  The main advantage of quantum metrology relies on the effective use of
entanglement, which indeed allows us to achieve strictly better estimation
performance over the standard quantum limit. In this paper, we propose an
analogous method uti- lizing entanglement for the purpose of feedback control.
The system considered is a general linear dynamical quantum system, where the
control goal can be systemati- cally formulated as a linear quadratic Gaussian
control problem based on the quantum Kalman filtering method; in this setting,
an entangled input probe field is effectively used to reduce the estimation
error and accordingly the control cost function. In par- ticular, we show that,
in the problem of cooling an opto-mechanical oscillator, the
entanglement-assisted feedback control can lower the stationary occupation
number of the oscillator below the limit attainable by the controller with a
coherent probe field and furthermore beats the controller with an optimized
squeezed probe field.
