We report experimental demonstration of the feasibility of reaching temperatures below 1 mK using cryogen-free technology. Our prototype system comprises an adiabatic nuclear demagnetization stage, based on hyperfineenhanced nuclear magnetic cooling, integrated with a commercial cryogenfree dilution refrigerator and 8 T superconducting magnet. Thermometry was provided by a current-sensing noise thermometer. The minimum temperature achieved at the experimental platform was 600 µK. The platform remained below 1 mK for over 24 h, indicating a total residual heat-leak into the experimental stage of 5 nW. We discuss straightforward improvements to the design of the current prototype that are expected to lead to enhanced performance. This opens the way to widening the accessibility of temperatures in the microkelvin regime, of potential importance in the application of strongly correlated electron states in nanodevices to quantum computing.

We describe the design and performance of a series of fast, precise current sensing noise thermometers. The thermometers have been fabricated with a range of resistances from 1.290 Ω down to 0.2 mΩ. This results in either a thermometer that has been optimised for speed, taking advantage of the improvements in superconducting quantum interference device (SQUID) noise and bandwidth, or a thermometer optimised for ultra-low temperature measurement, minimising the system noise temperature. With a single temperature calibration point, we show that noise thermometers can be used for accurate measurements over a wide range of temperatures below 4 K. Comparisons with a melting curve thermometer, a calibrated germanium thermometer and a pulsed platinum nuclear magnetic resonance thermometer are presented. For the 1.290 Ω resistance we measure a 1 % precision in just 100 ms, and have shown this to be independent of temperature.

The use of low-temperature platforms with base temperatures below 1 K is rapidly expanding, for fundamental science, sensitive instrumentation and new technologies of potentially significant commercial impact. Precise measurement of the thermodynamic temperature of these low-temperature platforms is crucial for their operation. In this paper, we describe a practical and user-friendly primary current-sensing noise thermometer (CSNT) for reliable and traceable thermometry and the dissemination of the new kelvin in this temperature regime. Design considerations of the thermometer are discussed, including the optimization of a thermometer for the temperature range to be measured, noise sources and thermalization. We show the procedure taken to make the thermometer primary and contributions to the uncertainty budget. With standard laboratory instrumentation, a relative uncertainty of 1.53% is obtainable. Initial comparison measurements between a primary CSNT and a superconducting reference device traceable to the PLTS-2000 (Provisional Low Temperature Scale of 2000) are presented between 66 and 208 mK, showing good agreement within the k=1 calculated uncertainty.

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