Single-junction quantum-circuit refrigerator

TL;DR

This paper introduces a quantum-circuit refrigerator (QCR) based on photon-assisted quasiparticle tunneling through a single NIS junction, which is immune to low-frequency charge noise and can initialize resonators into non-thermal states.

Contribution

The work presents a simplified QCR design with galvanic connection to a superconducting resonator, analyzing its behavior with a semiclassical model and a Born-Markov master equation, revealing non-linearities and state initialization capabilities.

Findings

QCR induces bias-dependent damping and frequency shifts.

High impedance regimes show significant non-linearities.

QCR can prepare non-thermal states without microwave drive.

Abstract

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 works with multiple junctions and an additional charge island for the QCR, we galvanically 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.