A cascade electronic refrigerator using superconducting tunnel junctions

TL;DR

This paper introduces a cascade electronic refrigerator using superconducting tunnel junctions, which improves cooling efficiency at sub-kelvin temperatures by actively evacuating quasiparticles with a second NIS cooling stage.

Contribution

The novel cascade design employs a second NIS stage to thermalize and evacuate quasiparticles, significantly enhancing cooling performance in superconducting electronic refrigerators.

Findings

Approaches ideal theoretical cooling performance.

Effectively evacuates quasiparticles from hot superconductor.

Maintains high cooling power at low temperatures.

Abstract

Micro-refrigerators that operate in the sub-kelvin regime are a key device in quantum technology. A well-studied candidate, an electronic cooler using Normal metal - Insulator - Superconductor (NIS) tunnel junctions offers substantial performance and power. However, its superconducting electrodes are severely overheated due to exponential suppression of their thermal conductance towards low temperatures, and the cooler performs unsatisfactorily - especially in powerful devices needed for practical applications. We employ a second NIS cooling stage to thermalize the hot superconductor at the backside of the main NIS cooler. Not only providing a lower bath temperature, the second stage cooler actively evacuates quasiparticles out of the hot superconductor, especially in the low temperature limit. The NIS cooler approaches its ideal theoretical expectations without compromising cooling power. This cascade design can also be employed to manage excess heat in other cryo-electronic devices.