Entanglement and the measurement problem

Abstract: The entangled "measurement state" (MS), predicted by von Neumann to arise during quantum measurement, seems to display paradoxical properties such as multiple macroscopic outcomes. But analysis of interferometry experiments using entangled photon pairs shows that entangled states differ surprisingly from simple superposition states. Based on standard quantum theory, this paper shows that (i) the MS does not represent multiple detector readings but instead represents nonparadoxical multiple statistical correlations between system states and detector readings, (ii) exactly one outcome actually occurs, and (iii) when one outcome occurs, the other possible outcomes simultaneously collapse nonlocally. Point (iii) resolves an issue first raised in 1927 by Einstein who demonstrated that quantum theory requires instantaneous state collapse. This conundrum's resolution requires nonlocal correlations, which from today's perspective implies the MS must be an entangled state. Thus, contrary to previous presumed proofs of the measurement problem's insolubility, we find the MS to be the collapsed state and just what we expect upon measurement.