Rémi Blandino

Implementation of a Nondeterministic Optical Noiseless Amplifier

Quantum Physics imposes that any phase independent amplification introduces excess noise. Nevertheless this limitation could be ignored with conditioning. We report fully characterization with homodyne tomography of a non-deterministic noiseless amplification of a coherent state.

Homodyne Estimation of Gaussian Quantum Discord

We address the experimental estimation of Gaussian quantum discord for a two-mode squeezed thermal state, and demonstrate a measurement scheme based on a pair of homodyne detectors assisted by Bayesian analysis, which provides nearly optimal estimation for small value of discord. In addition, though homodyne detection is not optimal for Gaussian discord, the noise ratio to the ultimate quantum limit, as dictated by the quantum Cramer-Rao bound, is limited to about 10 dB.

Non-Gaussianity of quantum states: An experimental test on single-photon-added coherent states

(Received 18 October 2010; published 28 December 2010)Non-Gaussian states and processes are useful resources in quantum information with continuous variables.An experimentally accessible criterion has been proposed to measure the degree of non-Gaussianity of quantumstates based on the conditional entropy of the state with a Gaussian reference. Here we adopt such a criterionto characterize an important class of nonclassical states: single-photon-added coherent states. Our studiesdemonstrate the reliability and sensitivity of this measure and use it to quantify how detrimental is the roleof experimental imperfections in our implementation.DOI: 10.1103/PhysRevA.82.063833 PACS number(s): 42

Improving the maximum transmission distance of continuous-variable quantum key distribution using a noiseless amplifier

We show that the maximum transmission distance of continuous-variable quantum key distribution in presence of a Gaussian noisy lossy channel can be arbitrarily increased using a heralded noiseless linear amplifier. We explicitly consider a protocol using amplitude and phase modulated coherent states with reverse reconciliation. Assuming that the secret key rate drops to zero for a line transmittance Tlim, we find that a noiseless amplifier with amplitude gain g can improve this value to Tlim/g2, corresponding to an increase in distance proportional to log g. We also show that the tolerance against noise is increased.

Channel purification via continuous-variable quantum teleportation with Gaussian postselection

We present a protocol based on continuous-variable quantum teleportation and Gaussian postselection that can be used to correct errors introduced by a lossy channel. We first show that the global transformation enacted by the protocol is equivalent to an effective system composed of a noiseless amplification (or attenuation), and an effective quantum channel, which can in theory have no loss and an amount of thermal noise arbitrarily small, hence tending to an identity channel. An application of our protocol is the probabilistic purification of quantum non-Gaussian states using only Gaussian operations.

Proposal for a loophole-free violation of Bell's inequalities with a set of single photons and homodyne measurements

We demonstrate that different kinds of mesoscopic quantum states of light can be efficiently generated from a simple iterative scheme. These states exhibit strong non-classical features, and could be of great interest for many applications such as quantum error-correcting codes or fundamental testings. Based on these states, we further propose a protocol allowing a large loophole-free violation of a Clauser-Horne-Shimony-Holt (CHSH)-type Bells inequality with significant losses, thus showing that the quantum properties of some of these states can exhibit a remarkable robustness to losses.

Characterization of a π-phase shift quantum gate for coherent-state qubits

We discuss the characterization of a π-phase shift quantum gate acting on a qubit encoded in superpositions of coherent states. We adopt a technique relying on some a priori knowledge about the physics underlying the functioning of the device. A parameter summarizing the global quality of the quantum gate is obtained by ‘virtually’ processing an entangled state. With such an approach, we can facilitate the characterization of our gate, focusing on the useful subspace rather than on the entire phase space.

Surpassing the no-cloning limit with a heralded hybrid linear amplifier for coherent states

The no-cloning theorem states that an unknown quantum state cannot be cloned exactly and deterministically. However, this limit can be circumvented by abandoning determinism and using probabilistic methods. Here, we report an experimental demonstration of probabilistic cloning of arbitrary coherent states that clearly surpasses the no-cloning limit. Our scheme is based on a hybrid linear amplifier that combines an ideal deterministic linear amplifier with a heralded measurement-based noiseless amplifier. We demonstrate the production of up to five clones with the fidelity of each clone clearly exceeding the corresponding no-cloning limit. Moreover, since successful cloning events are heralded, our scheme has the potential to be adopted in quantum repeater, teleportation and computing applications.

Heralded processes on continuous-variable spaces as quantum maps

Heralding processes, which only work when a measurement on a part of the system give the good result, are particularly interesting for continuous-variables. They permit non-Gaussian transformations that are necessary for several continuous-variable quantum information tasks. However if maps and quantum process tomography are commonly used to describe quantum transformations in discrete-variable space, they are much rarer in the continuous-variable domain. Also, no convenient tool for representing maps in a way more adapted to the particularities of continuous variables have yet been explored. In this paper we try to fill this gap by presenting such a tool.

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.