70 research outputs found
Quantum homodyne tomography of a two-photon Fock state
We present a continuous-variable experimental analysis of a two-photon Fock
state of free-propagating light. This state is obtained from a pulsed
non-degenerate parametric amplifier, which produces two intensity-correlated
twin beams. Counting two photons in one beam projects the other beam in the
desired two-photon Fock state, which is analyzed by using a pulsed homodyne
detection. The Wigner function of the measured state is clearly negative. We
developed a detailed analytic model which allows a fast and efficient analysis
of the experimental results.Comment: 4 pages, 6 figures Revised version : corrected typo and reference
Noiseless Linear Amplification and Quantum Channels
The employ of a noiseless linear amplifier (NLA) has been proven as a useful
tool for mitigating imperfections in quantum channels. Its analysis is usually
conducted within specific frameworks, for which the set of input states for a
given protocol is fixed. Here we obtain a more general description by showing
that a noisy and lossy Gaussian channel followed by a NLA has a general
description in terms of effective channels. This has the advantage of offering
a simpler mathematical description, best suitable for mixed states, both
Gaussian and non-Gaussian. We investigate the main properties of this effective
system, and illustrate its potential by applying it to loss compensation and
reduction of phase uncertainty.Comment: 8 pages, 3 figure
Nonclassical radiation from diamond nanocrystals
The quantum properties of the fluorescence light emitted by diamond
nanocrystals containing a single nitrogen-vacancy (NV) colored center is
investigated. We have observed photon antibunching with very low background
light. This system is therefore a very good candidate for the production of
single photon on demand. In addition, we have measured larger NV center
lifetime in nanocrystals than in the bulk, in good agreement with a simple
quantum electrodynamical model.Comment: 8 pages, 5 figures, revised version, to appear in PR
Increasing entanglement between Gaussian states by coherent photon subtraction
We experimentally demonstrate that the entanglement between Gaussian
entangled states can be increased by non-Gaussian operations. Coherent
subtraction of single photons from Gaussian quadrature-entangled light pulses,
created by a non-degenerate parametric amplifier, produces delocalized states
with negative Wigner functions and complex structures, more entangled than the
initial states in terms of negativity. The experimental results are in very
good agreement with the theoretical predictions
Single photon quantum cryptography
We report the full implementation of a quantum cryptography protocol using a
stream of single photon pulses generated by a stable and efficient source
operating at room temperature. The single photon pulses are emitted on demand
by a single nitrogen-vacancy (NV) color center in a diamond nanocrystal. The
quantum bit error rate is less that 4.6% and the secure bit rate is 9500
bits/s. The overall performances of our system reaches a domain where single
photons have a measurable advantage over an equivalent system based on
attenuated light pulses.Comment: 4 pages, 3 figure
Qubit-Programmable Operations on Quantum Light Fields
Engineering quantum operations is one of the main abilities we need for
developing quantum technologies and designing new fundamental tests. Here we
propose a scheme for realising a controlled operation acting on a travelling
quantum field, whose functioning is determined by an input qubit. This study
introduces new concepts and methods in the interface of continuous- and
discrete-variable quantum optical systems.Comment: Comments welcom
Heralded noiseless linear amplification and quantum channels
The employ of a heralded noiseless linear amplifier has been proven as a useful tool for mitigating imperfections in quantum channels. Its analysis is usually conducted within specific frameworks, for which the set of input states for a given protocol is fixed. Here we obtain a more general description by showing that a noisy and lossy Gaussian channel followed by a heralded noiseless linear amplifier has a general description in terms of effective channels. This has the advantage of offering a simpler mathematical description, best suited for mixed states, both Gaussian and non-Gaussian. We investigate the main properties of this effective system, and illustrate its potential by applying it to loss compensation and reduction of phase uncertainty
Intrication de champs quantiques mesoscopiques pour les communications quantiques
Cette thèse s inscrit dans le cadre de l information quantique avec des variables continues, en utilisant des états quantiques du champ électromagnétique. En combinant les outils propres aux variables discrètes, ou la lumière est décrite en termes de photons, avec les outils des variables continues, où la lumière est décrite en termes de quadratures, nous pouvons étudier théoriquement et produire expérimentalement des états non-classiques, ainsi que des protocoles élémentaires d information quantique. Ainsi, nous avons produit expérimentalement un état chat de Schrödinger , superposition quantique de deux états lumineux quasi-classiques, sur lequel nous avons appliqué une porte quantique introduisant une phase dans la superposition. Nous avons ensuite analysé la qualité de cette porte en utilisant un modèle simple de notre expérience. Nous nous sommes ensuite intéressés aux corrélations quantiques, mesurées par la discorde quantique, pour une classe d états particulièrement importants en information quantique. Nous avons quantifié la précision de nos mesures en les comparant aux bornes de Cramér-Rao classique et quantique. Enfin, nous avons étudié théoriquement l utilisation d un amplificateur quantique non-déterministe en cryptographie quantique. Cet amplificateur possède la propriété de pouvoir amplifier des états quantiques sans en amplifier le bruit quantique associé. Ainsi, nous avons montré qu il permet une amélioration de la distance maximale de transmission d une clé secrète, ainsi qu une amélioration de la résistance au bruit introduit par le canal quantique.This thesis is concerned with different aspects of quantum information with the continuous variables of quantum states of light. Through the combination of the continuous and discrete descriptions, where the light is either described in terms of quadratures or photons, non-classical quantum states and elementary quantum information protocols have been theoretically studied and experimentally implemented. We have experimentally implemented a quantum superposition of two quasi-classical states of light, a Schrödinger cat state , which was used to feed a quantum phase gate. We have analysed the quality of this implementation by using a simple model of the experiment. We have then studied quantum correlations, as captured by the quantum discord, for an important class of states in quantum information. We have compared the precision of our measurements by using the classical and quantum Cramér-Rao bounds. Finally, we have theoretically studied the use of a non-deterministic quantum amplifier in quantum cryptography. This amplifier has the property to amplify quantum states without amplifying their quantum noise. Using this property, we have shown that it is possible to increase the maximum distance of transmission of a secret key, as well as the tolerance to the noise added by the quantum channel.PARIS11-SCD-Bib. électronique (914719901) / SudocSudocFranceF
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