Toward a precise definition of branches in a many-body wavefunction

When the wavefunction of a large non-integrable quantum system unitarily evolves away from a low-entropy initial state, there is strong circumstantial evidence it develops “branches”: a preferred decomposition into orthogonal components that is indistinguishable from the corresponding incoherent mixture with feasible observations. We describe work in progress on a possible definition for wavefunction branches for many-body lattice systems based on spatially redundant correlations. If the wavefunction reliably “collapses” — in the sense that off-diagonal terms in the in the basis of branches of low-order N-point functions are suppressed — then branches can be classically sampled. And if branches have bounded spatial entanglement then non-integrable systems can be efficiently simulated for long times.