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Crueltyfree: Counting photons without killing them

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1:06pm, August 22, 2007

Disclaimer: No particles were harmed in the making of this experiment. Physicists have found a way to count photons as they zip along, without destroying them. The researchers say that the technique will enable scientists to probe quantum effects that so far have been the subject only of speculation.

In physics labs, detecting light has long been synonymous with absorbing photons. Typically, the photons cease to exist and the light's energy transforms into an electrical signal. Physicists can count single photons—but they haven't been able to count them and keep them.

"Up to now, when you measure light, it's a destructive process," says Serge Haroche of the École Normale Supérieure of Paris. Now, Haroche and his colleagues have shown how to count photons nondestructively while they bounce back and forth between two mirrors.

Haroche's team began by introducing small numbers of photons into the space between two niobium-coated screens. Kept at le

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