  The possible existence of closed timelike curves (CTCs) draws attention to
fundamental questions about what is physically possible and what is not. An
example is the "no cloning theorem" in quantum mechanics, which states that no
physical means exists by which an unknown arbitrary quantum state can be
reproduced or copied perfectly. Using the Deutsch approach, we have shown that
the no-cloning theorem can be circumvented in the presence of closed timelike
curves, allowing the perfect cloning of a quantum state chosen randomly from a
finite alphabet of states. Further, we show that a universal cloner can be
constructed that when acting on a completely arbitrary qubit state, exceeds the
no-cloning bound on fidelity. Since the no cloning theorem has played a central
role in the development of quantum information science, it is clear that the
existence of closed timelike curves would radically change the rules for
quantum information technology. Nevertheless, we show that this type of cloning
does not violate no-signalling criteria.
