Bipartite quantum states admitting a causal explanation

Abstract: The statistics of local measurements of joint quantum systems can sometimes be used to distinguish the temporal order in which they were measured. We first prove that every bipartite separable density matrix is temporally compatible with direct causal influence for arbitrary finite-dimensional quantum systems and measurements of a tomographically-complete class of observables, which includes all Pauli observables in the case of multi-qubit systems. Equivalently, if a bipartite density matrix is not temporally compatible with direct causal influence, then it must be entangled. We also provide an operational meaning for the temporal evolution consistent with such correlations in terms of a generalized dephasing channel and a pretty good measurement. The two temporal evolutions turn out to not be Petz recovery maps of each other, but are Bayesian inverses of each other. Finally, we prove necessary and sufficient conditions for an arbitrary bipartite quantum state to be temporally compatible, thereby providing a temporal analogue of the positive partial transpose criterion valid for quantum systems of any dimension.