Trapped light pulses interact

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 Review Letters.