J. Brendel

Violation of Bell inequalities by photons more than 10 km apart

A Franson-type test of Bell inequalities by photons 10.9 km apart is presented. Energy-time entangled photon pairs are measured using two-channel analyzers, leading to a violation of the inequalities by 16 standard deviations without subtracting accidental coincidences. Subtracting them, a two-photon interference visibility of 95.5% is observed, demonstrating that distances up to 10 km have no significant effect on entanglement. This sets quantum cryptography with photon pairs as a practical competitor to the schemes based on weak pulses.

PULSED ENERGY-TIME ENTANGLED TWIN-PHOTON SOURCE FOR QUANTUM COMMUNICATION

A pulsed source of energy-time entangled photon pairs pumped by a standard laser diode is proposed and demonstrated. The basic states can be distinguished by their time of arrival. This greatly simplifies the realization of 2-photon quantum cryptography, Bell state analyzers, quantum teleportation, dense coding, entanglement swapping, GHZ-states sources, etc. Moreover, the entanglement is well protected during photon propagation in telecom optical fibers, opening the door to few-photon applications of quantum communication over long distances.

Experimental demonstration of quantum correlations over more than 10 km

Energy and time entangled photons at a wavelength of 1310 nm are produced by parametric down-conversion in a

Experimental test of nonlocal quantum correlation in relativistic configurations

{\mathrm{KNbO}}_{3}

LONG-DISTANCE BELL-TYPE TESTS USING ENERGY-TIME ENTANGLED PHOTONS

crystal and are sent into all-fiber interferometers using a telecommunications fiber network. The two interferometers of this Franson-type test of the Bell inequality are located 10.9 km apart from one another. Two-photon fringe visibilities of up to 81.6% are obtained. These strong nonlocal correlations support the nonlocal predictions of quantum mechanics and provide evidence that entanglement between photons can be maintained over long distances.

Measurement of chromatic dispersion in optical fibers using pairs of correlated photons

We report on a new kind of experimental investigations of the tension between quantum nonlocally and relativity. Entangled photons are sent via an optical fiber network to two villages near Geneva, separated by more than 10 km where they are analyzed by interferometers. The photon pair source is set as precisely as possible in the center so that the two photons arrive at the detectors within a time interval of less than 5 ps (corresponding to a path length difference of less than 1 mm). One detector is set in motion so that both detectors, each in its own inertial reference frame, are first to do the measurement! The data always reproduces the quantum correlations, making it thus more difficult to consider the projection postulate as a compact description of real collapses of the wave-function

NON-LOCAL TWO-PHOTON CORRELATIONS USING INTERFEROMETERS PHYSICALLY SEPARATED BY 35 METERS

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, especially quantum key distribution. Tests using more than one analyzer on each side are also presented.

Quantum cryptography using entangled photons in energy-time bell states

Photon pairs generated by spontaneous parametric downconversion are used to implement a novel technique for measuring the chromatic dispersion of optical fibers. The time and energy correlation of the photons of a pair allow a measurement with a high time resolution which is only limited by the detection electronics. The device gives direct access to the differential group delays at several wavelengths around 1.3 μm. Our prototype setup is capable of measuring group delay differences with a resolution of about 5 ps, only limited by the upper delay limit of the time interval analyzer (presently 50 ns).

Experimental test of relativistic quantum state collapse with moving reference frames

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 Bells 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 Bells inequalities in order to examine quantum correlations over long distances.