Converting a coherent
light signal into a coherent atom state. (a) A light signal (consisting
of information encoded in a laser beam) approaches a cell containing
rubidium atoms. Normally the Rb atoms would absorb the signal light.
However, a control beam (also laser light) is adjusted to create a condition
of "electromagnetically induced transparency" (EIT) in the cell, and
the signal beam is not absorbed. (b) As the signal pulse enters the
cell it undergoes a dramatic spatial compression since the front edge
is slowed first. At the same time atomic spins are being flipped: the
signal and the spins form a coupled excitation called a "polariton."
(c) After the signal is entirely in the cell, one begins to reduce the
intensity of the control beam. As a result the velocity of the polariton
is being reduced, the amplitude of the signal light decreases while
more and more spins are flipped. (d) Control beam is off: no signal
photons left, all information is stored in the form of excited spins.
(e-f) The process can be reversed. The light pulse departs at the normal
speed of light, while the atomic vapor is left as it was before the
signal pulse arrived.
Phillips, Physics Review Letters, 29 January, 2001.
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