  Coherent feedback is a non-measurement based, hence a back-action free,
method of control for quantum systems. A typical application of this control
scheme is squeezing enhancement, a purely non-classical effect in quantum
optics. In this paper we report its first experimental demonstration that well
agrees with the theory taking into account time delays and losses in the
coherent feedback loop. The results clarify both the benefit and the limitation
of coherent feedback control in a practical situation.

Quantum Physics