Bifurcation in Quantum Measurement

We present a generic model of (non-destructive) quantum measurement. It consists of a two-level system μ interacting with a larger system A, in such a way that if μ is initially in one of the chosen basis states, it does not change but makes A change into a corresponding state (entanglement). The μA-interaction is described as a scattering process. Internal (unknown) variables of A may influence the transition amplitudes. It is assumed that the statistics of these variables is such that, in the mean, the μA-interaction is neutral with respect to the chosen basis states. It is then shown that, for a given initial state of μ, in the limit of a large system A, the statistics of the ensemble of available initial states leads to a bifurcation: those initial states of A that are efficient in leading to a final state, are divided into two separated subsets. For each of these subsets, μ ends up in one of the basis states. The probabilities in this branching confirm the Born rule.