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The possible existence of closed timelike curves (CTCs) draws attention tofundamental questions about what is physically possible and what is not. Anexample is the "no cloning theorem" in quantum mechanics, which states that nophysical means exists by which an unknown arbitrary quantum state can bereproduced or copied perfectly. Using the Deutsch approach, we have shown thatthe no-cloning theorem can be circumvented in the presence of closed timelikecurves, allowing the perfect cloning of a quantum state chosen randomly from afinite alphabet of states. Further, we show that a universal cloner can beconstructed that when acting on a completely arbitrary qubit state, exceeds theno-cloning bound on fidelity. Since the no cloning theorem has played a centralrole in the development of quantum information science, it is clear that theexistence of closed timelike curves would radically change the rules forquantum information technology. Nevertheless, we show that this type of cloningdoes not violate no-signalling criteria.

Quantum state cloning in the presence of a closed timelike curve