On-and-Off Chip Cooling of a Coulomb Blockade Thermometer down to 2.8 mK*

Reaching ultralow temperatures in electronic transport experiments can facilitate novel quantum matter as well as enhanced quantum coherence. Cooling nanoelectronic devices below 10 mK is a great challenge since thermal conductivities become very small, creating a pronounced sensitivity to heat leaks. Here, we overcome these difficulties by combining on-and-off chip adiabatic demagnetization. This provides direct cooling of the islands of a Coulomb blockade thermometer as well as the electrical leads connecting to the sample, thus reducing the external heat leak. The device comprises a linear array of Al/AlO_x/Al tunnel junctions with huge copper islands in between, serving as spin reservoirs for demagnetization, thus enabling on-chip cooling. This scheme results in a lowest electronic temperature of 2.8 ± 0.1 mK, setting a new record for a nanoelectronic device. We also present a thermal model which gives a good qualitative account and suggests how to overcome the main limitations to cool below 1 mK, thus opening the door for future microkelvin nanoelectronics.