Squeezing and Feedback

Momčilo M. Gavrilov

Howard M. Wiseman

Electro-optical feedback has many features in common with optical nonlinearities and hence is relevant to the generation of squeezing. First, I discuss theoretical and experimental results for traveling-wave feedback, emphasizing how the ``in-loop'' squeezing (also known as ``squashing'') differs from free squeezing. Although such feedback, based on ordinary (demolition) photodetection cannot create free squeezing, it can be used to manipulate it. Then I treat feedback based on nonlinear quantum optical measurements (of which non-demolition measurements are one example). These {\em are} able to produce free squeezing, as shown in a number of experiments. Following that I discuss theories showing that intracavity squeezing can be increased using ordinary feedback, and produced using QND-based feedback. Finally, I return to ``squashed'' fields and present recent results showing that the reduced in-loop fluctuations can suppress atomic decay in a manner analogous to the effect for squeezed fields.

Menno Poot, King Yan Fong, Hong X. Tang

Using a stabilizing quadrature-feedback scheme the thermal motion of an on-chip opto-electromechanical resonator is squeezed far beyond the limit of classical parametric squeezing. It is shown that feedback on the Y quadrature by itself can already squeeze the thermal motion of the resonator, but the maximum achievable squeezing level is limited by the imprecision noise. By combining the feedback and parametric pumping a record of 15.1 dB of classical noise squeezing is demonstrated. This not only largely exceeds the 3 dB limit for regular squeezing, but is also deeper than ever can be achieved with feedback cooling. The detector-resonator interaction is analyzed within the semi-classical framework and it is shown that using this feedback-stabilized parametric pumping technique true quantum-squeezed states can be prepared when the resonator starts off close to its ground state, and that the ultimate amount of squeezing depends on the minimum detuning that can be achieved.

George A. Brawley, Warwick P. Bowen

Yashuai Han, Juanzhong Qiang

In the paper, the intensity-difference squeezing enhancement from a two-port feedback optical parametric amplifier (TFPA) is theoretically discussed. The performance of squeezing enhancement with the TFPA is compared with that of a one-port feedback optical parametric amplifier (OFPA) and a phase-sensitive optical amplifier. For the ideal case, the infinite squeezing can be achieved for both TFPA and OFPA by manipulating the strength of the feedback and the phase delay induced by the feedback loops. Furthermore, the TFPA can realize the all-phase squeezing enhancement. For the lossy case, the TFPA shows unique advantages in squeezing enhancement against the other two schemes. Our results provide new ideas on the generation of high-quality intensity-difference squeezed light, which may find potential applications in quantum information and quantum metrology.