Observation of photons from an inhibited spontaneous emitter

David Branning and Alan Migdall
Optical Technology Division

A fun and timely problem in cavity quantum electrodynamics is the question of whether atoms are actually radiating anything when their spontaneous emission is inhibited by a cavity. Recent theoretical papers have indicated that the answer to this question, counterintuitively, is "yes" — or more accurately, in the quantum-mechanical sense, "yes and no." That is, even when spontaneous emission is inhibited by a cavity, there is still a nonzero probability for the energy to be found in the field instead of the atom: the two are in an entangled quantum state that contains both possibilities. We have planned a demonstration of this that will use the process of spontaneous parametric downconversion, in which a "pump" photon may be split into two lower energy "signal" and "idler" photons within a nonlinear crystal. It has already been shown that this process may be frustrated by forming a cavity around the signal and idler modes, in precise analogy to the inhibited spontaneous emission of atoms, but with the advantage that much larger cavities may be used. We will construct a time-dependent cavity that can be switched on or off in several ns, which is less than the round-trip time of the cavity. By detecting the immediate arrival of photons outside the cavity just after it is switched off, we can catch the downconverter in the act of having sometimes radiated signal and idler photons even when its emission was "inhibited" by the cavity. This amounts to projecting, or "collapsing," the entangled state of the system onto an eigenstate in which only the field contains the energy.