Squeezed states and subshot noise optical detection schemes

The use of second-order nonlinear processes in crystals for producing squeezed light are reviewed. With CW beams, an optical parametric oscillator (OPO) scheme is used, i.e., the nonlinear crystal is placed inside a resonant optical cavity. Below threshold, up to a 4-dB reduction in quantum noise has been obtained, allowing subshot noise measurements of very small optical phase shifts. Above threshold, quantum-correlated beams have been produced, i.e., the quantum fluctuations of the difference between the intensities of two light beams have been strongly reduced. In the pulsed regime, the crystal can be used without a resonant cavity, and wideband pulsed squeezed light has been generated. Practical detection schemes have been implemented, allowing full use of the subshot noise sensitivity.