  Quantum correlations between imprecision and back-action are a hallmark of
continuous linear measurements. Here we study how measurement-based feedback
can be used to improve the visibility of quantum correlations due to the
interaction of a laser field with a nano-optomechanical system. Back-action
imparted by the meter laser, due to radiation pressure quantum fluctuations,
gives rise to correlations between its phase and amplitude quadratures. These
quantum correlations are observed in the experiment both as squeezing of the
meter field fluctuations below the vacuum level in a homodyne measurement, and
as sideband asymmetry in a heterodyne measurement, demonstrating the common
origin of both phenomena. We show that quantum feedback, i.e. feedback that
suppresses measurement back-action, can be used to increase the visibility of
the sideband asymmetry ratio. In contrast, by operating the feedback loop in
the regime of noise squashing, where the in-loop photocurrent variance is
reduced below the vacuum level, the visibility of the sideband asymmetry is
reduced. This is due to feedback back-action arising from vacuum noise in the
homodyne detector. These experiments demonstrate the possibility, as well as
the fundamental limits of measurement-based feedback as a tool to manipulate
quantum correlations.
