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Quantum Electronics and Laser Science Conference

Splitting of single photons by an ordinary beamsplitter

J. D. Franson

Published Jun 7, 1996• 0 citations
OpenAlex

Abstract

Summary form only given. The author has recently demonstrated a metallic beamsplitter and ultraviolet photons from an argon ion laser. The main experimental difficulty is the high rate of accidental counts that result from fluorescence and similar effects. The accidental rate is proportional to the square of the incident laser intensity, while the effect of interest is proportional to the intensity itself. Thus coincidence measurements at sufficiently low incident intensities can reduce the accidental events to an acceptably small level. Roughly one event per day was observed in which an incident photon was split into two secondary photons by a metallic beamsplitter.

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Summary form only given. The author has recently demonstrated a metallic beamsplitter and ultraviolet photons from an argon ion laser. The main experimental difficulty is the high rate of accidental counts that result from fluorescence and similar effects. The accidental rate is proportional to the square of the incident laser intensity, while the effect of interest is proportional to the intensity itself. Thus coincidence measurements at sufficiently low incident intensities can reduce the accidental events to an acceptably small level. Roughly one event per day was observed in which an incident photon was split into two secondary photons by a metallic beamsplitter.
The quantum mechanical description of parametric fluorescence is the splitting of a single photon into two photons. This description has been verified by observing coincidences between photons emitted by an ammonium dihydrogen phosphate crystal pumped by a 325-nm He-Cd laser. The coincidence rate RC{R}_{C}RC​ decreases to the calculated accidental rate [<0.03RC(max)<0.03{R}_{C}(max)<0.03RC​(max)], unless the two detectors are arranged to satisfy energy and momentum conservation and have equal time delays.
While the classical, wavelike behavior of light (interference and diffraction) has been easily observed in undergraduate laboratories for many years, explicit observation of the quantum nature of light (i.e., photons) is much more difficult. For example, while well-known phenomena such as the photoelectric effect and Compton scattering strongly suggest the existence of photons, they are not definitive proof of their existence. Here we present an experiment, suitable for an undergraduate laboratory, that unequivocally demonstrates the quantum nature of light. Spontaneously downconverted light is incident on a beamsplitter and the outputs are monitored with single-photon counting detectors. We observe a near absence of coincidence counts between the two detectors—a result inconsistent with a classical wave model of light, but consistent with a quantum description in which individual photons are incident on the beamsplitter. More explicitly, we measured the degree of second-order coherence between the outputs to be g(2)(0)=0.0177±0.0026, which violates the classical inequality g(2)(0)⩾1 by 377 standard deviations.
The reported experiment is a two-photon optical imaging experiment1 in which a spontaneous-parametric-downconversion light beam, which consists of pairs of orthogonally polarized signal and idler photons, is split into two diverging beams by a polarization beam splitter, so that co- incidence detections may be performed between two distant photon-counting detectors. An aperture (mask) placed in front of one of the detectors is illuminated by the signal beam through a convex lens. Surprisingly, an image of this aperture is observed in the coincidence counting rate when the other detector is scanned in the transverse plane of the idler beam, even though both detectors’ single counting rates remain constant.
The relation between the two-quantum photocounting distribution p(n, T) and the intensity fluctuations of the radiation incident on the detector is obtained and compared with the one-quantum results. The actual statistics are evaluated for several cases of interest, including a chaotic source, an amplitude-stabilized wave, and the output of a single-mode laser (Van der Pol oscillator). The two-quantum count rate 〈n〉/T is found to depend on the mean-square intensity of the radiation, in contrast to the one-quantum count rate which is proportional to the mean intensity. The effects of photon correlations in the radiation beam become apparent since the two-quantum distributions manifest a distinctly more positive second derivative than the corresponding one-quantum distributions. Both the low-and the high-count probabilities are therefore increased at the expense of counts near the mean. The quantum-theoretical treatment is found to be equivalent to the semiclassical treatment for density operators possessing a positive-definite weight function in the P-representation. Some possible experiments to verify the theory are discussed and shown to be feasible.