The question of an efficient multimode description of optical pulses is<br> studied. We show that a relatively very small number of nonmonochromatic<br> modes can be sufficient for a complete quantum description of pulses<br> with Gaussian quadrature statistics. For example, a three-mode<br> description was enough to reproduce the experimental data of photon<br> number correlations in optical solitons [S. Spalter, N. Korolkova, F.<br> Konig, A. Sizmann, and G. Leuchs, Phys. Rev. Lett. 81, 786 (1998)]. This<br> approach is very useful for a detailed understanding of squeezing<br> properties of soliton pulses with the main potential for quantum<br> communication with continuous variables. We show how homodyne detection<br> and/or measurements of photon number correlations can be used to<br> determine the quantum state of the multimode field. We also discuss a<br> possible way of physical separation of the nonmonochromatic modes.
Continuous variable quantum cryptography: Beating the 3 dB loss limit
We demonstrate that secure quantum key distribution systems based on<br> continuous variable implementations can operate beyond the apparent 3 dB<br> loss limit that is implied by the beam splitting attack. The loss limit<br> was established for standard minimum uncertainty states such as coherent<br> states. We show that, by an appropriate postselection mechanism, we can<br> enter a region where Eve's knowledge on Alice's key falls behind the<br> information shared between Alice and Bob, even in the presence of<br> substantial losses.
Diversity detection of speckles for double-wavelength interferometry on rough surfaces
When the topography of a rough surface is measured with a double-wavelength interferometer, the phase error of the signal corresponding to the synthetic wavelength increases in the vicinity of dark speckles. To overcome this problem we perform an amplitude-dependent averaging of the synthetic phase over independent speckles (diversity detection). We either use spatially neighboring speckles or in the case of depolarizing surfaces, we use speckles of the same spatial mode, but with orthogonal polarizations. For the latter case the lateral resolution stays unaffected. The reduction of the speckle noise is demonstrated experimentally for a laterally scanning double-wavelength interferometer with superheterodyne detection of the synthetic phase. (C) 2002 Optical Society of America.
Soliton backaction-evading measurement using spectral filtering
F Konig,
B Buchler,
T Rechtenwald,
Gerd Leuchs,
A Sizmann
We report an a backaction-evading (BAE) measurement of the photon number<br> of fiber-optical solitons operating in the quantum regime. We employ a<br> recently proposed different detection scheme based on spectral filtering<br> of colliding optical solitons. The measurements of the BAE criteria<br> demonstrate significant quantum state, preparation and transfer of the<br> input signal to the signal and probe outputs exiting the apparatus,<br> displaying the quantum-nondemolition behavior of the experiment.
Polarization squeezing and continuous-variable polarization entanglement
N Korolkova,
Gerd Leuchs,
R Loudon,
TC Ralph,
Christine Silberhorn
A concept of polarization entanglement for continuous variables is<br> introduced. For this purpose the Stokes-parameter operators and the<br> associated Poincare sphere, which describe the quantum-optical<br> polarization properties of light, are defined and their basic properties<br> are reviewed. The general features of the Stokes operators are<br> illustrated by evaluation of their means and variances for a range of<br> simple polarization states. Some of the examples show polarization<br> squeezing, in which the variances of one or more Stokes parameters are<br> smaller than the coherent-state value. The main object of the paper is<br> the application of these concepts to bright squeezed light. It is shown<br> that a light beam formed by interference of two orthogonally polarized<br> quadrature-squeezed beams exhibits squeezing in some of the Stokes<br> parameters. Passage of such a primary polarization-squeezed beam through<br> suitable optical components generates a pair of polarization-entangled<br> light beams with the nature of a two-mode squeezed state. Implementation<br> of these schemes using the double-fiber Sagnac interferometer provides<br> an efficient method for the generation of bright nonclassical<br> polarization states. The important advantage of these nonclassical<br> polarization states for quantum communication is the possibility of<br> experimentally determining all of the relevant conjugate variables of<br> both squeezed and entangled fields using only linear optical elements<br> followed by direct detection.
Quantum key distribution with bright entangled beams
We suggest a quantum cryptographic scheme using continuous EPR-like<br> correlations of bright optical beams. For binary key encoding, the<br> continuous information is discretized in a novel way by associating a<br> respective measurement, amplitude, or phase, with a bit value "1" or<br> "0." The secure key distribution is guaranteed by the quantum<br> correlations. No predetermined information is sent through the quantum<br> channel contributing to the security of the system.
Direct experimental test of non-separability and other quantum techniques using continuous variables of light
N Korolkova,
Christine Silberhorn,
O Glockl,
S Lorenz,
Christoph Marquardt,
Gerd Leuchs
EUROPEAN PHYSICAL JOURNAL D
18
(2)
229-235
(2002)
| Journal
We present schemes for the generation and evaluation of continuous<br> variable entanglement of bright optical beams and give a brief overview<br> of a variety of optical techniques and quantum communication<br> applications on this basis. A new entanglement-based quantum<br> interferometry scheme with bright beams is suggested. The performance of<br> the presented schemes is independent of the relative interference phase<br> which is advantageous for quantum communication applications.
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