Towards quantum conversion of microwave and optical photons based erbium doped crystals

Diverse systems, such as atoms, photons, and macroscopic superconducting qubits, have shown their own advantages in building parts of a full quantum network. To combine the best features of those sub-systems that work exclusively either in microwave frequencies or optical frequencies, we proposed a quantum converter between microwave photons and optical photons by coupling an erbium doped crystal to both a microwave and optical cavity. Unit efficiency conversion can be achieved so long as the product of the optical and microwave cooperativity factors can be made large. An efficiency of photon number conversion of 10^-5 is demostrated in our experiment. Other applications of erbium doped crystals in a three-dimensional high-Q superconducting cavity at zero magnetic field are also discussed.