Using a method based on the detection of the Kerr phase shift by a self-aligned interferometer, we present measurements of the nonlinear coefficient n/sub 2//A/sub eff/ for standard single-mode fiber (SMF), dispersion-shifted fibers, and dispersion compensating fibers. The presence of a Faraday mirror in the interferometer makes the setup very robust, and different test fibers can be measured without any further readjustments. Interlaboratory comparisons show that the values found with our method are in good agreement with the other ones. Further, analysis of a SMF fiber with large chromatic dispersion shows a good reproducibility of the n/sub 2//A/sub eff/ measurements as a function of fiber length.
John Bell proposed an ontology for the GRW modification of quantum mechanics in terms of flashes occurring at space-time points. The paper spells out the motivation for this ontology, enquires into the status of the wave-function in it, critically examines the claim of its being Lorentz-invariant and considers whether it is a parsimonious, but nevertheless physically adequate ontology.
A new interferometric setup using a 3 x 3 coupler and two Faraday mirrors is presented. It has the advantages of being built only with passive components, of freedom from the polarization fading problem, and of operation with a LED. It is well suited for sensing time-dependent signals and does not depend on reciprocal or nonreciprocal constant perturbations.
The first quantum cryptography protocol, proposed by Bennett and Brassard in 1984 (BB84), has been widely studied in recent years. This protocol uses four states (more precisely, two complementary bases) for the encoding of the classical bit. Recently, it has been noticed that by using the same four states, but a different encoding of information, one can define a protocol which is more robust in practical implementations, specifically when attenuated laser pulses are used instead of single-photon sources [V. Scarani et al., Phys. Rev. Lett. 92, 057901 (2004), referred to as the SARG04 protocol]. We present a detailed study of SARG04 in two different regimes. In the first part, we consider an implementation with a single-photon source: we derive bounds on the error rate $Q$ for security against all possible attacks by the eavesdropper. The lower and the upper bound obtained for SARG04 ($Q\ensuremath{\lesssim}10.95%$ and $Q\ensuremath{\gtrsim}14.9%$, respectively) are close to those obtained for BB84 ($Q\ensuremath{\lesssim}12.4%$ and $Q\ensuremath{\gtrsim}14.6%$, respectively). In the second part, we consider a realistic source consisting of an attenuated laser and improve on previous analysis by allowing Alice to optimize the mean number of photons as a function of the distance. The SARG04 protocol is found to perform better than BB84, both in secret-key rate and in maximal achievable distance, for a wide class of Eve's attacks.
In the late 1960s, a young physicist was sailing along the coast of California towards Berkeley, where he got a post-doc position in astronomy. But his real goal was not astronomy, at least not immediately. First, John Clauser eagerly wanted to test some predictions of quantum theory that were at odds with a then recent and mostly ignored result by an Irish physicist John Stewart Bell, working at the celebrated CERN near Geneva.
Bell's theorem states that, to simulate the correlations created by measurement on pure entangled quantum states, shared randomness is not enough: some 'non-local' resources are required. It has been demonstrated recently that all projective measurements on the maximally entangled state of two qubits can be simulated with a single use of a 'non-local machine'. We prove that a strictly larger amount of this non-local resource is required for the simulation of pure non-maximally entangled states of two qubits |ψ(α)⟩ = cosα|00⟩ + sinα|11⟩ with .
Today's science provides quite a lean picture of time as a mere geometric evolution parameter. I argue that time is much richer. In particular, I argue that besides the geometric time, there is creative time, when objective chance events happen. The existence of the latter follows straight from the existence of free-will. Following the french philosopher Lequyer, I argue that free-will is a prerequisite for the possibility to have rational argumentations, hence can't be denied. Consequently, science can't deny the existence of creative time and thus that time really passes.
An improved "plug & play" interferometric system for quantum key distribution is presented. Self-alignment and compensation of birefringence remain, while limitations due to reflections are overcome. Original electronics implementing the BB84 protocol makes adjustment simple. Key creation with 0.1 photon per pulse at a rate of 325 Hz with a 2.9 percent QBER - corresponding to a net rate of 210Hz - over a 23 Km installed cable was performed.
Increasingly sophisticated quantum computers motivate the exploration of their abilities in certifying genuine quantum phenomena. Here, we demonstrate the power of state-of-the-art IBM quantum computers in correlation experiments inspired by quantum networks. Our experiments feature up to 12 qubits and require the implementation of paradigmatic Bell-State Measurements for scalable entanglement-swapping. First, we demonstrate quantum communication advantages in up to nine-qubit systems while only assuming that the quantum computer operates on qubits. Harvesting these communication advantages, we are able to certify 82 basis elements as entangled in a 512-outcome measurement. Then, we relax the qubit assumption and consider quantum nonlocality in a scenario with multiple independent entangled states arranged in a star configuration. We report quantum violations of source-independent Bell inequalities for up to ten qubits. Our results demonstrate the ability of quantum computers to outperform classical limitations and certify scalable entangled measurements.
We study machines that take N identical replicas of a pure qudit state as input and output a set of M_A clones of a given fidelity and another set of $M_B$ clones of another fidelity. The trade-off between these two fidelities is investigated, and numerous examples of optimal N -> M_A+M_B cloning machines are exhibited using a generic method. A generalisation to more than two sets of clones is also discussed. Finally, an optical implementation of some such machines is proposed. This paper is an extended version of [xxx.arxiv.org/abs/quant-ph/0411179].
No abstract is provided for this article.
No abstract is provided for this article.
