Connecting the Wave and Particle Aspects of Light

Connecting the wave and particle aspects of light by detecting a photon and then measuring the fluctuations of a closely associated electromagnetic field has been experimentally achieved for the first time. In most experiments, researchers focus upon either light's particle aspects (by counting photons, for instance) or wave aspects (by measuring an interference between electromagnetic fields, to cite a simple example).

Now, researchers at SUNY-Stony Brook and the University of Oregon (Luis Orozco, Stony Brook, 631-632-8138, lorozco@notes.cc.sunysb.edu) have demonstrated an experimental setup, which they call a "Wave-Particle Correlator," for determining the relationships between both aspects of the light that comes from a single physical process. The "light source" in their experiment consists of a beam of rubidium atoms passing in between a highly reflecting pair of mirrors (a "cavity QED system"). In their setup, a laser aims light into the cavity through one of its mirrors. Acting as a sort of "artificial molecule," the cavity absorbs the light and re-emits it. A photon occasionally escapes through an output mirror, only to be detected as a particle by a photodiode. The photon detection sets up a subsequent measurement of wavelike properties, as the cavity occasionally gets rid of a second photon to relax to a stable state.

The resulting electromagnetic field gets mixed with another known electromagnetic wave to produce an interference pattern. The pattern emerges only after averaging over many such "conditional" measurements triggered by photodiode detections. It reveals that the electromagnetic field inside the cavity after the first photon's departure contains, in effect, a tenth of a photon, since a second photon is only emitted about 10 percent of the time. Measuring such wave-particle correlations might bring about new microscopy techniques. (Foster et al., Physical Review Letters, 9 October 2000; Select Article).