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A closed timelike curve is a loop back in time (somewhat like the time portals in the Hollywood film Looper). That is, at certain "locations" in spacetime, there is a wormhole such that, if you jump in, you'll emerge at some point in the past. To the best of our knowledge, these time loops are not ruled out by the laws of physics.

Recent research of Todd Brun, Andreas Winter and I shows how a time traveler can copy quantum data at will, in violation of a fundamental principle of quantum mechanics often referred to as the "no quantum Xerox machine" theorem. The method involves looping a quantum particle back many times in the past and then reading out many copies of it in such a way that you don't disturb the past.

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