Magnetic Cooling for Nanoelectronics below 1 mK*

Temperatures below 1 mK in quantum transport experiments could open the door for new physics such as novel nuclear spin phases, fragile fractional QH states, topological phases and unprecedented coherence. However, this is a formidable challenge since the thermal coupling becomes weak, making devices susceptible to heat leaks, microwaves and electronic noise. Our approach is to provide a separate nuclear refrigerator in each sample wire, cooling the device through the electronic degree of freedom.
Combining on-and-off chip demagnetization provides cooling of the islands of a Coulomb blockade thermometer as well as the electrical leads connecting to the sample, thus reducing external heat leaks [1]. 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 1.8 ± 0.1 mK. We also present a model which gives a good match and suggests how to overcome the main limitations to cool below 1 mK, thus opening the door for future microkelvin nanoelectronics.
[1] Palma, Scheller et al., Appl. Phys. Lett. 111, 253101 (2017).