Cascade Electronic Refrigerator Using Superconducting Tunnel Junctions

Solid-state microrefrigerators operating in the subkelvin regime are key to quantum technology. An electronic cooler based on a normal-metal/insulator/superconductor ($N\phantom{\rule{0}{0ex}}I\phantom{\rule{0}{0ex}}S$) tunnel junction works well in principle, but typically its $S$ electrodes are severely overheated and performance sags\char22{}especially in the powerful devices needed for applications. The authors use a second $N\phantom{\rule{0}{0ex}}I\phantom{\rule{0}{0ex}}S$ cooling stage to thermalize the hot superconductor at the back side of the main cooler. This cascade design could also manage heat in other cryo-electronic devices.