A Superselection Rule for Quantum Causality

Abstract: We formalize the independent frame choice on connections between local laboratories as a symmetry principle for indefinite causal order. This induces a superselection rule that partitions process space into distinct sectors and leads to our main result: every bipartite covariant process$-$including marginals of multipartite circuits or reductions across cuts$-$is causally separable. Because covariant processes cannot be converted into noncovariant ones, all circuit-embeddable dynamics, such as the quantum switch, are forbidden from violating bipartite causal inequalities, even asymptotically.