npj Quant. Info. 1, 15007 (2015)

Time travel, it seems, is little more than fantasy — particularly for those of us with a healthy respect for causality. But in the quantum regime, solutions to the equations of general relativity, known as closed timelike curves, are capable of breaking causality. The implications are rather drastic: these solutions permit the violation of uncertainty principles and the cloning of quantum states. As long you respect causality though, these crimes against quantum mechanics are deemed null and void. But what if there were a way to get all these causality-breaking effects without actually breaking causality?

Xiao Yuan and colleagues have come up with a scenario for realizing this paradox. The team considered a time-travelling particle that is isolated from its causal past while it's going back in time. So, in principle, the particle itself can break causality — it just never actually does, as a result of its complete isolation. If the particle is initially entangled with another, it can still violate uncertainty principles and clone quantum states. The solutions corresponding to these particles may even afford quantum processors additional computing power — at least, in theory.