Quantum Temporal Marginal Problem

Abstract

Experiments on a multi-time quantum process may reveal its responses only on selected, possibly overlapping, sets of times. We ask whether these partial descriptions admit a common underlying process, and formulate this process tensor marginal problem as a semidefinite feasibility test. Agreement on overlaps is necessary but insufficient: three locally consistent qubit marginals can have no common extension. Another family is incompatible as a time-ordered process even though a single quantum channel satisfies both its channel and temporal marginal constraints. This isolates temporal causality as the obstruction. We quantify incompatibility by the mixing needed to restore compatibility and derive finite tests using separate interventions at each time and final measurements. We further show that incompatibility is equivalent to an advantage over every compatible family in a multi-time process-discrimination task. Together, these results test whether process descriptions obtained in different experiments can arise from one causally ordered quantum process.