Quantum leaps, these unexpected “jumps” from one state to another in microscopic quantum systems, have long been detected for atoms, electrons, ions, and other particles, but never for a photon. Indeed, this elemental particle of light is generally destroyed as soon as it is recorded. Most light receivers–including the eye–absorb the photons they detect irreversibly.
Yet there is nothing in nature that says that photons have to be destroyed to be measured. Researchers at the Kastler Brossel laboratory have thus managed to observe, hundreds of times, a single photon trapped in a “photon box.” Key to this experiment, this box is a cavity formed by two superconducting mirrors cooled to near absolute zero temperature. In it, a photon of the residual thermal radiation bounces back and forth over a billion times between the mirrors, which are placed 2.7 centimeters apart, before it disappears.
To detect this photon, it would be straightforward to use a process of atomic absorption. An atom, which can exist in different energy states, absorbs a photon while jumping from one energy state to a higher one. By feeding an atom in the box, and measuring its energy, researchers could know whether the box contains a photon. There is just one problem: The absorption destroys the photon in the process, so it can only be seen once. And that's not enough to observe a quantum leap.
To get around this hurdle, researchers chose atoms whose transition between the two states 0 and 1 corresponds to an energy which is different from that of the photons. In this case, the law of conservation of energy implies that the atom cannot absorb the light. The presence of the photon will only slightly alter the frequency of the atomic transition (which is measured using an auxiliary microwave field outside the cavity). As in the standard absorptive method, if the cavity contains a photon, the atom jumps to state 1, whereas it remains at state 0 if the box is empty. But this time, the energy absorbed by the atom is taken from the auxiliary field and not from that of the cavity. As a result, the photon is still there after having been seen, and is ready to be measured again.
With this method, researchers recorded a great number of sequences lasting several seconds in which thousands of atoms, crossing the cavity one by one, were detected either in state 0 or in state 1. The photons remain trapped from about a tenth of a second to up to half a second, until they suddenly disappear. The moments at which the photons appear and disappear reveal the quantum leaps of light.
Through this experiment, researchers showed that the information carried by a quantum of light can be transferred hundreds of times to a physical system without being lost. By observing such jumps over a period of several hours, the researchers directly confirmed the statistical properties of thermal radiation laid down a century ago by Planck and Einstein.
Claire Le Poulennec
1. CNRS / ENS / Collège de France / Université Paris 6.