A fascinating phenomenon with a forbidding name (parametric down
conversion) occurs in certain crystals. A single photon enters,
but two emerge. There are myriad pair combinations; but in each
case, the sum of the total energy and momentum of the two output
photons exactly equals those of the single input photon.
Parametric down conversion is finding a host of uses in physics,
from testing the efficiency of high-tech single-photon detectors
to cutting-edge experiments in quantum mechanics. Alan Migdall
of the National Institute of Standards and Technology explains
and demonstrates how it's done.