Speaker
Description
Sub-millikelvin (sub-mK) temperatures, or even lower, were first achieved in 1956 by adiabatic demagnetization cooling of nuclear spins in copper [1]. Since then, the use of such extremely low temperatures has long been limited to researchers in fundamental science such as superfluid 3He and nuclear magnetism [2]. In general, lowering temperature have their own advantages, e.g., improved measurement resolution due to reduced thermal noise and protection of pure (quantum) states from thermal disturbances. The recent rise of millikelvin environments down to 10–100 ｍK obtained by adiabatic demagnetization refrigerators (ADRs) using electronic spins or 3He-4He dilution refrigerators (DRs) for the development of quantum computers and quantum sensors is utilizing these advantages. If so, even lower temperatures may represent a new frontier for quantum technologies in the future.
Here we present the first successful construction and test results of a continuous sub-mK refrigerator capable of continuously maintaining a base temperature down to 0.57 mK with cooling powers of 19 nW and 290 nW at 1 mK and 5 mK, respectively. The continuous refrigeration mechanism [3] is based on independent demagnetization cooling of two nuclear magnetic refrigerants of PrNi5, a hyperfine enhanced nuclear magnet [2], with two zinc superconducting heat switches [4]. Precooling of this continuous nuclear demagnetization refrigerator (cNDR) is achieved by a commercially available cryogen-free DR. The size of the cNDR is 156W×84D×240H (mm each), excluding the Pt-wire NMR thermometer used for test cooling, and the total weight is currently less than 5 kg. Due to its compactness, the cNDR can be mounted on most DRs and possibly on the extended version of ADR without introducing any magnetic disturbance to the nearby measurement setup.
Our cNDR is a nuclear spin version of the cADR developed by NASA [5], which can maintain 50 mK and is operating successfully in space. However, due to the extreme temperature environment, we had to solve several technical difficulties for cNDR by developing a new type of thermal insulation support for the coolant, a compact shielded superconducting magnet (1.3 T) [6] and its flexible thermal link, thermalization techniques for the PrNi5 coolant [7], etc. Some of these will also be useful for other cryogenic devices at higher temperatures. In my presentation, I will provide details of the development and test results of this first continuous sub-mK refrigerator.
[1] N. Kurty et al., Nature 178, 450 (1956).
[2] F. Pobell, Matter and Methods at Low Temperatures, 3rd edn. (Springer, Berlin, 2007).
[3] R. Toda et al., J. Phys.: Conf. Ser. 969, 012093 (2018).
[4] R. Toda et al., arXiv:2209.08260v1.
[5] P. J. Shirron et al., Cryogenics 74, 2 (2016).
[6] S. Takimoto et al., J. Low Temp. Phys. 201, 179 (2020).
[7] S. Takimoto et al., J. Low Temp. Phys. 208, 492 (2022).
