Julien Niset

Multipartite nonlocality without entanglement in many dimensions

We present a generic method to construct a product basis exhibiting nonlocality without entanglement with n parties each holding a system of dimension at least n-1. This basis is generated via a quantum circuit made of controlled discrete Fourier transform gates acting on the computational basis. The simplicity of our quantum circuit allows for an intuitive understanding of this new type of nonlocality. We also show how this circuit can be used to construct unextendible product bases and their associated bound entangled states. To our knowledge, this is the first method which, given a general Hilbert space H=x{sub i=1}{sup n}H{sub d{sub i}} with d{sub i}{<=}n-1, makes it possible to construct (i) a basis exhibiting nonlocality without entanglement, (ii) an unextendible product basis, and (iii) a bound entangled state.

No-go Theorem for Gaussian Quantum Error Correction

We prove that Gaussian operations are of no use for protecting Gaussian states against Gaussian errors in quantum communication protocols. Specifically, we introduce a new quantity characterizing any single-mode Gaussian channel, called entanglement degradation, and show that it cannot decrease via Gaussian encoding and decoding operations only. The strength of this no-go theorem is illustrated with some examples of Gaussian channels.

Experimentally Feasible Quantum Erasure-Correcting Code for Continuous Variables

We devise a scheme that protects quantum coherent states of light from probabilistic losses, thus achieving the first continuous-variable quantum erasure-correcting code. If the occurrence of erasures can be probed, then the decoder enables, in principle, a perfect recovery of the original light states. Otherwise, if supplemented with postselection based on homodyne detection, this code can be turned into an efficient erasure-filtration scheme. The experimental feasibility of the proposed protocol is carefully addressed.

Tight bounds on the concurrence of quantum superpositions

The entanglement content of superpositions of quantum states is investigated based on a measure called concurrence. Given a bipartite pure state in arbitrary dimension written as the quantum superposition of two other such states, we find simple inequalities relating the concurrence of the state to that of its components. We derive an exact expression for the concurrence when the component states are biorthogonal and provide elegant upper and lower bounds in all other cases. For quantum bits, our upper bound is tighter than the previously derived bound [N. Linden et al., Phys. Rev. Lett. 97, 100502 (2006)].

Superiority of entangled measurements over all local strategies for the estimation of product coherent states

It is shown that the ensemble

Tests of multimode quantum nonlocality with homodyne measurements

\{p (\alpha),|\alpha>|\alpha^*>\}

Network distributed quantum random number generation

where

Random number distribution

p (\alpha)

Continuous-variable quantum erasure correcting code

is a Gaussian distribution of finite variance and

Génération d'un nombre aléatoire quantique réparti sur réseau

| \alpha>