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Optics Communications

Violation of classical probability in parametric down-conversion

X. Y. Zou, L.J. Wang, L. Mandeļ

Published Aug 1, 1991• 30 citations
Publisher

Abstract

A classical inequality relating to photoelectric coincidence counting with two light beams is derived. It is then demonstrated in a coincidence counting experiment with the signal and idler photons produced in the process of parametric down-conversion, that the classical inequality is violated by about 600 standard deviations.
DOI
10.1016/0030-4018(91)90101-i

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A number of optical experiments in which quantum mechanics is tested and the laws of classical probability are violated will be described. These all make use of the phenomenon in non-linear optics known as parametric down-conversion, in which an incident pump photon effectively splits into two correlated, lower frequency signal and idler photons in an entangled quantum state. We show that two-photon coincidence measurements of signal and idler photons violate classical probability by at least 600 standard deviations. Other measurements involving two down-converters lead to the realization of a ″quantum eraser″ and suggest a reinterpretation of the usual quantum wave function.
We investigated statistical fluctuations in the numbers of photons contained in the correlated twin photon beams generated by an optical parametric process. The coincidence between the signal- and idler-photoelectron pulses from a pair of avalanche photo-diodes was measured. The measured fluctuation in the number of signal-photons coinciding with idler-photons was equal to the standard deviation of a binomial distribution and was smaller than that of a Poisson distribution. We determined that even when the quantum efficiency of the photon detectors was much lower than unity, the uncertainty in the number of photons obtained with the twin photon beams was smaller, by a factor of 1/√2, than that with a classical light beam.
We report a new two-photon polarization correlation experiment for realizing the Einstein-Podolsky-Rosen-Bohm (EPRB) state and for testing Bell-type inequalities. We use the pair of orthogonally-polarized light quanta generated in Type 2 parametric down conversion. Using 1 nm interference filters in front of our detectors, we observe from the output of a 0.5mm beta - BaB2O4 (BBO) crystal the EPRB correlations in coincidence counts, and measure an associated Bell inequality violation of 22 standard deviations. The quantum state of the photon pair is a polarization analog of the spin-1/2 singlet state.
This chapter discusses two reported experiments in which the interference between signal and idler photons generated in the process of parametric down-conversion were investigated. Although two-photon interference effects have been treated theoretically before, it analyzes the two experiments in some detail to show how differences between the predictions of classical and quantum theories appear. The chapter compares the predictions of classical and quantum theories with the experimental results obtained. As practically all detectors of light rely on the photoelectric effect, in which a free electron is produced at the expense of a photon, the photon annihilation operator plays a key role in describing the measurement of light in the quantum theory. The time correlation between signal and idler photons produced in the parametric down-conversion process was first measured with a resolving time of about 10 nanoseconds. The interference effect can be made the basis of a measurement of the length of a photon wave packet with femtosecond time resolution.