  The advent of quantum optical techniques based on superconducting circuits
has opened new regimes in the study of the non-linear interaction of light with
matter. Of particular interest has been the creation of non-classical states of
light, which are essential for continuous-variable quantum information
processing, and could enable quantum-enhanced measurement sensitivity. Here we
demonstrate a device consisting of a superconducting artificial atom, the
Cooper pair transistor, embedded in a superconducting microwave cavity that may
offer a path toward simple, continual production of non-classical photons. By
applying a dc voltage to the atom, we use the ac Josephson effect to inject
photons into the cavity. The backaction of the photons on single-Cooper-pair
tunneling events results in a new regime of simultaneous quantum coherent
transport of Cooper pairs and microwave photons. This single-pair Josephson
laser offers great potential for the production of amplitude-squeezed photon
states and a rich environment for the study of the quantum dynamics of
nonlinear systems.
