976 publications from this institution
We review the status of Bell's inequalities in quantum information, stressing mainly the links with quantum key distribution and distillation of entanglement. We also prove that for all the eavesdropping attacks using one qubit, and for a family of attacks of two qubits, acting on half of a maximally entangled state of two qubits, the violation of a Bell inequality implies the possibility of an efficient secret-key extraction.
The present a new optical frequency domain reflectometer based on a tunable fiber laser with a very narrow linewidth (about 10 kHz). This instrument performs reflectivity measurements with -110 dB sensitivity and 80 dB dynamic range. The narrow linewidth allows long-range measurements, at 150 m, with a spatial resolution of 16 cm. At short range, about 5 m, the resolution increases to subcentimeter.
Characterization of single-mode optical fibers requires a basis for time, wavelength and polarization.Parametric downconversion in non-linear crystals naturally provide pairs of photons extremelyhighly correlated in time, energy (thus wavelength) and polarization. It is thus tempting to exploresuch photon pairs for fiber and fiber device characterization. Historically, entangled photon pairswhere first used in delicate tests of quantum mechanics [1]. Indeed, their correlation is higher thanclassically possible! However, nowaday photon pair sources can be made cheap and compact enoughto offer practical alternatives for many of the traditional measurement schemes. In addition photonpairs provide entirely new possibilities and open the door to new developments in metrology. Thepurpose of this contribution is first to present some concrete proposals and results along the abovelines, and next to draw the attention of the audience to these rather revolutionary new combinationsof quantum optics and communication.
We analyze various eavesdropping strategies on a quantum cryptographic channel. We present the optimal strategy for an eavesdropper restricted to a two-dimensional probe, interacting on-line with each transmitted signal. The link between safety of the transmission and the violation of Bell's inequality is discussed. We also use a quantum copying machine for eavesdropping and for broadcasting quantum information.
No abstract is provided for this article.
No abstract is provided for this article.
In earlier papers the authors introduced a quantum state diffusion model for the evolution of an individual open quantum system and proved localization theorems based on this model. This paper shows in more detail how the diffusion leads to localization in position and phase space, and to symmetry breaking for chiral molecules. The theory of radioactive decay of absorbers and detectors is described in the state diffusion picture. The Mott and Gurney theory of latent image formation in photography is presented in its state diffusion version. It is an example of quantum detection without significant amplification.
The generation of (Bell-)nonlocal correlations, i.e., correlations leading to the violation of a Bell-like inequality, requires the usage of a nonlocal resource, such as an entangled state. When given a correlation (a collection of conditional probability distributions) from an experiment or from a theory, it is desirable to determine the extent to which the participating parties would need to collaborate nonlocally for its (re)production. Here, we propose to achieve this via the minimal group size (MGS) of the resource, i.e., the smallest number of parties that need to share a given type of nonlocal resource for the above-mentioned purpose. In addition, we provide a general recipe --- based on the lifting of Bell-like inequalities --- to construct MGS witnesses for non-signaling resources starting from any given ones. En route to illustrating the applicability of this recipe, we also show that when restricted to the space of full-correlation functions, non-signaling resources are as powerful as unconstrained signaling resources. Explicit examples of correlations where their MGS can be determined using this recipe and other numerical techniques are provided.
In this Letter we show that quantum nonlocality can be superactivated.That is, one can obtain violations of Bell inequalities by tensorizing a local state with itself.In the second part of this work we study how large these violations can be.In particular, we show the existence of quantum states with very low Bell violation but such that five copies of them give very large violations.In fact, this gap can be made arbitrarily large by increasing the dimension of the states.
The authors develop the tool of quantum diffusion (i.e. Hilbert-space-valued stochastic differential equations) for dissipative quantum systems. The aims are to find possible limitations to this approach and to investigate new pictures of open quantum systems. They are guided by the relaxation process for arbitrary spin and the associated natural rotational symmetry. They also impose the condition that the spin-coherent states remain coherent during the dissipative evolution. They present a new quantum diffusion equation that satisfies the above conditions and that is the unique quantum diffusion satisfying Percival's condition (d psi )2=0.
Get PDF Email Share Share with Facebook Tweet This Post on reddit Share with LinkedIn Add to CiteULike Add to Mendeley Add to BibSonomy Get Citation Copy Citation Text Nicolas Gisin, Sébastien Tanzilli, Wolfgang Tittel, Matthäus Halder, Olivier Alibart, Pascal Baldi, Hugo Zbinden, Gabriel Biener, Avi Niv, Vladimir Kleiner, and Erez Hasman, "Quantum Information," Optics & Photonics News 16(12), 40-41 (2005) Export Citation BibTex Endnote (RIS) HTML Plain Text Citation alert Save article
A generalization of the quantum XOR-gate is presented which operates in arbitrary dimensional Hilbert spaces. Together with one-particle Fourier transforms this gate is capable of performing a variety of tasks which are important for quantum information processing in arbitrary dimensional Hilbert spaces. Among these tasks are the preparation of Bell states, quantum teleportation and quantum state purification. A physical realization of this generalized XOR-gate is proposed which is based on non-linear optical elements.
We review the long history of in physics with special emphasis on the conceptual breakthroughs over the last few years. For the first time it is possible to study nonlocality without signaling {\it from the outside}, that is without all the quantum physics Hilbert space artillery. We emphasize that physics has always given a nonlocal description of Nature, except during a short 10 years gap. We note that the very concept of nonlocality without signaling is totally foreign to the spirit of relativity, the only strictly local theory.
We investigate possible explanations of quantum correlations that satisfy the principle of continuity, which states that everything propagates gradually and continuously through space and time. In particular, following [J.D. Bancal et al, Nature Physics 2012], we show that any combination of local common causes and direct causes satisfying this principle, i.e. propagating at any finite speed, leads to signalling. This is true even if the common and direct causes are allowed to propagate at a supraluminal-but-finite speed defined in a Newtonian-like privileged universal reference frame. Consequently, either there is supraluminal communication or the conclusion that Nature is nonlocal (i.e. discontinuous) is unavoidable.
