Physical realization of an environment with squeezed quantum fluctuations via quantum-nondemolition-mediated feedback

Abstract

We show how a squeezed environment can be obtained by means of a suitable feedback of the output signal corresponding to a quantum-nondemolition (QND) measurement of an observable. As an example we show how the variance of a field quadrature of a cavity mode subject to QND-mediated feedback can be squeezed below the standard quantum limit and that this actually means that applying feedback is equivalent to coupling the mode to a squeezed environment