Abstract

Entangled multiparticle quantum states cannot be expressed as a product of singleparticle states; they can give rise to correlations between separated particles unaccountable within the framework of classical physics and are of considerable interest to those concerned with the foundations of quantum mechanics. Two separated identical atoms having, for example, two excited levels with energy interval h12 are brought into an entangled linear superposition of excited states by absorption of two correlated photons. The quantum description of either atom alone shows incoherently populated excited states; the single-atom emission probability shows no quantum interference. However, the joint probability for fluorescent emission manifests, as a function of the time interval between radiative decay of both atoms, quantum beats¹ at frequency 12. This quantum interference is insensitive to atomic motion and atomic density, and it can be engendered by photons whose spectral widths are smaller than the beat frequency.

© 1990 Optical Society of America