A quantum measurement usually perturbs the quantity being measured by adding noise in the system under study. The purpose of a quantum nondemolition (QND) measurement is to control this back-action noise by feeding it back exclusively into the observable complementary to the one being measured. Quantum optics is a particularly well-suited domain for performing QND measurements because quantum-noise-limited sensitivity can easily be achieved and good nonlinear media are available.

© 1994 Optical Society of America

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