microkelvin

The microkelvin is where laser cooling was supposed to stop. Doppler theory set a floor for each atom, fixed by the linewidth of its cooling transition: 240 μK for sodium, 146 μK for rubidium. In 1988 the sodium atoms in William Phillips' laboratory at NIST measured 40 μK, six times colder than the theory allowed. The measurement survived every check; the theory was incomplete, and the missing mechanism, polarisation-gradient cooling in the overlapping laser beams, was worked out within the year. It remains one of the few celebrated cases of an experiment beating its own theoretical limit in the happy direction.

The unit is also load-bearing for timekeeping. A caesium fountain clock tosses a cloud of atoms cooled to about a microkelvin, where caesium moves at centimetres per second and can be interrogated through a full second of free flight; the SI second is realised from atoms at this temperature, so the world's timescale rests on the bottom of this unit's range.

On this site the μK converts like any other multiple of the kelvin, with the standing reminder that an absolute temperature of a microkelvin and a microkelvin of temperature difference, the stability language of precision thermometry, are separate quantity types here.