We use a lithium iodate nonlinear crystal as a source of two quantum correlated beams at about 830 nm. A liquid-crystal cell is placed in one of the beams and is electrically driven to produce a very small modulation of its transmission coefficient. Low-noise high-efficiency photodiodes detect the light in each beam and the spectrum of the photocurrents, and the spectrum of the difference between the photocurrents, are measured. The difference spectrum shows a reduction of the noise below the shot-noise level of 4 dB compared with the classical difference technique, where the shot noise in the two detectors simply adds. However, the classical noise in one beam is low enough to make a single-beam shot-noise-limited measurement. Using an optimized difference technique, we show a reduction of the noise level compared to the single-beam shot noise of 2 dB.

• Received 8 January 1991

DOI:https://doi.org/10.1103/PhysRevA.44.3266