Jean Etesse

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.

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.

Multiplexed on-demand storage of polarization qubits in a crystal

A long-lived and multimode quantum memory is a key component needed for the development of quantum communication. Here we present temporally multiplexed storage of 5 photonic polarization qubits encoded onto weak coherent states in a rare-earth-ion doped crystal. Using spin refocusing techniques we can preserve the qubits for more than half a millisecond. The temporal multiplexing allows us to increase the effective rate of the experiment by a factor of 5, which emphasizes the importance of multimode storage for quantum communication. The fidelity upon retrieval is higher than the maximum classical fidelity achievable with qubits encoded onto single photons and we show that the memory fidelity is mainly limited by the memory signal-to-noise ratio. These results show the viability and versatility of long-lived, multimode quantum memories based on rare-earth-ion doped crystals.

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.

Towards highly multimode optical quantum memory for quantum repeaters

Long-distance quantum communication through optical fibers is currently limited to a few hundreds of kilometres due to fiber losses. Quantum repeaters could extend this limit to continental distances. Most approaches to quantum repeaters require highly multimode quantum memories in order to reach high communication rates. The atomic frequency comb memory scheme can in principle achieve high temporal multimode storage, without sacrificing memory efficiency. However, previous demonstrations have been hampered by the difficulty of creating high-resolution atomic combs, which reduces the efficiency for multimode storage. In this article we present a comb preparation method that allows one to increase the multimode capacity for a fixed memory bandwidth. We apply the method to a Eu3+151-doped Y2SiO5 crystal, in which we demonstrate storage of 100 modes for 51 μs using the AFC echo scheme (a delay-line memory) and storage of 50 modes for 0.541 ms using the AFC spin-wave memory (an on-demand memory). We also briefly discuss the ultimate multimode limit imposed by the optical decoherence rate, for a fixed memory bandwidth.

Generating non-classical correlations between photons and spins in a crystal

We perform the experimental generation of pairs of photons on a solid-state rare-earth ion doped crystal of Eu3+:Y2SiO5, by using a DLCZ-like protocol designed for inhomogeneously broadened media. The idea relies on the use of the atomic frequency comb technique, in order to rephase the atoms for the emission of the photons. A specificity of this protocol is its high temporal multimode capacity, as many pairs of photons can be emitted at different instants in time. A Cauchy-Schwarz inequality violation of 2.88>1 is witnessed, proving the non-classical correlations of the photon pairs that we produce. A detailed analysis of the source and detection imperfections is conducted, revealing ways of increasing the quality of the pairs that are produced.

Characterization of the hyperfine interaction of the excited

We characterize the europium (Eu3+) hyperfine interaction of the excited state (5D0) and determine its effective spin Hamiltonian parameters for the Zeeman and quadrupole tensors. An optical free induction decay method is used to measure all hyperfine splittings under a weak external magnetic field (up to 10 mT) for various field orientations. On the basis of the determined Hamiltonian, we discuss the possibility to predict optical transition probabilities between hyperfine levels for the 7F0⟷5D0 transition. The obtained results provide necessary information to realize an optical quantum memory scheme which utilizes long spin coherence properties of 151Eu3+:Y2SiO5 material under external magnetic fields.

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We present the implementation of a synchronized pulse optical cavity for short term storage, and iterative addition of single photons for generating Fock states and Schrodinger cat states in hybrid variable quantum optics domain.

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As they can travel long distances, free space optical quantum states are good candidates for carrying information in quantum information technology protocols. These states, however, are often complex to produce and require protocols whose success probability drops quickly with an increase of the mean photon number. Here we propose a new protocol for the generation and growth of arbitrary states, based on one by one coherent adjunctions of the simple state superposition α|0〉 + β|1〉. Due to the nature of the protocol, which allows for the use of quantum memories, it can lead to high performances.