976 publications from this institution
Polarization mode dispersion (PMD) sets an ultimate limit to bit rate in long-haul transmission systems. So far, the PMD has been measured using two main procedures based on different theoretical approaches. One works in the frequency domain, the other one in the time domain. The first method is based on the evolution of the state of polarization, as a function of the frequency, on the Poincare sphere and measures the differential group delay Δτ between the principal states of polarization (PSPs).1,2
Recently a new Bell inequality has been introduced by Collins et al. [Phys. Rev. Lett. 88, 040404 (2002)], which is strongly resistant to noise for maximally entangled states of two d-dimensional quantum systems. We prove that a larger violation, or equivalently a stronger resistance to noise, is found for a nonmaximally entangled state. It is shown that the resistance to noise is not a good measure of nonlocality and we introduce some other possible measures. The nonmaximally entangled state turns out to be more robust also for these alternative measures. From these results it follows that two von Neumann measurements per party may be not optimal for detecting nonlocality. For $d=3,4,$ we point out some connections between this inequality and distillability. Indeed, we demonstrate that any state violating it, with the optimal von Neumann settings, is distillable.
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.
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 < 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 < n=2. This even allows for secure multi-party computation with t < 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.
No abstract is provided for this article.
Optical time domain reflectometery (OTDR) is one of the most used measurement techniques in the characterization of optical fiber links. In this paper, we present a thorough investigation of an OTDR using a Peltier cooled photon-counting detector at 1.55 um. Due to its superior spatial resolution and the absence of classical dead-zones, it is well suited for detailed, high resolution analysis of problem zones. We give a detailed analysis of its performance, and also demonstrate that a polarization-sensitive photon-counting OTDR can be used to extract local birefringences too large to be measured with standard P-OTDRs.
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.
