Quantum correlated twin beams in cascaded optical parametric oscillator

We experimentally generate twin beams through cascaded quadratic processes in a nonlinear χ⁽²⁾ crystal, leading to an internally pumped optical parametric oscillation, in a doubly resonant second-harmonic generation system. The exploration of the non-classical properties of these beams enabled the observation of up to 5 dB of noise reduction in their intensity difference below the standard quantum limit.