By Peter Weiss
At the heart of an optical laser, a few photons flying in formation trigger an avalanche of other photons that join their ranks. The new recruits spring from atoms shedding energy. That amplification yields a laser beam in which hordes of these elementary particles of light, equivalent to electromagnetic waves, all line up, crest to crest and trough to trough. This coordination is known as phase coherence.
Now, U.S. and Japanese researchers have demonstrated coherent amplification of atoms much like the photon cascade in lasers.