Microkelvin
One millionth of a Kelvin. The temperature regime reached by laser cooling, where neutral atoms and trapped ions are held still enough to act as qubits.
A microkelvin (µK) is 0.000001 Kelvin, one thousand times colder than a millikelvin and about a millionth of a degree above absolute zero. It is the temperature regime at which individual atoms move slowly enough to be trapped and manipulated with laser light.
Microkelvin temperatures are reached by laser cooling, not by dilution refrigeration. Carefully tuned laser beams push against the motion of atoms in a vacuum chamber, dissipating their kinetic energy until they are nearly at rest. The technique was pioneered at NIST and other labs in the 1980s and 1990s and underpins most of modern atomic physics.
Neutral atom and trapped ion quantum computers operate in the microkelvin regime. Superconducting quantum computers, by contrast, operate roughly a thousand times warmer at millikelvin temperatures, but use bulk metal circuits rather than individual atoms.
Microkelvin physics is the foundation of Colorado's neutral atom and trapped ion companies. Atom Computing, Infleqtion, Quantinuum, and IonQ's Boulder lab all rely on laser cooling techniques developed at NIST Boulder and JILA. The 1995 Bose-Einstein condensate breakthrough by Eric Cornell and Carl Wieman at JILA required cooling rubidium atoms to less than 170 nanokelvin, a thousand times colder still.