Time-delayed quantum coherent Pyragas feedback control of photon   squeezing in a degenerate parametric oscillator
  Quantum coherent feedback control is a measurement-free control method fully preserving quantum coherence. In this paper we show how time-delayed quantum coherent feedback can be used to control the degree of squeezing in the output field of a cavity containing a degenerate parametric oscillator. We focus on the specific situation of Pyragas-type feedback control where time-delayed signals are fed back directly into the quantum system. Our results show how time-delayed feedback can enhance or decrease the degree of squeezing as a function of time delay and feedback strength. 
Manuel Kraft, Sven Moritz Hein, Judith Lehnert, Eckehard Sch\"oll,   Stephen Hughes, and Andreas Knorr
quant-ph