Josephson-photonics devices as source of non-classical microwave radiation

Sources of non-classical photons have important applications in quantum communication and sensing technologies. With non-classical microwave sources these are extended to circuit-QED setups extensively used for various quantum information tasks. Here, we report recent experimental result, demonstrating that a dc-biased Josephson junction embedded in a carefully engineered electromagnetic environment constitutes a new source of bright non-classical radiation. We will explain, why in such a ''Josephson-photonics'' device with a single mode of large impedance strongly anti-bunched photons are produced, opening the path to a single-photon source in the microwave range. A Cooper-pair crossing a junction, which is coupled to two resonators, under the proper dc-bias emits a pair of photons into the two resonators and thus produces correlated light with strongly reduced noise. Measurements of this noise reduction factor demonstrate the non-classical nature of the light source.