Locally unitary quantum state evolution is local

We study the localization properties of bipartite channels, whose action on a subsystem yields a unitary channel. In particular we show that, under such channel, the subsystem must evolve independent of its environment. This point of view is another way to verify certain well-known conservation laws of quantum information in a generalized way. A no-go theorem for non classical conditional semantics in quantum computation is obtained as an intermediate result.