The Problem with Time Travel: Physicists Tackle Cloning Quantum Information in the Past

The Doctor from "Doctor Who" isn't the only one who deals with time travel. Physicists have also tackled the idea. Now, scientists have found that it would be theoretically possible for time travelers to copy quantum data from the past.

The latest findings all started when David Deutsch, a pioneer of quantum computing, came up with a simplified model of time travel to deal with the paradoxes that would occur if time travel were possible. For example, would it be possible to travel back in time to kill one's grandfather?  In fact, some researchers have used this paradox to argue that it is actually impossible to change the past.

"The question is, how would you have existed in the first place to go back in time and kill your grandfather?" said Mark Wilde, one of the researchers, in a news release.

So how is this solved? Originally, Deutsch solved this paradox by using a slight change to quantum theory. He proposed that you could change the past as long as you did so in a self-consistent manner. This means that if you kill your grandfather, you do it with only probability one-half; this means he's dead with probability one-half and you are not born with probability one-half. The opposite is also true.

Yet there's another issue with time travel--and that's the no-cloning theorem. This theorem, which is related to the fact that one cannot copy quantum data at will, is a consequence of Heisenberg's famous Uncertainty Principle, by which one can measure either the position of a particle or its momentum, but not both with unlimited accuracy. According to this principle, it is impossible to take one particle and spit out two particles with the same position and momentum since you would know too much about both particles at once.

Yet now, this new research has looked at whether a Deutschian closed timelike curve could allow for copying of quantum data to many different points in space. The new approach allows for a particle, or a time traveler, to make multiple loops back in time-something like Bruce Willis' travels in the Hollywood film "Looper."

"That is, at certain locations in spacetime, there are wormholes such that, if you jump in, you'll emerge at some point in the past," said Wilde in a news release. "To the best of our knowledge, these time loops are not ruled out by the laws of physics. But there are strange consequences for quantum information processing if their behavior is dictated by Deutsch's model."

A single looping path back in time behaving according to Deutsch's model would have to allow for a particle entering the loop to remain the same each time it passed through a particular point in time. In other words, the particle would have the maintained self-consistency. In order to be consistent with Deutsch's model, the researchers had to come up with a solution that would allow for a looping curve back in time, and copying of quantum data based on a time traveling particle, without disturbing the past.

"That was the major breakthrough, to figure out what could happen at the beginning of this time loop to enable us to effectively read out many copies of the data without disturbing the past," said Wilde in a news release. "It just worked."

The findings reveal that it's possible to copy quantum data from the past. While this finding may not seem all that practical, it does have implications for quantum computing in the future.

The findings are published in the journal Physical Review Letters.