Quantum Cascade Refrigeration: A Novel Strategy for Efficient Sub-Kelvin Cooling Using Quantum Magnets

The increasing demand for sub-Kelvin temperatures in quantum computing, cryogenic instrumentation, space technologies, and quantum materials research has renewed interest in adiabatic demagnetization refrigeration (ADR) as a sustainable alternative to conventional ³He-based refrigeration. While significant progress has been made in discovering materials capable of reaching ultra-low temperatures, a major persistent challenge is maintaining long holding times under the unavoidable fixed heating power (non-adiabaticity) of a cryogenic system. Effective spin-½ quantum magnets can achieve exceptionally low temperatures owing to strong quantum fluctuations and suppressed magnetic ordering, but their limited entropy reservoir often results in rapid warming. Conversely, classical high-spin systems exhibit long holding times but are less effective in reaching the lowest temperatures due to stronger magnetic interactions. Overcoming this fundamental trade-off between minimum temperature and holding time is crucial for next-generation cryogenic technologies.

In this talk, I will present a novel Quantum Cascade Refrigeration (QCR) strategy that addresses this challenge by combining quantum magnets with complementary entropy landscapes. The approach utilizes two effective spin-½ quantum magnets CeVO₄ and NdVO₄, where persistent short-range magnetic correlations in CeVO₄ and field-induced critical fluctuations in NdVO₄ generate enhanced and complementary low-temperature entropy distribution. Individually, CeVO₄ reaches temperatures as low as 130 mK, while NdVO₄ exhibits an extended holding time of nearly 12 hours. By cascading these materials in a stacked refrigeration architecture, the QCR system achieves a minimum temperature of approximately 160 mK together with a remarkably enhanced holding time of about 14 hours under identical thermal conditions and fixed heat load.

By examining the complementary cooling behavior of quantum magnets exhibiting short-range correlations and field-induced critical fluctuations, this talk explores how low-temperature entropy can be engineered to enhance holding time under a fixed heat load while maintaining efficient sub-Kelvin cooling.