Quantum state cloning in the presence of a closed timelike curve

TL;DR

This paper demonstrates that the presence of closed timelike curves (CTCs) can enable perfect quantum state cloning, challenging the no-cloning theorem and potentially transforming quantum information science, while still respecting no-signalling constraints.

Contribution

It shows that CTCs allow perfect and universal quantum cloning, circumventing the no-cloning theorem under the Deutsch approach.

Findings

Perfect cloning of states from a finite alphabet is possible with CTCs.

A universal quantum cloner exceeding the no-cloning fidelity bound is constructed.

Cloning with CTCs does not violate no-signalling principles.

Abstract

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.