Catching light in a box has always been an alluring aspiration. Young children will slam the lid down on a ray of sunlight, hoping to capture its magic glow. The problem is, how do you know the light is in there if the lid is closed? And if you lift the lid to peek, the sunlight disappears. Scientists solved the first part of the problem by learning how to detect a single photon. But that detection was still destructive, obliterating the photon in the process. Now physicists at the Ecole Normale Superieure have completed the equation, devising a way to see a single photon without destroying it. The researchers' superconducting cavity can trap a photon for up to 1 ms. They use a beam of excited rubidium atoms to probe the cavity and determine whether they have indeed caught that elusive particle of light. The experiment, detailed in the July 15 issue of Nature, could lead to quantum logic gates based on cavity quantum electrodynamics, the authors noted. The procedure works much like optical quantum nondemolition measurements, but the French physicists use atomic rather than optical interferometry. A meter atom probes the cavity to detect whether a photon is present. A phase shift in the system indicates that there is indeed a photon in the cavity. Same photon, two times An important aspect of this research is that the system can detect a photon more than once, showing that the photon was not destroyed by the detection procedure. The scientists have performed repeated measurements by sending two identical atom probes -- 75 ms apart -- to detect a photon in the cavity. The authors describe their system in terms of quantum logic: It is comparable to a quantum gate, with the photon acting as a control qubit and the atom as the target qubit.