NORMALLY you can鈥檛 detect a single photon without destroying it, but Serge
Haroche of the Ecole Normale Sup茅rieur in Paris and his colleagues have
found a trick that lets them have their cake and eat it.
The researchers built a superconducting cavity that can trap a photon for up
to a millisecond. The researchers were able to tell if their trap had caught a
photon by probing the box with a beam of excited rubidium atoms (Nature, vol 400, p 239).
When one of the rubidium atoms absorbs a photon, it re-emits an identical one
but slightly changes its quantum properties in the process. By measuring that
shift, Haroche says he can determine whether or not the cavity contains a
photon. He calls the experiment 鈥渁 striking example of quantum logic鈥�.
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