Theory of environment engineering for superconducting circuits by photon-assisted electron tunneling

Aalto Quantum Physics Seminar (Nanotalo). Speaker: Dr. Matti Silveri (QCD, Aalto University, Finland).

Tunable environments have applications in fast and accurate initialization of quantum devices as well as in stabilizing interesting quantum states. We consider a tunable broadband environment for superconducting quantum circuits formed by a photon-assisted electron tunneling in a capacitively coupled superconductor--insulator--normal-metal--insulator--superconductor (SINIS) junction.  When the voltage bias of the junction is below the energy gap of the superconductor, the normal-metal electron needs to receive an additional energy from the coupled electric circuit to overcome the gap. By considering these photon-assisted tunneling processes, we provide a first-principles derivation of the coupling strength and temperature of the environment.  The developed model accurately describes the physics of the quantum-circuit refrigerator and the microwave absorption and emission. We are also able to capture fine details, such as multi-photon processes and the broadband Lamb shift.