Researchers at Harvard University have
showed that light pulses can be trapped and held in a rubidium vapor and
made to interact with one another.
The method could eventually be used in quantum cryptographic and
quantum computing schemes. Quantum cryptography and quantum computers
use attributes of particles like photons to represent the 1s and 0s of
binary numbers. Quantum cryptography promises perfectly secure communications.
Quantum computers can theoretically solve certain very large problems
many orders of magnitude faster than classical computers, including those
that underpin today's security methods.
The method builds on the researchers' existing pulse trapping
technique, which involves firing a pulse and a pair of control beams that
are aimed at each other into the vapor. The control beams create a standing
wave pattern in the rubidium atoms that acts like a set of microscopic
mirrors oriented parallel to each other. This mirror standing wave pattern
holds the pulse in place.
Ordinarily light pulses barely interact, passing through each
other essentially unaltered. Coaxing light pulses to interact usually
requires special crystals and high-power lasers.
The researchers were able to used the rubidium vapor method to
trap a pair of pulses at once, forcing them to interact. The researchers
showed that the phase of one pulse can be shifted proportionally to the
intensity of the second pulse, and that a shift of 180 degrees in a pulse
can be caused by a single photon.
The method should also work with pairs of individual photons rather
than pulses made up of many photons, according to the researchers. Toward
that end they are working on trapping pulses precisely enough to hold
a pair of photons in the same spot and thus force them to interact.
Researchers generally agree that practical quantum computers are
one to two decades away.
The work appeared in the February 18, 2005 issue of Physical
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