  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 show here that
this theorem can be circumvented in the presence of closed timelike curves,
allowing the cloning of an unknown arbitrary quantum state chosen from a finite
alphabet of states. Since the "no cloning theorem" has played a central role in
the development of quantum information science, it is clear that the existence
of CTCs would radically change the rules for quantum information technology.
Nevertheless we show that this type of cloning does not violate no-signalling
criteria.
