Development of dilution refrigerators—A review

Abstract Subkelvin refrigeration refers to the refrigeration below 1 K and is widely used in the fields of condensed-matter physics, quantum computing, and astronomical observation. Among the three conventional technologies, adsorption refrigeration, adiabatic demagnetization refrigeration, and dilution refrigeration, dilution refrigeration is the most popular with the advantages of lowest temperature down to several mK, continuous cooling, and relatively large cooling power. The method was first proposed in the 1950s and the first complete system was built in 1960s, followed by a successful commercialization. Since then, with the ever-increasing demands of low temperature physics research and due to its apparent advantages, it quickly becomes the most important technique to reach the millikelvin temperature range. The lowest temperature achievable nowadays by a conventional dilution refrigeration is 1.75 mK, and largest cooling power at 100 mK is 2 mW. In addition to the commercialization of dilution refrigeration, lots of other research, tailored to special applications, have also been done or going on, which include cold-cycle dilution refrigeration without room-temperature pumping system and zero-g dilution refrigeration for space applications, etc. The cold-cycle dilution refrigerator on the ground, driven by two adsorption pumps, can reach a minimum temperature of 8 mK and the corresponding cooling power was 60 μW at 110 mK. For zero-g space applications, an open-cycle dilution refrigerator was successfully operated in space for 2.5 years with a cooling power of 0.1 μW at 92.6 mK. Closed-cycle dilution refrigerators for zero-g environment are still under investigation. With a brief introduction on the thermodynamics of dilution refrigeration, this paper reviews the development of the conventional 3He-circulating dilution refrigerator, the 4He-circulating dilution refrigerator, the cold-cycle dilution refrigerator on the ground, and dilution refrigerators for space. Key issues related to non-conventional dilution refrigeration are pointed out with some discussions on their prospects.