Towards a Microwave Single Photon Detector Using Inelastic Cooper Pair Tunneling

The detection of single photons is a fundamental quantum measurement, complementary to linear amplification. However, in the microwave domain this is a difficult task due to the low energy of the photons. We present here a photo-multiplier using the energy of a Cooper pair tunneling across a voltage-biased Josephson junction to convert one microwave photon into several photons at a different frequency. This process relies on the strong non-linearity provided by the interaction between a Josephson junction and its high-impedance electromagnetic environment. We have fabricated and measured a device composed of a low critical current SQUID galvanically coupled to two high-impedance resonators. It showed conversion from one to two photons with an efficiency of 80% and also exhibited conversion from one to three photons. By cascading two of these multiplication stages and adding a quantum limited amplifier, it should be possible to discriminate itinerant single photon states from vacuum without dead time.