Quantum measurement theory of optical heterodyne detection

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Abstract
An analysis is presented of optical heterodyne detection of an intracavity field within the context of quantum measurement theory. We find the intracavity density operator becomes diagonal in the basis which diagonalizes the measured quantity, a quadrature phase amplitude of the field. This representation is the ‘‘pointer basis,’’ and this result is the continuous-measurement equivalent of state reduction. The model illustrates a general feature of continuous measurement; one parameter given by the product of the system and measuring device coupling bandwidth and the fluctuations in the measuring device completely characterizes the measurement. This constant determines both the rate of diagonalization of the density operator and the rate of growth of fluctuations in the system quantity conjugate to the measured observable.