Single-junction quantum-circuit refrigerator*

Superconducting quantum circuits constitute one of the most promising platforms for building a scalable quantum computer. One of the acute problem is a fast qubit initialization to the ground state, which in theory can be solved using a quantum-circuit refrigerator (QCR). This device consists of two normal-metal–insulator–superconductor (NIS) junctions controlled by a bias voltage which allows to tune the effective temperature and the dissipation rate of the rest of the circuit. Here, we propose a simpler design of a QCR consisting of a single biased NIS junction galvanically coupled to a cooled system. Using a semiclassical approach we analytically obtain the Lamb shift and the decay rate of a low-impedance resonator caused by the coupling to the QCR. For high-impedance resonators we numerically calculate a response function and show the distinctive features of multiphoton processes which become pronounced in this limit. Such a simple device has surprisingly rich physics and may open a possibility for study of complicated interaction of microwave photons.