The process of making a measurement on a quantum-mechanical system introduces quantum noise to that system. A quantum nondemolition measurement scheme seeks to make a measurement of an observable to better than this quantum limit, by feeding all the introduced noise into a conjugate variable to that under consideration. In this paper three quantitative criteria are suggested for the evaluation of nonideal quantum nondemolition measurements. The application of these criteria is illustrated using both a simple beam splitter and the ${\mathrm{\chi }}^3$ interaction in a Raman-active medium.

• Received 21 March 1990

DOI:https://doi.org/10.1103/PhysRevA.42.2995