To detect fine energies like cosmic radiation or to maintain the quantum state of superconducting qubits installed in quantum computers stably, it is necessary to keep the temperature very low to minimize thermal disturbances. Domestic researchers have successfully developed a cooling device that can achieve ultra-cold temperatures in a compact size without using expensive refrigerants.
A research team led by Professor Jeong Sang-kwon from the Korea Advanced Institute of Science and Technology (KAIST) proposed a structure that integrates an adiabatic demagnetization refrigerator utilizing magnetic field changes with an adsorption-type refrigerator, achieving a cooling temperature of 0.3K (-272.85 degrees) close to absolute zero, they noted on the 19th. The adsorption refrigerator uses the phenomenon of gas adhering to the surface of solid adsorbents like activated carbon to produce a cooling effect.
Until now, dilution refrigerators that used mixtures of helium-3 and helium-4 isotopes have been employed for ultra-cold cooling. The dilution refrigerator is a cooling method that utilizes the principle of heat absorption during the dilution of helium-3. However, dilution refrigerators have the drawback of requiring very expensive helium-3, and because it is a system where low-density helium-3 circulates, a massive gas circulation device is required at room temperature, resulting in a large system size.
The research team developed a cooling device that can achieve ultra-cold temperatures in a relatively compact size without using the expensive helium-3 as a refrigerant. Unlike the existing cooling methods that use gas compression and expansion through an adiabatic demagnetization refrigerator utilizing magnetic field changes, they replaced the existing large gas circulation device with a superconducting magnet capable of compressing and expanding magnetic materials, thereby miniaturizing the system.
However, the adiabatic demagnetization refrigerator is composed without mechanical movement, providing high reliability and cooling efficiency, but it has the drawback of a limited operating temperature range. To compensate for this, the researchers adopted a structure that integrates a commercial ultra-low temperature refrigerator capable of achieving a cooling temperature of 4K (-269.15 degrees Celsius) with an adsorption-type refrigerator utilizing the evaporative cooling effect of liquid helium-4. They operated the adiabatic demagnetization refrigerator with a domestically produced superconducting magnet to achieve an absolute temperature of 0.3K (-272.85 degrees). After conducting dozens of continuous operation tests, they confirmed that the developed refrigerator operates stably without performance degradation.
Professor Jeong remarked, "The integrated adiabatic demagnetization refrigerator developed this time is an innovative ultra-cold cooling method that combines both miniaturization and simplicity, and it is expected to be utilized for cooling various quantum devices," adding, "If we can select magnetic materials capable of achieving even lower temperatures in the future, it could completely replace the existing dilution refrigerators."
The results of this research will be presented at the Space Cryogenics Workshop (SCW) organized by the National Aeronautics and Space Administration (NASA) in May next year.