  We propose to generate photon-pair blockade in a Josephson-photonics circuit
that consists of a dc voltage-biased Josephson junction in series with a
superconducting charge qubit and two nondegenerate microwave resonators. The
two-level charge qubit is utilized to create anticorrelations of the charge
transport; that is, the simultaneous Cooper pair tunneling events are
inhibited. When the Josephson frequency matches the sum of resonance
frequencies of the charge qubit and the two resonators, we demonstrate that the
two resonators can release their energies in the form of antibunched pairs of
two strongly correlated photons. The present work provides a practical way for
producing a bright microwave source of antibunched photon pairs, with potential
applications ranging from spectroscopy and metrology to quantum information
processing.
