Josephson-Photonics Devices as a Source of Entangled Microwave Photons*

The realization and characterization of efficient sources of entangled microwave photons is of paramount importance for many future applications of quantum technology. Josephson-photonics devices are very promising candidates for this task since they allow one to create a broad range of different entangled states in a surprisingly simple and robust way. These devices consist of a dc-voltage–biased Josephson junction which is placed in series to several microwave cavities. Steady states with multifaceted entanglement properties are reached here naturally due to the interplay of multiphoton creation processes by the Josephson current and subsequent individual photon leakage from the cavities. In this talk, we present a detailed theoretical study of the bipartite entanglement between photon pairs in the output transmission lines. Numerical simulations, taking into account low-frequency fluctuations of the bias voltage and the finite bandwidth of microwave signal detectors, show excellent agreement with recent experimental data.