Cascade Electronic Refrigerator Using Superconducting Tunnel Junctions

Microrefrigerators that operate in the subkelvin regime are key devices in quantum technology. A well-studied candidate, an electronic cooler using normal-metal-insulator-superconductor (N-I-S) 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 N-I-S cooling stage to thermalize the hot superconductor at the backside of the main N-I-S 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. We demonstrate the apparent advantage of our approach. This cascade design can also be employed to manage excess heat in other cryoelectronic devices.