Title: Heisenberg's Introduction of the `Collapse of the Wavepacket' into Quantum Mechanics

Abstract: 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.