Obama Taps Steven Chu, Physics Nobel Laureate

The Science Behind Designated Energy Secretary Chu's 1997 Prize

Dec 16, 2008

Barack Obama's designated energy secretary shared the 1997 Nobel prize in physics. What is the science behind Chu's Nobel prize in physics?

President elect, Barack Obama, selected Nobel laureate Steven Chu as energy secretary signaling the importance of science to the Obama administration. Steven Chu strongly advocates alternative energy sources and as a Nobel laureate obviously understands scientific and technical issues involved. Hence his appointment should contribute to Obama's goal of decreasing the United States' reliance on imported oil.

Chu however did not win his Nobel prize for work on alternative energy. Rather it was, according to the Nobel citation, for "development of methods to cool and trap atoms with laser light." Chu shared this prize with Claude Cohen-Tannoudji and William D. Phillips who along with many collaborators contributed to the work. How can lasers cool and trap atoms?

Manipulating Particles

Manipulating charged particles, such as protons and electrons, is relatively easy. Both electric and magnetic forces act on charged particles, so combinations of electric and magnetic fields can manipulate charged particles. Manipulating neutral particles, such as atoms, is more difficult because electric and magnetic forces do not affect uncharged particles.

Physicists needed another force to act on neutral atoms. Light can supply this force. Photons of light have, according to quantum mechanics, a momentum proportional to their frequency. Hence photons can exert a force on atoms and other neutral particles.

Laser Cooling and Trapping of Atoms

The force from photons is however too small to significantly affect rapidly moving warm atoms. Chu and his collaborators developed techniques to cool atoms with laser light so that the lasers could trap and manipulate the atoms.

To trap atoms lasers first cool them. Chu's method for cooling atoms uses six laser beams surrounding the atoms. To apply a force on an atom a laser beam must be tuned to a specific frequency, the resonant frequency. However if the atom is moving, the frequency shifts a small amount because of the Doppler effect.

Chu's group tuned their lasers's frequencies so that atoms moving towards the laser would be slowed. As the atomic motion slowed, the temperature of the atoms decreased. Remember, thermal energy causes faster random atomic motions.

In 1985 Chu's group used this technique to lower the temperature of sodium atoms to 0.000240 Kelvin degrees.

By analogy to high viscosity fluids, like molasses, that slow motion, the laser photons surrounding the sodium atoms are called optical molasses. The optical molasses serves the dual purpose of both cooling and trapping the sodium atoms.

Applications of this Work

Fundamental scientific work, with no obvious applications often leads to applications the original workers never dreamed of. Chu's work is no exception.

Techniques used to manipulate neutral atoms can also be used to manipulate single cells and smaller structures, such as DNA molecules, inside cells. This work therefore provides biologists with a new tool to study fundamental principles of life.

The ability to manipulate atoms allows physicists to make fountains of individual atoms that shoot upward and descend under the force of gravity. This work may lead to atomic clocks much more accurate than we have now.

Perhaps the application most relevant to Chu's new job is the use of atomic fountains to very accurately measure the acceleration of gravity. Such measurements are used to prospect for resources. Places where gravity is stronger than normal may contain dense mineral deposits. Places where it is weaker may contain less dense oil deposits.