Proposal for an Optomechanical Microwave Sensor at the Subphoton Level
  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. 
Keye Zhang, Francesco Bariani, Ying Dong, Weiping Zhang, Pierre   Meystre
quant-ph