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arXiv:quant-ph/9703023 · Europhysics Letters (EPL)

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

Wolfgang Tittel, J Brendel, T Herzog, H Zbinden, N Gisin

Published Dec 15, 1997• 15 citations
arXivHTMLPDF

Abstract

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 KNbO 3 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.

DOI
10.1209/epl/i1997-00509-0

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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 KNbO 3 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.
Energy and time entangled photons at a wavelength of 1310 nm are produced by parametric down-conversion in a KNbO3{\mathrm{KNbO}}_{3}KNbO3​ 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.
Summary form only given.In this presentation experimental results of a two-photon interference experiment will be presented and discussed. The two photons, produced by parametric downconversion in a KNbO/sub 3/ crystal, were analyzed with two interferometers physically separated by more than 10 km with the source roughly at the center. The results clearly indicate that the famous spooky action at a distance extends over very large distances. Indeed, from the measured two-photon fringe visibility of 81.6% we infer a violation of the Bell inequality by 10 standard deviations.
A periodically poled MgO – doped LiNbO3 (MgO:LN) non-degenerate photon pair source is utilized for spontaneous parametric down-conversion of 532 nm photons into time-energy entangled pairs of 794 and 1614 nm photons. The entangled photons are separated using previously detailed sorting optics, such that each wavelength is independently directed through one of two modified Mach-Zehnder interferometers – also known as a Franson interferometer – after which they are fiber-optically guided to high-efficiency photon detectors. Output from the detectors is sent to a high resolution time tagger, where coincidences between the entangled photons are recorded. By varying the length of the long path in one Mach-Zehnder interferometer, it is possible to observe high visibility sinusoidal fringes in the measured coincidence rates (while no variation is seen in single photon detection rates). These fringes – due to interference between the photon probability amplitudes – are indicative of a violation of the Bell inequality, and confirm inconsistencies with local hidden variable theory for the correlations of the time-energy entangled photon pairs.
We report the experimental realization of entanglement swapping over long distances in optical fibers. Two photons separated by more than 2km2\phantom{\rule{0.3em}{0ex}}\mathrm{km}2km of optical fibers are entangled, although they never directly interacted. We use two pairs of time-bin entangled qubits created in spatially separated sources and carried by photons at telecommunication wavelengths. A partial Bell-state measurement is performed with one photon from each pair, which projects the two remaining photons, formerly independent onto an entangled state. A visibility high enough to infer a violation of a Bell inequality is reported, after both photons have each traveled through 1.1km1.1\phantom{\rule{0.3em}{0ex}}\mathrm{km}1.1km of optical fiber.
An interference experiment with photon pairs produced by degenerate parametric fluorescence is performed. The pairs are split and directed to two spatially separated Michelson interferometers. The interferometers are set to path differences which cause time delays far exceeding the coherence time of the light. We investigate photon coincidences between the output arms of the interferometers and observe nonlocal fourth-order interferences in the absence of second-order interferences. Maximum visibilities up to 86% are obtained, leading to a violation of Bell's inequality for energy and time by several standard deviations.