  With a levitodynamics experiment in the strong and coherent quantum
optomechanical coupling regime, we demonstrate that the oscillator acts as a
broadband quantum spectrum analyzer. The asymmetry between positive and
negative frequency branches in the displacement spectrum traces out the
spectral features of the quantum fluctuations in the cavity field, which are
thus explored over a wide spectral range. Moreover, in our two-dimensional
mechanical system the quantum back-action, generated by such vacuum
fluctuations, is strongly suppressed in a narrow spectral region due to a
destructive interference in the overall susceptibility.
