Abstract

Quantum nondemolition (QND) measurements enable quantum limited information on a beam of light to be detected with minimal back-action. The light beam and signal therefore remain intact after the measurement is complete. One QND technique is to combine the information carrying beam with a strongly squeezed state at a beamsplitter [1] . The two outputs then constitute meter and signal output beams. We show experimental QND results obtained using the strongly squeezed light from our optical parametric amplifier (OPA). Further experiments show how this system can be improved significantly by using the photocurrent of the meter beam to control the signal output beam. We confirm our predictions [2] and prove the reliability of the combined squeezing/feed-forward technique which is a key component of future quantum information experiments such as teleportation.

© 2000 IEEE