976 publications from this institution
An upper bound for the fidelity of quantum teleportation explainable by local hidden variables is derived. This bound is larger than the fidelity corresponding to product states, i.e. to local quantum states. This is relevant for the study of mixed states. In particular, the fidelity of Werner's mixed state, known to be larger than the fidelity of product states, is found to be smaller than the fidelity explainable by local hidden variables. Hence the fidelity of Werner's mixed state does not exhibit nonclassical aspects.
Any Bell test consists of a sequence of measurements on a quantum state in spacelike separated regions. Thus, a state is better than others for a Bell test when, for the optimal measurements and the same number of trials, the probability of existence of a local model for the observed outcomes is smaller. The maximization over states and measurements defines the optimal nonlocality proof. Numerical results show that the required optimal state does not have to be maximally entangled.
Entangled coherent states can be prepared remotely by subtracting non-locally a single photon from two quantum superpositions of coherent states, the so-called "Schroedinger's cat" state. Such entanglement can further be distributed over longer distances by successive entanglement swapping operations using linear optics and photon-number resolving detectors. The aim of this paper is to evaluate the performance of this approach to quantum repeaters for long distance quantum communications. Despite many attractive features at first sight, we show that, when using state-of-the-art photon counters and quantum memories, they do not achieve higher entanglement generation rates than repeaters based on single-photon entanglement. We discuss potential developments which may take better advantage of the richness of entanglement based on continuous variables, including in particular efficient parity measurements.
Quantum key distribution (QKD) is stepping out of the lab. We present a commercial fibre-optic QKD-prototype based on faint laser pulses and the results of field tests. Faint laser, single photon and entangled-photon based systems are compared with respect to possible bit rates, detector noise and security and their possible implementation in a commercial apparatus.
We present the optimal collective attack on a quantum key distribution protocol in the "device-independent" security scenario, where no assumptions are made about the way the quantum key distribution devices work or on what quantum system they operate. Our main result is a tight bound on the Holevo information between one of the authorized parties and the eavesdropper, as a function of the amount of violation of a Bell-type inequality.
No abstract is provided for this article.
We compare, from a theoretical and an experimental point of view, the interferometric and the fixed analyzer (also known as the wavelength scanning) techniques for measuring polarization mode dispersion. The information provided by both techniques is shown to be identical, up to a Fourier transform. This information is related to a natural definition of polarization mode delay, /spl Delta//spl tau/. For standard communication fibers, /spl Delta//spl tau/ is itself related to the mean delay between the principal polarization modes, /sub /spl omega//, by a simple numerical factor.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">></ETX>
Optical frequency domain reflectometry (OFDR) measurements with 60 dB dynamics on the 1.55 µm Rayleigh backscattering signal with 2 cm spatial resolution and 10 m maximum range are presented. A sensitivity of –152.5 dB is demonstrated, representing the highest sensitivity reported to date using the OFDR technique. This sensitivity level was achieved with <3 min total acquisition time.
Twenty-five years after the invention of quantum teleportation, the concept of entanglement gained enormous popularity. This is especially nice to those who remember that entanglement was not even taught at universities until the 1990s. Today, entanglement is often presented as a resource, the resource of quantum information science and technology. However, entanglement is exploited twice in quantum teleportation. Firstly, entanglement is the "quantum teleportation channel", i.e., entanglement between distant systems. Second, entanglement appears in the eigenvectors of the joint measurement that Alice, the sender, has to perform jointly on the quantum state to be teleported and her half of the "quantum teleportation channel", i.e., entanglement enabling entirely new kinds of quantum measurements. I emphasize how poorly this second kind of entanglement is understood. In particular, I use quantum networks in which each party connected to several nodes performs a joint measurement to illustrate that the quantumness of such joint measurements remains elusive, escaping today's available tools to detect and quantify it.
In computing the spectra of quantum mechanical systems one encounters the Fourier transforms of time correlation functions, as given by the quantum regression theorem for systems described by master equations. Quantum state diffusion (QSD) gives a useful method of solving these problems by unraveling the master equation into stochastic trajectories; but there is no generally accepted definition of a time correlation function for a single QSD trajectory. In this paper we show how QSD can be used to calculate these spectra directly; by formally solving the equations which arise, we arrive at a natural definition for a two-time correlation function in QSD, which depends explicitly on both the stochastic noise of the particular trajectory and the time of measurement, and which agrees in the mean with the ensemble average definition of correlation functions.
We present a setup for quantum secret sharing using pseudo-GHZ states based on energy-time entanglement. In opposition to true GHZ states, our states do not enable GHZ-type tests of nonlocality, however, they bare the same quantum correlations. The relatively high coincidence count rates found in our setup enable for the first time an application of a quantum communication protocoll based on more than two qubits.
It is well-known that n players, connected only by pairwise secure channels, can achieve unconditional broadcast if and only if the number t of cheaters satis es t &lt; n=3. In this paper, we show that this bound can be improved | at the sole price that the adversary can prevent successful completion of the protocol, but in which case all players will have agreement about this fact. Moreover, a rst time slot during which the adversary forgets to cheat can be reliably detected and exploited in order to allow for future broadcasts with t &lt; n=2. This even allows for secure multi-party computation with t &lt; n=2 after the rst detection of such a time slot.
Several mechanisms that affect one and two photon coherence in optical fibers and their remedies are discussed. The results are illustrated on quantum cryptography experiments and on long distance Bell inequality tests.
An interferometer using Faraday mirrors is presented. It is self-balanced and shows extraordinarily good stability. A quantum cryptographic key has been created using the BB92 protocol1 with a total error rate as low as 0.5 +/- 0.1 %.
