Time Travel Computing: How to Use Physics to Solve Currently Intractable Problems

Why send a message back in time, but then lock it so that no one can ever read the contents? This may be the key to solving currently intractable problems.

Time travel is just theory. However, working on that theory may allow researchers to better understand how to perform currently intractable communications. If an experimenter entangles a message with some other system in the lab before sending it, he or she may create correlations between the time-traveling message and the lab system, which may fuel quantum computation.

About ten years ago, researcher Dave Bacon showed that a time-travelling quantum computer could quickly solve a group of problems, known as NP-complete, which mathematicians have lumped together as being hard. The problem was that the quantum computer was travelling around "closed timelike curves." These are paths through the fabric of spacetime that loop back on themselves. General relativity allows such paths to exist through contortions in spacetime known as wormholes.

Physicists argue something must stop such opportunities arising because it would threaten "causality." For example, someone could travel back in time and kill their grandfather, negating their own existence.

Breaking the causal flow of time has consequences for quantum physics, as well. Over the past two decades, researchers have shown that foundational principles of quantum physics break in the presence of closed timelike curves: you can beat the uncertainty principle, which is an inherent fuzziness of quantum properties, and the no-cloning theorem, which says quantum states can't be copied.

This new work, though, shows that a quantum computer can solve insoluble problems even if it is travelling alone "open timelike curves," which don't create causality problems. This is because they don't allow direct interaction with anything in the object's own past: the time travelling particles, or data they contain, never interact with themselves.

"We avoid 'classical' paradoxes, like the grandfathers paradox, but you still get all these weird results," said Mile Gu, one of the researchers, in a news release.

Quantum particles sent on a timeloop could gain super computational power, even though the particles never interact with anything in the past. The reason that there's an effect is because some information is stored in the entangling correlations; this is what the researchers are harnessing.

The findings are published in the journal npj Quantum Information.

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