A ruthenium oxide thermometer for dilution refrigerators operating down to 5 mK

TL;DR

This paper presents a ruthenium oxide thermometer capable of measuring down to 5 mK in dilution refrigerators, addressing sensitivity loss at ultra-low temperatures through RF filtering and analyzing parasitic heating effects.

Contribution

The authors developed a new ruthenium oxide thermometer with enhanced low-temperature sensitivity using RF filtering, and investigated parasitic heating sources affecting its performance.

Findings

RF filter encased with sensor is essential for effectiveness.

Parasitic heating is mainly due to black body radiation.

Thermometer's equilibration time increases sharply below 5 mK.

Abstract

At the lowest temperatures achieved in dilution refrigerators, ruthenium oxide resistance thermometers often saturate and therefore lose their sensitivity. In an effort to extend the range of such temperature sensors, we built a thermometer which maintains sensitivity to 5~mK. A key component of this thermometer is an in situ radio frequency filter which is based on a modern rf absorption material. We show that the use of such a filter is only effective when it is encased in the same rf-tight enclosure as the ruthenium oxide sensor. Our design delivers an attenuation level that is necessary to mitigate the effects of parasitic heating of a fraction of pW present in our circuit. Furthermore, we show that the likely origin of this parasitic heating is the black body radiation present within the experimental space of the refrigerator. We found that the equilibration time of the thermometer increases rapidly as the temperature is lowered; below 5~mK this thermometer becomes impractical because of the prohibitively long equilibration times.