SUB-SHOT-NOISE HIGH-SENSITIVITY SPECTROSCOPY WITH OPTICAL PARAMETRIC OSCILLATOR TWIN BEAMS

Nondegenerate optical parametric oscillators generate above-threshold signal and idler beams that have intensity fluctuations correlated at the quantum level (twin beams). We describe what is to our knowledge t he first high-sensitivity spectroscopy experiment using twin beams emi tted by a cw optical parametric oscillator: a very weak two-photon abs orption signal, in the 10(-7) range, is recorded on the 4S(1/2)-5S(1/2 ) transition of atomic potassium with a noise background that is reduc ed by 1.9 dB with respect to the shot noise limit of the light used in the experiment. (C) 1997 Optical Society of America.