Review on innovative small refrigeration methods for sub-Kelvin cooling

Abstract

Sub-Kelvin cooling has been generally demanded for the fields of low temperature physics, such as physical propertymeasurements, astronomical detection, and quantum computing. The refrigeration system with a small size can be appropriatelyintroduced when the measurement system does not require a high cooling capacity at sub-Kelvin temperature. The dilutionrefrigerator which is a common method to reach sub-Kelvin, however, must possess a large 3He circulation equipment at roomtemperature. As alternatives, a sorption refrigerator and a magnetic refrigerator can be adopted for sub-Kelvin cooling. This paperdescribes those coolers which have been developed by various research groups. Furthermore, a cold-cycle dilution refrigerator ofwhich the size of the 3He circulation system is minimized, is also introduced. Subsequently, a new concept of dilution refrigerator isproposed by our group. The suggested cooler can achieve sub-Kelvin temperature with a small size since it does not require anyrecuperator and turbo-molecular vacuum pump. Its architecture allows the compact configuration to reach sub-Kelvin temperatureby integrating the sorption pump and the magnetic refrigerators. Therefore, it may be suitably utilized in the low temperatureexperiments requiring low cooling capacity.