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Physical Review A

Experimental evidence for nonclassical fourth-order interferences in the quasielastic light scattering of water

Ulrich K. Krieger, F. Aurich, C. A. Chatzidimitriou‐Dreismann

Published Sep 1, 1995• 4 citations
Publisher

Abstract

Experimental results of quasielastic light scattering on a water-HCl mixture at T=297 K are reported, which demonstrate the existence of a different kind of nonclassical two-photon (fourth-order) correlation, in the absence of second-order interferences. The normalized two-photon correlation function ${\ensuremath{\lambda}}^{(2)}$(1,2) shows an oscillation with an amplitude of about 25%, depending on the relative position of the two detectors and the scattering source. The equivalent measurement on ${\mathrm{CS}}_{2}$ shows no such effect. The possible physical significance of the observations is briefly mentioned.

DOI
10.1103/physreva.52.r1827

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Experimental results of quasielastic light scattering on a water-HCl mixture at T=297 K are reported, which demonstrate the existence of a different kind of nonclassical two-photon (fourth-order) correlation, in the absence of second-order interferences. The normalized two-photon correlation function \ensuremathλ(2){\ensuremath{\lambda}}^{(2)}\ensuremathλ(2)(1,2) shows an oscillation with an amplitude of about 25%, depending on the relative position of the two detectors and the scattering source. The equivalent measurement on CS2{\mathrm{CS}}_{2}CS2​ shows no such effect. The possible physical significance of the observations is briefly mentioned.
Although we tend to think of optical interference as a classical wave phenomenon, recent experiments have revealed a number of effects that are not describable in classical terms. This is particularly true of interference effects involving the detection of a photon pair. We shall refer to them as fourth order interference, on the grounds that the joint probability density for the detection of one photon at r sub 1 at time t and another r sub 2 at time t is proportional to the fourth order correlation function of the field. This probability is readily measured when two photodetectors are positioned at r sub 1 and r sub 2 and the signals from the two detectors are fed to a coincidence counter that registers 'simultaneous' detections by the two detectors in coincidence. The topics covered include: fourth order interference measurements; the Franson experiment; and experimental test of the de Broglie guided wave theory.
The quantum theory of fourth-order interference of light is presented in a general format and compared with classical wave theory. The conditions under which nonclassical phenomena occur are discussed. In particular, the interference between the quantum field and classical field may give rise to a nonclassical effect. For some special states of light, the interference pattern does not disappear even though one field is much stronger than the other, for which no classical analog exists. Fourth-order effects in the interference between two independent fields are analyzed in detail. It is pointed out that the fourth-order interference between independent fields will not disappear when the integration time of detection is of the order of the reciprocal bandwidth of the two light fields as long as the spectra of the two fields are symmetric around the same center frequency, and for some correlated fields, the interference does not vanish even if the detection time is much larger than the reciprocal bandwidth of the fields. A new type of fourth-order interference experiment involving a beam splitter is proposed in which local realism of the Einstein-Podolsky-Rosen form is violated for quantum mechanics. This general argument is then applied to the interference between two photons generated in the parametric down-conversion process. The possibility of violations of Bell's inequalities in interference experiments is investigated.
Water is one of the most prevalent chemicals on our planet, an integral part of both our environment and our existence as a species. Yet it is also rich in anomalous behaviors. Here we reveal that water is a novel-yet ubiquitous-source for quantum correlated photon pairs at ambient conditions. The photon pairs are produced through Raman scattering, and the correlations arise from the shared quantum of a vibrational mode between the Stokes and anti-Stokes scattering events. We confirm the nonclassical nature of the produced photon pairs by showing that the cross-correlation and autocorrelations of the signals violate a Cauchy-Schwarz inequality by over 5 orders of magnitude. The unprecedented degree of violating the inequality in pure water, as well as the well-defined polarization properties of the photon pairs, points to its usefulness in quantum information.
A theoretical treatment is given of the process in which the two photons produced simultaneously in the parametric frequency splitting of light are allowed to interfere. It is shown that, while there is no interference in the usual sense involving quantities that are of the second order in the field, fourth-order interference effects are present. These may be revealed by measuring the joint probability of detecting two photons at two points x,x' in the interference plane with photoelectric detectors as a function of the separation x-x'. The probability exhibits a cosine modulation with x-x', with visibility that can approach 100%, even though the integration time in the experiment may greatly exceed the reciprocal bandwidth of the photons. The interference effect has a nonclassical origin and implies a violation of local realism in the highly correlated two-photon state.
Two methods for studying nonclassical correlations at the output of a two-mode parametric amplifier—first, the measurement of normalized Glauber correlation functions and, second, the measurement of the variance of the photon-number difference in conjugate modes—are compared. It is shown theoretically and experimentally that the corresponding quantities, the normalized correlation function, and the noise reduction factor have significantly different dependences on the main system parameters such as the parametric gain, optical losses, photo-detector quantum efficiency, and the detected angular and frequency bands.
A new fourth-order interference experiment based on detecting photon pairs in coincidence has been carried out. A Mach-Zehnder type of interferometer is used, with the two photons generated simultaneously in the process of down-conversion as inputs and two photon counting detectors at the two outputs. The optical path difference can be varied piezoelectrically. It is found that the coincidence counting rate, after subtraction of accidentals, exhibits a sinusoidal variation with the optical path difference as predicted by quantum mechanics, but in violation of the laws of classical optics. This is another non-classical interference effeet. By contrast, two light beams derived from a He:Ne laser used as inputs to the interferometer, do not produce any fourth order interference.