Single-junction quantum-circuit refrigerator

We propose a quantum-circuit refrigerator (QCR) based on photon-assisted quasiparticle tunneling through a single normal-metal-insulator-superconductor (NIS) junction. In contrast to previous studies with multiple junctions and an additional charge island for the QCR, we directly connect the NIS junction to an inductively shunted electrode of a superconducting microwave resonator making the device immune to low-frequency charge noise. At low characteristic impedance of the resonator and parameters relevant to a recent experiment, we observe that a semiclassical impedance model of the NIS junction reproduces the bias voltage dependence of the QCR-induced damping rate and frequency shift. For high characteristic impedances, we derive a Born-Markov master equation and use it to observe significant non-linearities in the QCR-induced dissipation and frequency shift. We further demonstrate that, in this regime, the QCR can be used to initialize the linear resonator into a non-thermal state even in the absence of any microwave drive. (c) 2022 Author(s).

Automated summary

This study proposes a quantum-circuit refrigerator based on photon-assisted quasiparticle tunneling through a single NIS junction. Compared to previous studies using multiple junctions and an additional charge island, this research directly connects the NIS junction to the electrode of a superconducting microwave resonator, making it immune to low-frequency charge noise. The results show that the bias voltage dependence of QCR-induced damping rate and frequency shift can be reproduced by a semiclassical impedance model for low characteristic impedance resonators.