single-photon detectors, which, in coincidence, herald the preparation event. By varying the amplitudes and phases of the ancillae, we can prepare any arbitrary superposition of zero-, one- and two-photon states.

Demonstration of cluster-state shaping and quantum erasure for continuous variables

Yoshichika Miwa, Ryuji Ukai, Jun-ichi Yoshikawa, Radim Filip, Peter van Loock, Akira Furusawa

We demonstrate experimentally how to remove an arbitrary node from a continuous-variable cluster state and how to shorten any quantum wires of such a state. These two basic operations, performed in an unconditional fashion, are a manifestation of quantum erasure and can be employed to obtain various graph states from an initial cluster state. Starting with a sufficiently large cluster, the resulting graph states can then be used for universal quantum information processing. In the experiment, all variations of this cluster shaping are demonstrated on a four-mode linear cluster state through homodyne measurements and feedforward.

Pressure-controlled phase matching to third harmonic in Ar-filled hollow-core photonic crystal fiber

J. Nold, P. Hoelzer, N. Y. Joly, G. K. L. Wong, A. Nazarkin, A. Podlipensky, M. Scharrer, P. St J. Russell

We report tunable third-harmonic generation (THG) in an Ar-filled hollow-core photonic crystal fiber, pumped by broadband < 2 mu J, 30 fs pulses from an amplified Ti:sapphire laser system. The overall dispersion is precisely controlled by balancing the negative dielectric susceptibility of the waveguide against the positive susceptibility of the gas. We demonstrate THG to a higher-order guided mode and show that the phase-matched UV wavelength is tunable by adjusting the gas pressure. (C) 2010 Optical Society of America

Optics in Curved Space

Vincent H. Schultheiss, Sascha Batz, Alexander Szameit, Felix Dreisow, Stefan Nolte, Andreas Tuennermann, Stefano Longhi, Ulf Peschel

We experimentally study the impact of intrinsic and extrinsic curvature of space on the evolution of light. We show that the topology of a surface matters for radii of curvature comparable with the wavelength, whereas for macroscopically curved surfaces only intrinsic curvature is relevant. On a surface with constant positive Gaussian curvature we observe periodic refocusing, self-imaging, and diffractionless propagation. In contrast, light spreads exponentially on surfaces with constant negative Gaussian curvature. For the first time we realized two beam interference in negatively curved space.

Optimized generation of heralded Fock states using parametric down-conversion

Agata M. Branczyk, T. C. Ralph, Wolfram Helwig, Christine Silberhorn

The generation of heralded pure Fock states via spontaneous parametric down-conversion (PDC) relies on perfect photon-number correlations in the output modes. Correlations in any other degree of freedom, however, degrade the purity of the heralded state. In this paper, we investigate spectral entanglement between the two output modes of a periodically poled waveguide. With the intent of generating heralded one-and two-photon Fock states, we expand the output state of the PDC to second order in photon number. We explore the effects of spectral filtering and inefficient detection, of the heralding mode, on the count rate, g((2)), and purity of the heralded state, as well as the fidelity between the resulting state and an ideal Fock state. We find that filtering can decrease spectral correlations, however, at the expense of the count rate and increased photon-number mixedness in the heralded output state. As a physical example, we model a type II PP-KTP waveguide pumped by lasers at wavelengths of 400 nm, 788 nm and 1.93 mu m. The latter two allow the fulfillment of extended phase-matching conditions in an attempt to eliminate spectral correlations in the PDC output state without the use of filtering; however, we find that, even in these cases, some filtering is needed to achieve states of very high purity.

Discrimination of optical coherent states using a photon number resolving detector

Christoffer Wittmann, Ulrik L. Andersen, Gerd Leuchs

The discrimination of non-orthogonal quantum states with reduced or without errors is a fundamental task in quantum measurement theory. In this work, we investigate a quantum measurement strategy capable of discriminating two coherent states probabilistically with significantly smaller error probabilities than can be obtained using non-probabilistic state discrimination. We find that appropriate postselection of the measurement data of a photon number resolving detector can be used to discriminate two coherent states with small error probability. We compare our new receiver to an optimal intermediate measurement between minimum error discrimination and unambiguous state discrimination.

On the experimental investigation of the electric and magnetic response of a single nano-structure

P. Banzer, U. Peschel, S. Quabis, G. Leuchs

We demonstrate an experimental method to separately test the optical response of a single sub-wavelength nano-structure to tailored electric and magnetic field distributions in the optical domain. For this purpose a highly focused y-polarized TEM10-mode is used which exhibits spatially separated longitudinal magnetic and transverse electric field patterns. By displacing a single sub-wavelength nano-structure, namely a single split-ring resonator (SRR), in the focal plane, different coupling scenarios can be achieved. It is shown experimentally that the single split-ring resonator tested here responds dominantly as an electric dipole. A much smaller but yet statistically significant magnetic dipole contribution is also measured by investigating the interaction of a single SRR with a magnetic field component perpendicular to the SRR plane (which is equivalent to the curl of the electric field) as well as by analyzing the intensity and polarization distribution of the scattered light with high spatial resolution. The developed experimental setup as well as the measurement techniques presented in this paper are a versatile tool to investigate the optical properties of single sub-wavelength nano-structures. (C) 2010 Optical Society of America