microkelvin

Microkelvin (µK) is a unit of temperature equal to one millionth of a kelvin. In physics, µK temperatures describe systems cooled to a few ×10^-6 kelvin, where thermal energy kB T is very small and quantum energy scales become prominent. This regime is used to study ultracold atomic and molecular gases, as well as certain solid-state systems, where quantum phenomena such as coherence, entanglement, and quantum degeneracy emerge.

Reaching microkelvin temperatures typically involves staged cooling techniques. Laser cooling, including optical molasses and sub-Doppler cooling, reduces atomic motion to the microkelvin range. Evaporative cooling in magnetic or optical traps continues the process, lowering the temperature toward and into the microkelvin regime and, in some cases, toward even lower temperatures. In solid-state contexts, microkelvin temperatures are often achieved with dilution refrigerators or nuclear demagnetization, enabling studies of quantum materials and low-noise devices.

Thermometry at the microkelvin scale employs methods such as time-of-flight imaging and velocity distribution analysis for ultracold atoms, spectroscopy-based thermometry, and fluctuations-based (noise) thermometry. In solid-state experiments, nuclear spin, electron spin, or calibrated resistance measurements can provide temperature estimates in the microkelvin range.

Applications of microkelvin physics include probing quantum degenerate gases, quantum simulation of many-body systems, precision measurements, and the investigation of superfluidity and quantum phase transitions. Although nanoKelvin temperatures are required for true Bose-Einstein condensation in some systems, the microkelvin regime remains important for preparing low-entropy states and exploring interaction regimes near quantum criticality.