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

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

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

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

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

Controlling the quantum state of a single photon emitted from a single polariton

We investigate in detail the optimal conditions for a high fidelity transfer from a single-polariton state to a single-photon state and subsequent homodyne detection of the single photon. We assume that, using various possible techniques, the single polariton has initially been stored as a spin-wave grating in a cloud of cold atoms inside a low-finesse cavity. This state is then transferred to a single-photon optical pulse using an auxiliary beam. We optimize the retrieval efficiency and determine the mode of the local oscillator that maximizes the homodyne efficiency of such a photon. We find that both efficiencies can have values close to one in a large region of experimental parameters.Comment: 10 pages, 8 figure