Long-distance Bell-type tests using energy-time entangled photons

Long-distance Bell-type experiments are presented. The different experimental challenges and their solutions in order to maintain the strong quantum correlations between energy-time entangled photons over more than 10 km are reported and the results analyzed from the point of view of tests of fundamental physics as well as from the more applied side of quantum communication, specially quantum key distribution. Tests using more than one analyzer on each side are also presented.Comment: 22 pages including 7 figures and 5 table

Twin-photon techniques for fiber measurements

The potential of twin photons generated by parametric down-conversion for metrological applications are discussed. We present several experimental results like the measurement of chromatic dispersion and polarization mode dispersion in optical fibers.Comment: 6 pages, 5 figures, Invited paper for the Symposium on Optical Fiber Measurements, sponsored by NIST, Boulder, Co, September 15-17, 199

Leaders of neuronal cultures in a quorum percolation model

We present a theoretical framework using quorum-percolation for describing the initiation of activity in a neural culture. The cultures are modeled as random graphs, whose nodes are excitatory neurons with kin inputs and kout outputs, and whose input degrees kin = k obey given distribution functions pk. We examine the firing activity of the population of neurons according to their input degree (k) classes and calculate for each class its firing probability \Phi_k(t) as a function of t. The probability of a node to fire is found to be determined by its in-degree k, and the first-to-fire neurons are those that have a high k. A small minority of high-k classes may be called "Leaders", as they form an inter-connected subnetwork that consistently fires much before the rest of the culture. Once initiated, the activity spreads from the Leaders to the less connected majority of the culture. We then use the distribution of in-degree of the Leaders to study the growth rate of the number of neurons active in a burst, which was experimentally measured to be initially exponential. We find that this kind of growth rate is best described by a population that has an in-degree distribution that is a Gaussian centered around k = 75 with width {\sigma} = 31 for the majority of the neurons, but also has a power law tail with exponent -2 for ten percent of the population. Neurons in the tail may have as many as k = 4, 700 inputs. We explore and discuss the correspondence between the degree distribution and a dynamic neuronal threshold, showing that from the functional point of view, structure and elementary dynamics are interchangeable. We discuss possible geometric origins of this distribution, and comment on the importance of size, or of having a large number of neurons, in the culture.Comment: Keywords: Neuronal cultures, Graph theory, Activation dynamics, Percolation, Statistical mechanics of networks, Leaders of activity, Quorum. http://www.weizmann.ac.il/complex/tlusty/papers/FrontCompNeuro2010.pd

Non-local two-photon correlations using interferometers physically separated by 35 meters

An experimental demonstration of quantum correlations is presented. Energy and time entangled photons at wavelengths of 704 and 1310 nm are produced by parametric downconversion in KNbO3 and are sent through optical fibers into a bulk-optical (704 nm) and an all-fiber Michelson-interferometer (1310 nm), respectively. The two interferometers are located 35 meters aside from one another. Using Faraday-mirrors in the fiber-interferometer, all birefringence effects in the fibers are automatically compensated. We obtained two-photon fringe visibilities of up to 95 % from which one can project a violation of Bell's inequality by 8 standard deviations. The good performance and the auto-aligning feature of Faraday-mirror interferometers show their potential for a future test of Bell's inequalities in order to examine quantum-correlations over long distances.Comment: 9 pages including 3 postscript figures, to be published in Europhys. Let

On conjugate symplecticity of B-series integrators

The long-time integration of Hamiltonian differential equations requires special numerical methods. Symplectic integrators are an excellent choice, but there are situations (e.g., multistep schemes or energy-preserving methods), where symplecticity is not possible. It is then of interest to study whether the methods are conjugate symplectic and thus have the same long-time behaviour as symplectic methods. This question is addressed in this work for the class of B-series integrators. Algebraic criteria for conjugate symplecticity up to a certain order are presented in terms of the coefficients of the B-series. The effect of simplifying assumptions is investigated. These criteria are then applied to characterize the conjugate symplecticity of implicit Runge-Kutta methods (Lobatto IIIA and Lobatto IIIB) and of energy-preserving collocation method

Experimental Test of Relativistic Quantum State Collapse with Moving Reference Frames

An experimental test of relativistic wave-packet collapse is presented. The tested model assumes that the collapse takes place in the reference frame determined by the massive measuring detectors. Entangled photons are measured at 10 km distance within a time interval of less than 5 ps. The two apparatuses are in relative motion so that both detectors, each in its own inertial reference frame, are first to perform the measurement. The data always reproduces the quantum correlations and thus rule out a class of collapse models. The results also set a lower bound on the "speed of quantum information" to 0.66 x 10^7 and 1.5 x 10^4 times the speed of light in the Geneva and the background radiation reference frames, respectively. The very difficult and deep question of where the collapse takes place - if it takes place at all - is considered in a concrete experimental context.Comment: 4 pages + 2 ps figure

Experimental Quantum Teleportation with a 3-Bell-state Analyzer

We present a Bell-state analyzer for time-bin qubits allowing the detection of three out of four Bell-states with linear optics, two detectors and no auxiliary photons. The theoretical success rate of this scheme is 50%. A teleportation experiment was performed to demonstrate its functionality. We also present a teleportation experiment with a Fidelity larger than the cloning limit of F=5/6.Comment: 11 pages, 14 figure

Femtosecond Time-Bin Entangled Qubits for Quantum Communication

We create pairs of non-degenerate time-bin entangled photons at telecom wavelengths with ultra-short pump pulses. Entanglement is shown by performing Bell kind tests of the Franson type with visibilities of up to 91%. As time-bin entanglement can easily be protected from decoherence as encountered in optical fibers, this experiment opens the road for complex quantum communication protocols over long distances. We also investigate the creation of more than one photon pair in a laser pulse and present a simple tool to quantify the probability of such events to happen.Comment: 6 pages, 7 figure