Page "International Temperature Scale of 1990" Paragraph 7

ITS – 90 does not address the highly specialized equipment and procedures used for measuring temperatures extremely close to absolute zero.

For instance, to measure temperatures in the nanokelvin range ( billionths of a kelvin ), scientists using optical lattice laser equipment to adiabatically cool atoms, turn off the entrapment lasers and simply measure how far the atoms drift over time to measure their temperature.

A cesium atom with a velocity of 7 mm per second is equivalent to temperature of about 700 nK ( which was a record cold temperature achieved by the NIST in 1994 ).