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Quantum Electronics and Laser Science Conference

Why biphoton but not two photons?

Dmitry Strekalov, T. B. Pittman, Morton H. Rubin, Yanhua H. Shih

Published May 18, 1997• 0 citations
Publisher

Abstract

What is the difference between two-photon phenomena and phenomena involving two photons? We address this question by reporting two two-photon interference experiments. In the first experiment we show that two-photon interference does not require the “indistinguishability” of two photons, and hence is not interference of two photons.
DOI
10.13016/m2pqus-vszl

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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.