Recent Developments in Quantum-Circuit Refrigeration

The recent progress in direct active cooling of the quantum-electric degrees of freedom in engineered circuits, or quantum-circuit refrigeration is reviewed. In 2017, the discovery of a quantum-circuit refrigerator (QCR) based on photon-assisted tunneling of quasiparticles through normal-metal-insulator-superconductor junctions inspired a series of experimental studies demonstrating the following main properties: i) the direct-current (dc) bias voltage of the junction can change the QCR-induced damping rate of a superconducting microwave resonator by orders of magnitude and give rise to nontrivial Lamb shifts, ii) the damping rate can be controlled in nanosecond time scales, and ii) the dc bias can be replaced by a microwave excitation, the amplitude of which controls the induced damping rate. Theoretically, it is predicted that state-of-the-art superconducting resonators and qubits can be reset with an infidelity lower than 10(-4) in tens of nanoseconds using experimentally feasible parameters. A QCR-equipped resonator has also been demonstrated as an incoherent photon source with an output temperature above 1 K yet operating at millikelvin. This source has been used to calibrate cryogenic amplification chains. In the future, the QCR may be experimentally used to quickly reset superconducting qubits, and hence assist in the great challenge of building a practical quantum computer.

Automated summary

This article reviews the recent progress in direct active cooling of the quantum-electric degrees of freedom in engineered circuits, known as quantum-circuit refrigeration. Experimental studies have demonstrated the controllability of the damping rate of a superconducting microwave resonator and the ability to reset superconducting qubits in nanosecond timescales using quasiparticles and microwave excitation. The use of quantum-circuit refrigeration as an incoherent photon source with an output temperature above 1 K has also been demonstrated, providing potential applications in calibrating cryogenic amplification chains.