976 publications from this institution
Entanglement is a main resource for various applications in quantum information processing and quantum communication. Increasing complexity of entanglement structure paves the way for both deeper fundamental tests of nature and utterly new methods for quantum information tasks. Quantum memory which is capable to store complex entanglement is essential tool for this direction since it allows to use increased quantum information capacity. Particularly, it is necessary to realize quantum repeater scheme to perform long-distance quantum communication [1]. In this context, quantum memories with high temporal multimode capacity offer a tool to realize efficient multiplexing and open a way for various applications in quantum communication studies.
The quantum state diffusion model describes the evolution of an individual open quantum system by an Itô diffusion equation. In this paper we show two examples of open systems which can be analytically treated, the damped harmonic oscillator and a pumped two-level system with spontaneous emission. The coherent states of the oscillator remain coherent through the stochastic evolution and undergo a classical Ornstein-Uhlenbeck process. We also consider time correlation spectra of the solutions of these stochastic equations.
We discuss three problems of practical quantum cryptography: continuous alignment, nose of the photon counters and eavesdropping. We present a new self-balanced interferometric setup using Faraday mirrors. This phase- coding setup needs no alignment of the interferometer nor polarization control, and therefore considerably facilitates the experiment. Moreover it features excellent fringe visibility. The importance of the detector noise is illustrated and means of reducing it are presented. Maximal distances and bit rates achievable with present day technologies are evaluated. Finally, practical eavesdropping strategies taking advantages of the optical fiber that could open a gate into the transmitter's receiver's offices are discussed.
Network nonlocality allows one to demonstrate nonclassicality in networks with fixed joint measurements, that is, without random measurement settings. The simplest network in a loop, the triangle, with four outputs per party is especially intriguing. The ``elegant distribution'' [Gisin, Entropy 21, 325 (2019)] still resists analytic proofs, despite its many symmetries. In particular, this distribution is invariant under any output permutation. The Finner inequality, which holds for all local and quantum distributions, has been conjectured to be also valid for all no-signaling distributions with independent sources (NSI distributions). Here we provide evidence that this conjecture is false by constructing a four-output network box that violates the Finner inequality and prove that it satisfies all NSI inflations up to the enneagon. As a first step toward the proof of the nonlocality of the elegant distribution, we prove the nonlocality of the distributions that saturates the Finner inequality by using geometrical arguments.
We show that standard nonlocal boxes, also known as Popescu-Rohrlich machines, are not sufficient to simulate any nonlocal correlations that do not allow signalling. This was known in the multipartite scenario, but we extend the result to the bipartite case. We then generalize this result further by showing that no finite set containing any finite-output-alphabet nonlocal boxes with uniform outputs can be a universal set for nonlocality.
No abstract is provided for this article.
Reproducing with elementary resources the correlations that arise when a quantum system is measured (quantum state simulation), allows one to get insight on the operational and computational power of quantum correlations. We propose a family of models that can simulate von Neumann measurements in the x-y plane of the Bloch sphere on n-partite GHZ states using only bipartite nonlocal boxes. For the tripartite and fourpartite states, the models use only bipartite nonlocal boxes; they can be translated into classical communication schemes with finite average communication cost.
No abstract is provided for this article.
No abstract is provided for this article.
No abstract is provided for this article.
Small-core photonic crystal fibers are often strongly birefringent, and consequently lend themselves to polarization maintaining applications or to polarimetric fiber sensors. For both types of use, it is important to have detailed knowledge not only of the fiber's polarization characteristics, but also of its sensitivity to changes in environmental conditions. For the first time, experimental results of the dependence of phase and group delay on temperature are shown for the important 1500-1600 nm wavelength range. Both the polarization characteristics and its temperature dependence are found to be qualitatively different in a photonic crystal fiber compared to a standard PANDA fiber.
