On-and-off chip cooling of a Coulomb blockade thermometer down to 2.8 mK

Cooling nanoelectronic devices below 10 mK is a great challenge since thermal conductivities become very small, thus creating a pronounced sensitivity to heat leaks. Here, we overcome these difficulties by using adiabatic demagnetization of both the electronic leads and the large metallic islands of a Coulomb blockade thermometer. This reduces the external heat leak through the leads and also provides on-chip refrigeration, together cooling the thermometer down to 2.8 ± 0.1 mK. We present a thermal model which gives a good qualitative account and suggests that the main limitation is heating due to pulse tube vibrations. With better decoupling, temperatures below 1 mK should be within reach, thus opening the door for μK nanoelectronics.
Reaching ultralow temperatures in electronic transport experiments can be key to novel quantum states of matter such as helical nuclear spin phases,1–3 full nuclear spin polarization,4 quantum Hall ferromagnets,4 or fragile fractional quantum Hall states.5,6 In addition, the coherence of semiconductor and superconducting qubits7–9 as well as hybrid Majorana devices10–13 could benefit from lower temperatures. With this motivation in mind, we built a parallel network of nuclear refrigerators14 to adapt the very well established technique of Adiabatic Nuclear Demagnetization (AND)15–17 for electronic transport experiments. In this approach, the concept is to cool a nanoelectronic device directly through the electronic leads, which remain effective thermal conductors also below 1 mK. Each wire is cooled by its own, separate nuclear refrigerator in the form of a large Cu plate. However, despite recent progress,18–25 it remains very challenging to cool nanostructures even below 10 mK. Due to reduced thermal coupling, these samples are extremely susceptible to heat leaks such as vibrations,25 microwave radiation,26,27 heat release,17 and electronic noise.20
Metallic Coulomb blockade thermometers (CBTs) have been established as precise and reliable electronic thermometers,18,28,29 operating down to 10 mK and slightly below.21,22,24,30 These typically consist of linear arrays of Al/AlOx/Al tunnel junctions with metallic islands in-between, consisting mainly of copper, see Fig. 1. The array divides the electronic noise per island by the number of junctions in series. This makes them less susceptible to electronic noise, but thermal conduction via Wiedemann-Franz cooling is not very effective through a series of resistive tunnel junctions. For this reason, the islands were enlarged into giant cooling fins,29 providing a huge volume for effective electron-phonon coupling and cooling through the substrate. At low temperatures, however, this eventually fails due to the very strong T5 temperature dependence of the electron phonon coupling.