Abstract

When a frequency stabilized dye laser (linewidth ~1 MHz) is passed through a 15.25-MHz Bragg cell, a Doppler-shifted beam emerges at a small angle. Lenses are used to cross this beam with the unshifted beam to form moving interference fringes at the photocathode of a streak camera. By using a large gain of the streak camera intensifier and an intensified low-background readout of the phosphor, single photons are detectable at the photocathode. This forms a novel Young’s experiment conducted with two resolvable frequency beams passing through one known pinhole each. If the photocathode could discriminate the frequency of the detected photon, and hence which beam it came from, or if either beam were blocked, zero visibility results. The experiment gives a direct measurement of the first-order cross-coherence at single-photon detection levels of two-frequency resolvable beams. By contrast, in quantum beat experiments only the photon probability intensity vs time is measured, and the beats are then attributed to a first-order cross-coherence of the quantum pathways. The question of the frequency spectrum and cross-spectral density of each detected photon is addressed with reference to a time-dependent physical spectrum of light.¹

© 1987 Optical Society of America