Quantum-limited amplification based on Josephon photonics*

Superconducting quantum devices usually operate Josephson junctions in the zero-voltage state as nonlinear inductors to form anharmonic oscillators, with strong anharmonicity being used in qubits and weak anharmonicity in wave-mixing processes. In this talk I will show that Josephson junctions can also yield useful quantum devices when operated in the finite voltage state. In this configuration, called Josephson photonics, Cooper pairs tunneling through a junction, biased below the gap at a voltage V, can transfer their energy 2eV into one or more photons in the harmonic modes of the circuit in which the junction is embedded [1]. The junction then acts as a nonlinear drive, which can be used to replace the pump tones that are usually required to operate quantum devices relying on wave mixing, such as parametric amplifiers. Therefore, Josephson photonics devices avoid the hardware overhead of generating, routing, and filtering these pump tones. In this talk, I will discuss how the nonlinearity in Josephson photonics can be controlled via the characteristic impedance of the harmonic modes of the circuit. I will then show how we use weak nonlinearity in Josephson photonics to implement quantum-limited linear amplifiers, which we call Inelastic Cooper-pair Tunneling Amplifiers (ICTAs) [2]. Stong nonlinearity can be used to implement photon-number amplifiers, which multiply photon number by an integer factor and allow for added photon noise below the standard quantum limit, at the cost of deamplifying or losing phase information [3].