Experimental demonstration of nanoelectronics below 1 mK electron temperature

Extending the experimental temperature domain of nanoelectronic devices into the microkelvin regime would make novel quantum states of matter accessible with great potential for quantum technologies. Cooling bulk metals to microkelvin temperatures by adiabatic demagnetization of nuclear spins has been an established technique for decades, however reaching electron temperatures below 1 mK in nanofabricated devices remained an unsolved challenge up until now. This is due to the thermal decoupling of electrons in micro- and nanostructures from the cold substrate in combination with high frequency electronic noise directly heating the electron system. We demonstrate the first successful nuclear magnetic cooling of electrons in a nanofabricated device to microkelvin temperatures, achieved by integrating indium as nuclear refrigerant onto the metallic islands of a Coulomb blockade thermometer with the device leads attached to bulk indium nuclear cooling stages. By performing the nuclear demagnetization process, we demonstrate electron temperatures below 500 μK by primary electron thermometry, and show that the device stays below 1 mK for several days.