NORMALLY you can’t detect a single photon without destroying it, but Serge
Haroche of the Ecole Normale Supérieur 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 “a striking example of quantum logic”.
