Resolving the problem of definite outcomes of measurements

Abstract

The entangled "Schrodinger's cat state" of a quantum and its measurement apparatus is not a paradoxical superposition of states but is instead a non-paradoxical superposition of nonlocal coherent correlations between states: An un-decayed nucleus is correlated with a live cat, and a decayed nucleus is correlated with a dead cat. This elucidation of entanglement is demonstrated by quantum-theoretical analysis and by experiments performed in 1990 using entangled photon pairs. Thus the cat state does not predict a dead-and-alive cat. Instead of indefinite superpositions, it predicts mixtures of definite eigenvalues even though the subsystems are not actually in the corresponding eigenstates, a situation that implies a (trivial) revision of the standard eigenvalue-eigenstate rule. Because the subsystem states are not mixed even though the subsystem eigenvalues are mixed, this analysis avoids two common objections to such a resolution, namely improper density operators and basis ambiguity. Thus, entanglement transfers coherence from the superposed quantum to correlations between the quantum and its measuring apparatus, permitting instantaneous collapse without interrupting the global unitary evolution. This resolves a key part of the measurement problem.