Wolfgang Ketterle of the Massachusetts Institute of Technology, who won the Nobel Prize in Physics for his work with ultracold atoms, explains.

First, let me introduce the scientific meaning of temperature: it is a measure of the energy content of matter. When air is hot, the molecules move fast and they have high kinetic energy. The colder the molecules are, the smaller their velocities are and, subsequently, their energy. Temperature is simply a way to characterize the energy of a system.

Temperature can be measured in different units. In everyday life the Celsius and Fahrenheit scales are common, but they both lack the natural property that the zero of the temperature scale should correspond to zero velocity of the gas particles (that is, zero energy). Thus the natural temperature scale is the absolute temperature measured in Kelvin. Zero kelvin is the lowest possible temperature. At absolute zero, all motion comes to a standstill. It is obvious that a lower temperature is not feasible because there is no velocity smaller than zero and no energy content less than nothing. (As a side remark, energy in this instance means only the energy that can be taken away from the particles and does not include the rest mass or quantum mechanical zero-point energies for confined particles.) Absolute zero corresponds to –273 degrees Celsius and –460 degrees Fahrenheit.

Cooling an object requires extracting energy from it and depositing it somewhere else. In household refrigerators, for example, the heat exchanger at the back gets warm because the energy extracted from the objects inside is deposited there. (In addition, there is some heat created just from running the refrigerator.)

In the 1980s and 1990s new methods for cooling atomic gases were developed: laser cooling and evaporative cooling. By combining these methods, temperatures below one nanokelvin (one billionth of a degree Kelvin) have been achieved. The lowest temperature recorded so far, described in a publication from our group in the September 12, 2003 issue of Science, is 450 picokelvins, which beat the previous record holder by a factor of six. Two recent Nobel prizes (in 1997 and 2001) were awarded for these developments.

In laser cooling, atoms scatter laser light. An incoming laser photon is absorbed and then reemitted in a different direction. On average, the color of the scattered photon is slightly shifted to the blue relative to the laser light. That is, a scattered photon has a slightly higher energy than an absorbed photon did. Because total energy is conserved, the difference in photon energy is extracted from the atomic motion--the atoms slow down. Shifts in wavelength can occur because of the Doppler effect (which is a shift proportional to the atomic velocity) or because of Stark shifts (due to the electric field of the laser beams) and offers one explanation of how the atoms lose energy.

A second description emphasizes how momentum is transferred to the atoms. If atoms are exposed to several laser beams with carefully chosen polarization and frequency values, then they preferentially absorb photons from the forward hemisphere, where the photon angular momentum and the atomic velocity are at an angle larger than 90 degrees. The photon momentum has a component that is opposite to the atomic motion and, as a result, the momentum kick of the absorbed photon slows the atom down. The subsequent emission of a photon occurs at random angles and as a result, averaged over several absorption-emission cycles, there is no momentum transferred due to the photon emission. The crucial step is making the atoms absorb photons preferentially from the forward direction and is achieved by utilizing the Doppler shift. When the photon momentum and the atomic velocity are at an angle larger than 90 degrees, the atom and the light are counterpropagating and the Doppler shift is an upward shift in frequency. When the laser light is detuned to the red of the atomic resonance, the Doppler shift brings the light closer into resonance and enhances the light absorption. For light coming from the backward direction, for which the angle between the photon momentum and the atomic velocity is smaller than 90 degrees, the Doppler shift is opposite and shifts the light even further away from the atomic resonance, decreasing its absorption.

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