Credit: Adapted from npj Quantum Information, doi:10.1038/npjqi.2015.7 (2015)
Why send a message back in time, but lock it so that no one can ever read the contents? Because it may be the key to solving currently intractable problems. That's the claim of an international collaboration who have just published a paper in npj Quantum Information. It turns out that an unopened message can be exceedingly useful. This is true if the experimenter entangles the message with some other system in the laboratory before sending it. Entanglement, a strange effect only possible in the realm of quantum physics, creates correlations between the time-travelling message and the laboratory system. These correlations can fuel a quantum computation. If the universe allows 'open timelike curves', particles travelling back in time along them could help to perform currently intractable computations. Even though such curves don't allow for interaction with anything in the past, researchers writing in npj Quantum Information show there is a gain in computational power as long as the time-travelling particle is entangled with one kept in the present. Around ten years ago researcher Dave Bacon, now at Google, showed that a time-travelling quantum computer could quickly solve a group of problems, known as NP-complete, which mathematicians have lumped...
