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Observation of two-photon interference in a dual-beam Michelson interferometer

Paul G. Kwiat, William A. Vareka, R. Y. Chiao, C. K. Hong, Howard Nathel

Published May 21, 1990• 0 citations
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Abstract

Nonlocal effects in quantum mechanics associated with entangled states have been the subject of much recent interest.1,2 Franson1 suggested that one can observe a violation of Bell's inequalities for position and time by use of two spatially separated interferometers. We report on the observation of a closely related two-photon interference. A signal photon at frequency ωs and an idler photon at frequency ωi = ωp – ωs are created simultaneously, i.e., as a conjugate pair, in parametric fluorescence inside a KDP crystal from the annihilation of a single UV pump photon with frequency ωp3.

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We have experimentally demonstrated two-photon interference between biphoton amplitudes arising from two well-separated pump pulses. It is important to note the following. 1) The pump pulse intensity was low enough so that single counting rates of the detectors were kept much smaller than pulse repetition rates: the probability of having one spontaneous parametric down conversion (SPDC) photon pair per pulse in our experiment is negligible. Hence the interference cannot be explained as two SPDC photons from two pump pulses (one SPDC photon pair from each pulse) interfering at the detectors. The two pump pulses simply provide two biphoton amplitudes which could result in coincidence counts, and interference occurs between these two biphoton amplitudes only when they are indistinguishable. 2) The BBO crystal in our experiment was only 100 /spl mu/m thick, so the two-pulse did not exist in BBO at the same time at any moment. 3) Due to the delays in the interferometer (/spl tau/ /spl tau//sub 1/), the signal-idler never met at the beamsplitter. Therefore, the existence of two-photon interference cannot be viewed as the interference between the signal and idler photons. These three points again emphasize the fact that it is the indistinguishability of the two-photon Feynman alternatives for the biphoton amplitudes which is responsible for the two-photon quantum interference effects.

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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.

Europhysics Letters (EPL)•Dec 1, 1992•238 citations
Nonlocal effects in quantum mechanics associated with entangled states have been the subject of much recent interest.1,2 Franson1 suggested that one can observe a violation of Bell's inequalities for position and time by use of two spatially separated interferometers. We report on the observation of a closely related two-photon interference. A signal photon at frequency ωs and an idler photon at frequency ωi = ωp – ωs are created simultaneously, i.e., as a conjugate pair, in parametric fluorescence inside a KDP crystal from the annihilation of a single UV pump photon with frequency ωp3.
We report on an interference effect arising from a two-photon entangled state produced in a potassium dihydrogen phosphate (KDP) crystal pumped by an ultraviolet argon-ion laser. Two conjugate beams of signal and idler photons were injected in a parallel configuration into a single Michelson interferometer, and detected separately by two photomultipliers, while the difference in its arm lengths was slowly scanned. The coincidence rate exhibited fringes with a visibility of nearly 50%, and a period given by half the ultraviolet (not the signal or idler) wavelength, while the singles rate exhibited no fringes.
We present experimental demonstrations of two-photon interference involving temporally separated photons within two types of interferometers: a Mach-Zehnder interferometer and a polarization-based Michelson interferometer. The two-photon states are probabilistically prepared in a symmetrically superposed state within the two interferometer arms by introducing a large time delay between two input photons; this state is composed of two temporally separated photons, which are in two different or the same spatial modes. We then observe two-photon interference fringes involving both the Hong-Ou-Mandel interference effect and the interference of path-entangled two-photon states simultaneously in a single interferometric setup. The observed two-photon interference fringes provide simultaneous observation of the interferometric properties of the single-photon and two-photon wavepackets. The observations can also facilitate a more comprehensive understanding of the origins of the interference phenomena arising from spatially bunched/anti-bunched two-photon states comprised of two temporally separated photons within the interferometer arms.
We perform a two-photon interference experiment in a Michelson interferometer with photon pairs produced by spontaneous parametric down-conversion. The interferometer is set to a path difference much larger than the coherence length of the light field. Application of a time-resolved coincidence detection scheme reveals two-photon interference fringes with a visibility of 87%. This high visibility clearly demonstrates the nonclassical nature of the observed interference and should allow further spin-free tests of Bell's inequalities.
We have experimentally demonstrated two-photon interference between biphoton amplitudes arising from two well-separated pump pulses. It is important to note the following. 1) The pump pulse intensity was low enough so that single counting rates of the detectors were kept much smaller than pulse repetition rates: the probability of having one spontaneous parametric down conversion (SPDC) photon pair per pulse in our experiment is negligible. Hence the interference cannot be explained as two SPDC photons from two pump pulses (one SPDC photon pair from each pulse) interfering at the detectors. The two pump pulses simply provide two biphoton amplitudes which could result in coincidence counts, and interference occurs between these two biphoton amplitudes only when they are indistinguishable. 2) The BBO crystal in our experiment was only 100 /spl mu/m thick, so the two-pulse did not exist in BBO at the same time at any moment. 3) Due to the delays in the interferometer (/spl tau/ /spl tau//sub 1/), the signal-idler never met at the beamsplitter. Therefore, the existence of two-photon interference cannot be viewed as the interference between the signal and idler photons. These three points again emphasize the fact that it is the indistinguishability of the two-photon Feynman alternatives for the biphoton amplitudes which is responsible for the two-photon quantum interference effects.
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.