  We show that three fundamental information-theoretic constraints--the
impossibility of superluminal information transfer between two physical systems
by performing measurements on one of them, the impossibility of broadcasting
the information contained in an unknown physical state, and the impossibility
of unconditionally secure bit commitment--suffice to entail that the
observables and state space of a physical theory are quantum-mechanical. We
demonstrate the converse derivation in part, and consider the implications of
alternative answers to a remaining open question about nonlocality and bit
commitment.
