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 the first high-sensitivity spectroscopy experiment using twin beams emitted by a cw optical parametric oscillator: a very weak two-photon absorption 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 reduced by 1.9 dB with respect to the shot-noise limit of the light used in the experiment.