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Counting photons in a cleaned-up crystal.

Expressing data - whether scientific measurements or the sounds of a symphony orchestra - as strings of ones and zeros has proved a remarkably versatile means of handling and conveying information. Such digital information can be readily encoded as electrical signals or light pulses.

It's possible to conceive of using the presence or absence of a single electron or photon to represent a bit-either 1 or 0. But quantum effects limit the efficacy of such a strategy, even when the pulses contain more than one electron or photon. For example, because of quantum effects, laser-generated light pulses typically display random fluctuations in the number of photons present in each pulse (SN: 5/30/92, p.356).

Now, theorist Gershon Kurizki of the Weizmann Institute of Science in Rehovot, Israel, and his collaborators have proposed a scheme that could lead to precise control of the number of photons in light pulses. By reducing the energy required to extract a given pulse from a noisy background, such a technique represents a potential means of transmitting digital information more efficiently than at present.

The researchers describe one version Of their scheme in the February JOURNAL oF THE OPTICAL SOCIETY OF AMERICA B.

To circumvent the intrinsic noisiness of optical devices, Kurizki and his coworkers take advantage of strong interactions between an electromagnetic field and a stream of atoms passing through a special, porous structure fabricated from an electrical insulator. Known as a photonic crystal, this kind of structure prevents atoms embedded within it from spontaneously absorbing and reemitting light at wavelengths that fall within a certain range, creating a band gap (SN: 11/2/91, p.277).

The researchers suggest that by introducing a defect that disrupts the structure's orderliness in Just the right way, they can use the defect to trap photons of a particular wavelength. The number of trapped photons initially present varies because of random fluctuations in the intensity of the laser light bathing the crystal.

But the uncertainty in the number of photons present can be removed by sending a beam of excited atoms, one by one, through the region of the defect. Alternately emitting and reabsorbing photons during their passage through such a structure, the atoms interact strongly with and modify the electromagnetic field associated with the defecttrapped photons.

Such interactions rapidly convert the electromagnetic field within the photonic crystal into a so-called photon-number state, meaning that a fixed number of photons is stored at the defect. In other words, the atomic beam acts as a kind of cleaning agent.

"The atoms remove the undesirable parts of the information, which is stored in the field," Kurizki says. "The remaining information within the field then conforms to what we would like to have."

The researchers can ascertain the existence of a particular photon-number state by looking for sequences in which each atom emerges from its interaction in an excited state. "One of the advantages of [our] strategy is that we can not only generate a photon-number state but also know in advance and adjust the conditions to get the right number of photons," Kurizki says. By establishing a well-defined photon-number state in a photonic crystal, researchers can exploit the ensuing certainty in the number of photons present for possible application in signal processing or optical computing.

However, no one has yet confirmed experimentally that it's possible to create a photon number state. Even the design of materials with photonic band gaps remains rudimentary (SN: 3/28/92, p. 206).

"From a technological point of view, this is still in its infancy," Kurizki says. "But these photon-number states are becoming a real possibility, and I think we are bound to see them within a relatively short time."
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Title Annotation:photonic crystal used to control number of photons in light pulses
Author:Peterson, Ivars
Publication:Science News
Date:Feb 6, 1993
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