subkelvin

Subkelvin is a regime of extremely low temperatures that falls below one kelvin (1 K). Temperatures in this range are achieved using specialized cooling techniques such as dilution refrigeration, adiabatic demagnetization, and nuclear demagnetization. Dilution refrigerators create a mixed phase of helium‑3 and helium‑4, which efficiently extracts heat from the evaporating gas. Adiabatic demagnetization cools magnetic dipoles in a paramagnetic salt as a magnetic field is slowly removed, while nuclear demagnetization exploits the nuclear spin system of certain materials, allowing the attainment of temperatures near a few millikelvin or less.

Achieving subkelvin conditions requires precise thermal isolation, ultra‑clean materials, and control of vibrational and electromagnetic noise. Critical heat exchangers are designed to maintain temperature stability and enable rapid cycle times. The lowest routinely attainable temperatures in laboratories are now below 1 µK using laser and magnetic trapping techniques that cool atoms to the quantum ground state.

Scientific research in the subkelvin domain enables observation of quantum phenomena that are impossible at higher temperatures. Superfluidity of helium‑4 becomes pronounced below 2.17 K, and superconductivity of many metals only manifests when the thermal energy is less than their superconducting gap, often requiring subkelvin temperatures. In condensed matter physics, quantum phase transitions can be studied at millikelvin temperatures where thermal fluctuations vanish. Quantum computing relies on maintaining qubits in coherent states, and many qubit architectures operate near 10‑100 mK to suppress decoherence. Fundamental tests of physics, such as measurements of the cosmic background radiation and searches for violations of symmetries, also benefit from subkelvin instrumentation.

In addition to fundamental science, subkelvin temperatures are used in metrology, for example in defining the electrical resistance standard via the quantum Hall effect, which is stable only at low temperatures. The versatility and sensitivity of subkelvin techniques continue to drive progress across physics, materials science, and quantum technology.