Squeezing Intensity Noise on Laser-like Beams

Abstract

Parametric down-conversion generates beams having strong quantum correlations. This property can be easily understood by reasoning in terms of photons : let us consider a non-linear crystal irradiated by light at frequency ω0. For each pump photon absorbed at frequency ω0, it simultaneously emits two “twin” signal photons with frequencies ω1 and ω2 (such that ω0 = ω1 + ω2). Such non-classical correlations have been observed by photon coincidence techniques |1|. However, with the available cw lasers and non-linear crystals, the parametric process generates only very weak correlated beams (typically a few photons/second), because the pump power is spread into an infinity of twin modes. To concentrate the output energy into a few pair of modes only, the non-linear crystal can be inserted in an optical cavity having mirrors with a high reflectivity for the signal frequencies ω1 and ω2. Above some pump power threshold, the system can oscillate and yields a pair of intense, laser-like, “twin” beams.