Making Light Work Of Photons

OPTICAL COMPUTING HAS LONG SHONE as the theoretical hope for faster number-crunchers when electronics bumps up against physical limits around 2020. A pair of breakthroughs makes that prospect seem far more likely.

A problem has been storing a digital bit with a light pulse. It's tricky; light, by nature, is always moving at the speed of light. So keeping a photon in one spot means bouncing it back and forth between mirrors, or pumping it through coils of optical fiber. Neither approach is practical outside a lab because it takes up far more space than today's memory chips.

Now, researchers at Ludwig-Maximilians-University in Munich and nearby Munich Technical University have devised a promising tactic. They use semiconductor structures to transform photons into "electron-hole pairs." Those are an outgrowth of quantum physics: Electrons have a negative charge, and "holes" carry a positive charge. When they meet, they annihilate one another and give off a flash of light. The trick is to control the timing of that annihilation. The researchers report in the Feb. 26 issue of Science that they can temporarily delay recombination and thus retrieve a photon at a predetermined instant.

Meanwhile, in the Feb. 11 issue of Nature, physicists at Stanford University unveiled a chip that uses electron-hole annihilation to create photons in vast numbers. Their chip is studded with thousands of posts, each of which can emit millions of photons a second. The posts also act as turnstiles, ensuring that only single photons pop out on schedule. Computers capable of processing single-photon signals would leave today's supercomputers in the dust.

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