  Because of their low energy content, microwave signals at the single-photon
level are extremely challenging to measure. Guided by recent progress in
single-photon optomechanics and hybrid optomechanical systems, we propose a
multimode optomechanical transducer that can detect intensities significantly
below the single-photon level via adiabatic transfer of the microwave signal to
the optical frequency domain where the measurement is then performed. The
influence of intrinsic quantum and thermal fluctuations is also discussed.
