The low-temperature extremes of continuous cooling have long been the domain of dilution refrigerators, exploiting a phase separation in a mixture of helium isotopes to produce cooling power at millikelvin temperatures. The development and demands of quantum computing and quantum technologies have led to now-commonplace commercial cryogen-free dilution refrigerators offering simple access to stable temperatures down to 5 mK.

Recent developments in nanomechanics, novel superconductors and proposals for dark-matter detectors have highlighted a desire for maintaining temperatures below 1 mK for increasingly-long measurement times. While bespoke dilution refrigerators have approached 1 mK, microkelvin temperatures in bulk materials are almost exclusively reached by demagnetising paramagnetic systems which are first pre-cooled by a dilution refrigerator. This is a “single-shot” process of reaching a minimum temperature and warming under parasitic heating with no further cooling power being produced. A system capable of continually maintaining submillikelvin temperatures would be of great benefit to low temperature research and applications, opening the microkelvin temperature range to a host of previously-unfeasible experiments and systems.

In this work we present our progress towards realising a continuous nuclear demagnetisation cooler (CNDR), designed to cool to below 1 mK. This cooler uses two stages of aluminium nuclear refrigerant which are magnetised and pre-cooled by a dilution system and then demagnetised to refrigerate an experimental platform. An array of heat switches controls the thermal links between the refrigerant, platform and host dilution unit. We can program a repeatable process of independently cycling the two aluminium stages and heat switches to maintain a continuous temperature below 1 mK. With a single aluminium stage, running in “single-shot mode” we have measured temperatures on a noise thermometer below 700 micro-kelvin and we report here on running two stages and the heat-switch array needed for continuous operation. Our CNDR interfaces with a standard cryogen-free commercial dilution refrigerator, extending its operating temperature range to the microkelvin regime.

Keywords
Demagnetisation|millikelvin|microkelvin|refrigeration