Magnetic cooling of electrons to 311 μK in a cryo-free dilution refrigerator*

Magnetic cooling has the potential to reduce electrons' temperature in a nanostructure far below dilution refrigerator's base temperature. Cooling materials to such levels, analogous to using a more powerful microscope, enables the investigation of interesting novel physics such as exotic quantum phases, new topological quasiparticles and unprecedented quantum coherence in transport experiments.

Here, we employ on- and off-chip cooling and reduce the heat leak during demagnetization by turning off the pulsed tube. We have developed an improved Coulomb blockade thermometer (CBT) with giant Cu reservoirs and greatly enhanced island ground capacitance, giving superior immunity to charge noise and random background charges, providing accurate thermometry down to 100 μK. With these advances, we demonstrate electronic temperatures as low as 311 +/- 7 μK and remain below 1 mK for over 2.5 hours. On-chip coolers and CBTs as thermometers may be integrated with other devices, e.g. semiconductors, opening new avenues for exploring microkelvin nanostructures.