Physicists in several different laboratories have performed experiments to
manipulate light – in this particular case, their attempt is in order to
stop light particles completely, and then restart them on their journey. This
requires slowing light from a speed of approximately 186,000 miles per second to an
absolute stop. In a previous experiment in 1999, Lene Vestergaard Hau (pronounced
lee-NUH) of
Harvard (who leads these recent experiments) was able to slow light
to only 38 miles per hour. She has gained a formidable reputation in the field
of
Physics for this and other experiments in her field.
To perform this experiment, they utilize a container where they magnetically
chill sodium atoms of gas to only a few-millionths of a degree of absolute zero,
and a consistency they like to call, “optical molasses.” In essence,
they are firing finely tuned lasers into a condensate of sodium atoms. Another
group working with Hau’s group, led by Ronald L. Walsworth and Mikhail
D. Lutkin of the Harvard-Smithsonian Institute for Astrophysics, used the alkaline
metal rubidium in place of the sodium atoms. In a normal case, the gasses would
absorb the light that is directed into the container, however scientists were
able to prevent this by putting the gasses into a state of “electromagnetically induced transparency” by firing a “control” laser beam into
the container. Then they were able to try using their probe laser (at a different
frequency) into the container, where it slows down dramatically. Finally, to
stop the probe light, they waited until the light had encountered the gas atoms
and left a pattern in the spinning atoms, then reducing the intensity of the
control beam. This effectively kept the information in the beam imprinted in
the spinning atoms. Then they restored the control beam, and the light stored
on the spinning atoms continued to go through the vessel. Fifty per cent of
the light that went into the vessel was retrieved in the regenerated light pulse.
Whether the light was actually stopped is left to interpretation. The probe
laser is a bundle of light waves that form a single wave. This is known to physicists
as the “group velocity”. But stopping
the group velocity doesn’t necessarily mean that the light waves themselves
were stopped.
There is also competition in this field of light manipulation. Physicists in
Princeton, N.J. pushed a laser pulse through a vapor of Cesium atoms to travel
faster than the conventional speed of light, rather than slowing it down.