Closed timelike curves make quantum and classical computing equivalent

@article{Aaronson2008ClosedTC,
  title={Closed timelike curves make quantum and classical computing equivalent},
  author={Scott Aaronson and John Watrous},
  journal={Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences},
  year={2008},
  volume={465},
  pages={631 - 647}
}
  • S. Aaronson, J. Watrous
  • Published 19 August 2008
  • Computer Science
  • Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences
While closed timelike curves (CTCs) are not known to exist, studying their consequences has led to non-trivial insights into general relativity, quantum information and other areas. In this paper, we show that, if CTCs existed, quantum computers would be no more powerful than classical computers: both would have the (extremely large) power of the complexity class polynomial space (), consisting of all problems solvable by a conventional computer using a polynomial amount of memory. This solves… 

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