Counteracting phase diffusion through a realistic optical parametric oscillator.

We address the use of an optical parametric oscillator (OPO) to counteract phase diffusion and to demonstrate phase-noise reduction for coherent signals traveling through a suitably tuned OPO. We theoretically and experimentally show that there is a threshold value on the phase noise, above which the OPO can be exploited to "squeeze" phase noise. The threshold depends on the energy of the input coherent state, and on the relevant parameters of the OPO (gain and input-output and crystal loss rates). Applications to phase estimation assisted by a realistic OPO and in the presence of phase diffusion are also discussed. Here two scenarios are considered: the first one is the measurement of the phase of a noisy optical field, while the second involves a quantum estimation scheme of a phase shift imposed on a noisy probe. In both cases, we prove that an OPO may lead to a partial or full compensation of the noise.