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.