Abstract

During recent years, various schemes have been implemented in order to overcome the shot-noise limit in optical measurements. One possibility is to measure the quantum noise of a light beam, using a scheme which evades the “back-action” noise, which is usually added when a quantum measurement is performed. When such a “quantum non-demolition” (QND) measurement is achieved, the measured quantity (e.g., the intensity) is kept unperturbed, and the measurement noise appears on the other quadrature component of the beam (e.g., the phase). Several QND schemes have been demonstrated experimentally, using either solid-state ^{[1, 2]} or atomic non-linear media.^[3]

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