  The quantum state of Schroedinger's cat is usually incorrectly described as a
superposition of "dead" and "alive," despite an argument by Rinner and Werner
that, locally, the cat should be considered to be in a mixture of
non-superposed states. Here, it is rigorously proven that the cat is not in a
superposition. This is central to the measurement problem. Nonlocal two-photon
interferometry experiments throw further light on the measurement state by
probing the effect of a variable phase factor inserted between its superposed
terms. These experiments demonstrate that both subsystems really are in locally
mixed states rather than superpositions, and they tell us what the measurement
state superposition actually superposes. They show that measurement transfers
the coherence in Schroedinger's nuclear superposition neither to the cat nor to
the nucleus, but only to the correlations between them. This explains the
collapse process--but not its subsequent irreversible dissipation--within the
context of unitary dynamics with no need for external entities such as the
environment, a human mind, other worlds, or collapse mechanisms.
