Towards microwave photon counting via frequency up-conversion

We have demonstrated nonlinear optical up-conversion of 100 GHz signal into optical range using high-Q whispering gallery mode (WGM) resonators with quadratic nonlinearity. This approach has allowed us to achieve the record power conversion efficiency of 0.5% with as low as 16 mW of CW optical pump power. We furthermore show that by implementing the WGM resonator for the microwave as well as for the optical modes, the unity photon-number conversion efficiency can be in principle achieved. Combining such a converter with a state-of-the-art optical photon counter it will be possible to count the microwave photons whose frequency is as low as 0.1 THz, at room temperature.