Putting the Brakes on Light

Daniel C. McCarthy

Two teams of researchers, working independently, have put the brakes on light, stopping it and holding it still, if for only a moment.

One team, led by Lene Vestergaard Hau, combined physicists from Harvard University and the Rowland Institute for Science, and published its results in the Jan. 25, 2001, issue of Nature. Ronald Walsworth, Mikhail Lukin and collaborators, working on separate experiments at the Harvard-Smithsonian Center for Astrophysics, described their results in the Jan. 29, 2001, issue of Physical Review Letters.

While each experiment differed in the details, they both followed similar approaches by compressing a pulse of light within a gas for a controlled period before releasing it again.

Each experiment used a laser split into signal and control beams with opposing circular polarizations. Destructive interference created by the two beams formed a transparency window in the normally opaque vapors and allowed the signal pulse to pass through. This, however, compressed the length of the signal pulse from kilometers to a millimeter scale so that it propagated at reduced group velocity.

Then, by turning off the control beam, the signal pulse speed dropped to zero, scattering the energy of the signal light into the outgoing control beam and converting the information distinguishing the signal and control pulses into a pattern of spins, known as a polariton, in the vapor atoms.

Holding Light

In each experiment, the gas held the encoded light information for a period of milliseconds. By switching the control laser back on, the researchers reconstituted the signal pulse from the polariton and let it depart the chamber.