NIST Physicists Measure Coldest Temperature Ever

GAITHERSBURG, Md.—If it's too hot outside, a visit to the National Institute of Standards and Technology might provide some relief. Physicists here recently cooled atoms to 700 nanokelvins, the coldest temperature ever recorded for matter.

NIST scientists chilled a cloud of cesium atoms very close to absolute zero using lasers to catch the atoms in an optical lattice. The atoms reached 700 nanokelvins, or 700 billionths of a degree above absolute zero. Zero kelvin (minus 273 degrees Celsius), or absolute zero, is the temperature at which atomic thermal motion would cease.

Since the late 1970s, physicists have sought to use lasers to cool atoms to as close to absolute zero as possible, primarily for improving atomic timekeeping. Super-cold atoms also can be used to improve certain experimental measurements. They are manipulated more easily than room-temperature atoms, and they may improve lithography processes for the semiconductor industry.

Since laser cooling was first demonstrated at the NIST laboratories in Boulder, Colo. in 1978, scientists around the world have been steadily pushing to lower temperatures. In 1985, researchers thought a theoretical limit had been reached when sodium atoms were cooled to 240 microkelvins (240 millionths of a kelvin).

Surprisingly, a few years later, a team at NIST in Gaithersburg measured sodium temperatures as low as 25 microkelvins. New theories led to a better understanding of laser cooling and allowed a collaboration, between NIST and researchers at the Ecole Normale Sup‚rieure in Paris, to cool cesium atoms to 2.5 microkelvins in Paris.

Their record stood as the coldest steady-state temperature for three-dimensional motion of atoms until the recent NIST work, although other researchers achieved lower temperatures transiently, or in one or two dimensions.

The new record low temperature at NIST was achieved with a technique borrowed from the Paris laboratory: an arrangement of four laser beams interfering to produce a regular array, or "optical lattice" of microscopic hills and valleys for the atoms.

The NIST team, which includes physicists Anders Kastberg, Steven Rolston, Robert Spreeuw, Poul Jessen and group leader William Phillips, found that atoms became trapped in the valleys of the optical lattice and reached temperatures close to 1 microkelvin. The trapped atoms oscillate back and forth around the bottoms of the valleys. To reduce the temperature of the atoms even more, the scientists reduced the intensity of the light. As the laser light fades, the terrain of the optical lattice becomes less steep, slowing the frequency of the oscillations. This phenomenon, known as adiabatic expansion, drives the atomic temperature even lower, where the typical atomic velocity is only 7 millimeters per second.

So how did the NIST team measure such a cold temperature? After the atoms reached the lowest temperature possible in the optical lattice, the scientists abruptly turned off the lasers. By measuring how far the atoms moved in a specified amount of time, the scientists were able to calculate their temperature. Now, the NIST researchers are working to apply their discovery in new and better atomic clocks.

As a non-regulatory agency of the Commerce Department's Technology Administration, NIST promotes U.S. economic growth by working with industry to develop and apply technology, measurements and standards.