Light Does Some Weird Math

From October 13th, 2007; Vol.172 #15 (p. 238)

Light is made of photons. Add a photon to a light pulse, then take one out, and you’d think you’d be back where you started. But in the world of quantum physics, things aren’t so straightforward.

For one thing, quantum uncertainty means that a pulse doesn’t necessarily have a well-defined number of photons. And acting on such a pulse may have counterintuitive consequences.

When Marco Bellini of the University of Florence in Italy and his collaborators used a laser to add a new photon to an existing light pulse, the pulse usually ended up with more photons, as expected. But when they subtracted a photon by passing a light pulse through a glass plate in such a way that a single photon bounced out, the pulse typically emerged with more photons rather than fewer. When the team performed the two operations in a row, the most likely number of photons in the pulse depended on the order in which the two operations were performed, the researchers report in the Sept. 28 Science.

Confirmation that the order of the operations of adding and subtracting a photon makes a difference to the outcome demonstrates a basic prediction of quantum theory.

“Although [this law] underlies the entire quantum behavior of light, it had never been checked directly,” Bellini says.

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