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Physical Review A

Probing the two-photon phase coherence of parametrically down-converted photons by a local oscillator

Masato Koashi, K. Kono, Masaya Matsuoka, Takuya Hirano

Published Nov 1, 1994• 16 citations
Publisher

Abstract

A degenerate beam of down-converted photon pairs from a nonlinear crystal pumped by pulsed light was mixed with coherent local oscillator pulses by a beam splitter, and the coincidence rate for detections of photons at the two output beams was measured. The coincidence rate showed interference fringes as the phase difference between the pump light and the local oscillator was varied. It showed a negative correlation in some range of the phase difference.

DOI
10.1103/physreva.50.r3605

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A degenerate beam of down-converted photon pairs from a nonlinear crystal pumped by pulsed light was mixed with coherent local oscillator pulses by a beam splitter, and the coincidence rate for detections of photons at the two output beams was measured. The coincidence rate showed interference fringes as the phase difference between the pump light and the local oscillator was varied. It showed a negative correlation in some range of the phase difference.
A different way of producing antibunched light was experimentally demonstrated in which the production process is difficult to understand by a naive photon picture. A degenerate beam of down-converted photon pairs from a nonlinear crystal pumped by pulsed light was mixed with coherent local oscillator pulses by a beam splitter, and the intensity autocorrelation of one of the output beams was measured. It showed two-photon interference fringes as the phase difference of the two input fields was varied, and showed antibunching when the interference was destructive. \textcopyright{} 1996 The American Physical Society.
A proposed experiment is analyzed theoretically. In the proposed experiment two coherent pump waves fall on two identical nonlinear crystals, down-converted signal and idler beams from the two crystals are mixed by two beam splitters, and the coincidence counting rate for photons leaving the beam splitters is measured. We show that this counting rate depends on the phase difference between the two coherent pump waves, and results from the interference of the vacuum with the down-converted photons. The experiment could be used to look for locality violations along the lines recently proposed by Grangier, Potasek, and Yurke [Phys. Rev. A 38, 3132 (1988)], but without the need for a coherent reference beam for homodyning.
A two-photon state can be selected by coincidence detection of photon pairs created in parametric downconversion. We demonstrate a reduction in the coincidence-count rate when pairs of photons are combined in a beam splitter. This reduction occurs only when photon wave functions overlap and can permit accurate (subpicosecond) relative timing measurements. We relate the width of the coincidence reduction to the bandwidth of the downconverted light and thence to the detailed phase-matching conditions in the parametric downconversion crystal. We also demonstrate that the loss of coincidences is associated with an increased probability of detection of photon pairs in each output of the beam splitter.
We study the Hong-Ou-Mandel interferometer in the regime of spontaneous parametric down-conversion with high pump beam power at the crystal. In this regime one and two photons from a pump pulsed laser beam generate one and two pairs of photons, respectively. These photons are then directed to the beam splitter of the interferometer and detected at its exit in coincidence. An interesting phenomenon is observed: The reduction of the visibility of the Hong-Ou-Mandel coincidence peak (or dip) with the increase of pump power. We study the relation between the visibility of the fourth-order interference pattern and the power of the pumping laser beam for type I and type II phase-matching crystals. Our theoretical calculations are in good agreement with the experimental results.
An analysis is presented of the interference between two photon pairs produced in the process of downconversion from two identical crystals, which are then mixed by beam splitters. Because the downconverted photons are entangled with the vacuum, they carry information about the phase of the pump field. As a result the two-photon detection probability is shown to depend on the phase of the pump, even when the resolving time of the detector electronics is far longer than the coherence time of the light.