Realization of a paradigm for quantum measurements: The squeezed light beam splitter

A beam splitter is the simplest and dearest example of a coupling device with which quantum limited measurements can be performed. The limitations due to the vacuum fluctuations entering the usually unused port are circumvented by injection of a 3.7 dB squeezed wave into this port. Thereby phase-sensitive measurements that satisfy the quantum nondemolition criteria are realized (signal transfer T-s + T-m = 1.29, 32% correlation). The dependences of the signal transfer and correlation on beam splitter transmissivity are measured and found to be in quantitative agreement with a calculation that takes into account various inefficiencies. Continuous stable operation for 36 hours is achieved.