  Heisenberg in 1929 introduced the "collapse of the wavepacket" into quantum
theory. We review here an experiment at Berkeley which demonstrated several
aspects of this idea. In this experiment, a pair of daughter photons was
produced in an entangled state, in which the sum of their two energies was
equal to the sharp energy of their parent photon, in the nonlinear optical
process of spontaneous parametric down-conversion. The wavepacket of one
daughter photon collapsed upon a measurement-at-a-distance of the other
daughter's energy, in such a way that the total energy of the two-photon system
was conserved. Heisenberg's energy-time uncertainty principle was also
demonstrated to hold in this experiment.
