  We develop a general framework to describe interferometric coherent feedback
loops and prove that, under any such scheme, the steady-state squeezing of a
bosonic mode subject to a rotating wave coupling with a white noise environment
and to any quadratic Hamiltonian must abide by a noise-dependent bound that
reduces to the 3dB limit at zero temperature. Such a finding is contrasted, at
fixed dynamical parameters, with the performance of homodyne continuous
monitoring of the output modes. The latter allows one to beat coherent feedback
and the 3dB limit under certain dynamical conditions, which will be determined
exactly.
