We report an immediate generalization of the argument by Sen(De) and Sen proving that quantum dynamics, if linear, must be unitary, under the hypothesis of no-faster-than-light signaling between noninteracting entangled parties. The assumption of linearity is unnecessary and the conclusion is even stronger: not only are supraquantum cloning or deleting machines forbidden by the no signaling condition, but also any nonuniversal trace-preserving evolution.

• Received 13 October 2005

DOI:https://doi.org/10.1103/PhysRevA.73.034304