  A model of single photon detection, illustrated by a photon-absorbing
superfluid or superconducting microvolume, is formulated as a cascading pair of
quantum phase transitions. In the first, the microvolume transitions to the
normal state upon photon absorption, resulting in a superposition of
macrostates depending on whether the photon is absorbed or not. The second
enables subsequent "wavefunction collapse," producing a density matrix
implementing the Born probability rule. Next, EPR-type measurements on
space-like separated entangled photon pairs are considered. It is argued that
macro-entangled superposition indeed survives until such time as the component
states come into causal contact, following which the state rapidly collapses to
one or the other expected outcome. Apparent superluminal communication effects
are entirely avoided.
