Art Hobson, Emeritus Professor of Physics, U Arkansas.

The heart of the measurement puzzle, namely the problem of definite outcomes, remains unresolved. This paper shows that Josef Jauch’s 1968 reduced density operator approach is the solution, even though many question it: The entangled “Measurement State” implies local mixtures of definite but indeterminate eigenvalues even though the MS continues evolving unitarily. A second, independent, argument based on the quantum’s nonlocal entanglement with its measuring apparatus shows that the outcomes must be definite eigenvalues because of relativity’s ban on instant signaling. Experiments with entangled photon pairs show the MS to be a non-paradoxical superposition of correlations between states rather than a “Schrodinger’s cat” superposition of states. Nature’s measurement strategy is to shift the superposition–the coherence–from the detected quantum to the correlations between the quantum and its detector, allowing both subsystems to collapse locally to mixtures of definite eigenvalues. This solution implies an innocuous revision of the standard eigenvalue-eigenstate link. Three frequent objections to this solution are rebutted. Full text