Imagine trapping less than half a photon’s worth of energy between two perfect mirrors. Most of the time, the electromagnetic field will be in its zero-photon ground state. But every now and again, the EM field will jump to its one-photon excited state, linger, then jump back down.
Recording a string of such jumps lies at the single-photon limit of quantum electrodynamics. To reach it, an experimenter needs not only to perform a delicate quantum nondemolition (QND) measurement, but also to sustain the feeble field for long enough to see at least one up-and-down jump. Both feats are hard to pull off; combining them is even harder.
A team from the École Normale Supérieure (ENS) in Paris has done just that. 1 Michel Brune, Serge Haroche, Jean-Michel Raimond, and their students sent a stream of specially prepared rubidium atoms through an optical cavity that contained on average 0.06 microwave photons....
