The usual methods of measuring photons destroy them, so they can be measured only once. Now researchers at l’Ecole Normale Supérieure in Paris have developed a nondestructive measurement technique that can be repeated hundreds of times to record the entire history of a single photon.The heart of the experimental setup is a “photon box,” a cavity made of two superconducting niobium mirrors facing each other in the Fabry-Perot configuration (see figure). It can store a photon for ~0.13 seconds, about the time it takes for a photon to travel a tenth of the distance to the moon.During the measurement process, a stream of atoms travels between the mirrors, and the presence of a photon slows down the electrons orbiting the atoms. The difference in the rate of the orbiting atoms is measured by two external microwave pulses outside the mirrors.Unlike conventional detectors, this setup does not absorb photons because the transition energy of the stream of atoms does not match the energy of the photon. This measurement technique is Ramsey interferometry, a method used in modern atomic clocks to compare the clock frequency to the atomic transition frequency. The researchers described this experiment in the March 15 issue of Nature.As one would expect from the Heisenberg uncertainty principle, the researchers could not know both the amplitude of the field and the phase of the wave with absolute certainty. Of the two variables, they measured the amplitude of the field exactly.In this experiment, a single photon controls the state of a large number of atoms, an important step toward quantum computing. In the future, the device may record a larger number of photons. It also could be used to prepare quantum superpositions of mesoscopic states that will be useful to study the border between the quantum and classical worlds.