Repeated Quantum Nondemolition Measurements of Continuous Optical Waves

Repeated quantum nondemolition (QND) measurements for continuous optical waves were performed by combining a degenerate optical parametric amplifier and a squeezed-light beam splitter. Two-step quantum state preparation and three-beam entanglement of the individually squeezed output beams were demonstrated. The experiment is analyzed using a set of generalized criteria for nonideal individual and repeated QND measurements. The system was operated fully stabilized for up to 36 hours. {copyright} {ital 1997} {ital The American Physical Society}