Single Cooper pair photonics

The Josephson junction is the key ingredient in all superconducting quantum circuits, because it is the only nonlinear dissipationless circuit element we know. It can be used either in the superconducting state, where it acts as a nonlinear inductor or in the voltage state, but so far only the superconducting state has been used in quantum circuits.

We use the junction in the voltage state instead, at voltages V below the gap, 2Δ/e, where the junction can become an active nonlinear element, transforming DC power into microwave photons, but still stays dissipationless. In this regime a tunneling Cooper pair has to give away its energy 2eV in order to reach the condensate on the other side of the junction. In the simplest case this energy is emitted into the electromagnetic environment of the junction (described by an impedance Z(ω) in series with the junction) in the form of a single photon at the Josephson frequency 2eV/h. This is the well known ac Josephson effect. In small Josephson junctions, however, quantum fluctuations in the electromagnetic environment can also trigger multiphoton processes where the Cooper pair energy 2eV is divided among two or more photons. In certain environments, these multiphoton processes can even dominate over single photon processes.

We want to tailor the electromagnetic environment of the junction in different ways with the superconducting circuit surrounding the junction and thereby employ the physics of inelastic Cooper pair tunneling to build useful devices for quantum optics with microwave and THz photons. We expect these devices to have complementary properties to existing devices based on the Josephson junction used as nonlinear inductor: