Photon, photon, who's got the photon?

Quantum mechanics has the quality of introducing uncertainties where classical physics would be certain. New instances of such behavior all contribute their bit to the overall philosophical question of whether anything in fact is certain or precise. The latest example, which could be called an instance of quantum chemistry more than of quantum physics, is reported in the Feb. 4 PHYSICAL REVIEW LETTERS. Philippe Grangier and Alain Aspect of the Institute of Optics of the University of Paris-South at Orsay, France, and Jacques Vigue of the Laboratory of Hertzian Spectroscopy of the Ecole Normale Superieure in Paris did the experiment.

The action concerns the dissociation, by a beam of laser light, of a molecule consisting of two calcium atoms. The energy delivered by the laser light dissociates the molecule, and the two atoms fly apart. One of them gets slightly more energy than the other and, a very short time later, emits a single photon of light. The question is: Can experiment tell which of the two atoms emitted this photon of fluorescence?

Classically it should be able to. By measuring the momenta of the recoiling atoms and the wavelength and polarization of the emitted photon, one should be able to tell which atom emitted the photon. Unfortunately, in real life, quantum mechanics rules this instance, and one cannot measure those things precisely. There are two ways in which the action can go: Either the one atom or the other gets the extra energy and emits the light. In quantum mechanical theory, the probabilities of following the two paths interfere with each other and so it is impossible to tell which one an actual case has followed.

Experiment bears this out. "There is no way," the experimenters conclude, "to know 'which atom emitted the photon.'"