Abstract

Nondegenerate parametric downconversion is used to produce two identical photon trains. Shot-noise fluctuations in one photon train are fed back to reduce the noise in the other to below the shot-noise limit. A watt of 413-nm light illuminates a deuterated potassium dihydrogen phosphate (KD*P) crystal. The intensity of this light is classically stabilized by a feedback loop using an electrooptic modulator (EOM). Apertures are used to select two downconverted beams from the cone of downconverted light produced in the crystal. The aperture positions are adjusted to satisfy the phase matching conditions for 826 ± 50-nm photon pairs. The beams are detected using silicon PIN diodes with low-noise (<1-fA/Hz^1/2) preamplifiers. When the apertures are correctly aligned a high degree of shot-noise correlation is observed between detectors. One of the detector outputs is fed back to the EOM, and the noise in both detectors falls below the shot noise limit.

© 1987 Optical Society of America