QUANTUM-NONDEMOLITION MEASUREMENTS IMPROVED BY A SQUEEZED METER INPUT

Continuous quantum nondemolition (QND) measurements of the amplitude q uadrature of a 1064 nm wave are performed with a monolithic dual-port degenerate optical parametric amplifier (OPA). The quality of the meas urement is limited in part by the quantum fluctuations carried by the vacuum meter input wave. Improved QND measurements are realized by inj ecting 3.4 dB amplitude-squeezed light into the meter input port of th e OPA. This achieves increased squeezing and correlation of the output signal and meter waves, and both improved operation as a quantum opti cal tap and as a quantum state preparator. The all-solid-state system was operated for 5 h with high stability.