Time warp: Researchers show possibility of cloning quantum information from the past

Dec 06, 2013 by Paige Brown

(Phys.org) —Popular television shows such as "Doctor Who" have brought the idea of time travel into the vernacular of popular culture. But problem of time travel is even more complicated than one might think. LSU's Mark Wilde has shown that it would theoretically be possible for time travelers to copy quantum data from the past.

It all started when David Deutsch, a pioneer of quantum computing and a physicist at Oxford, came up with a simplified model of travel to deal with the paradoxes that would occur if one could travel back in time. For example, would it be possible to travel back in time to kill one's grandfather? In the Grandfather paradox, a time traveler faces the problem that if he kills his grandfather back in time, then he himself is never born, and consequently is unable to travel through time to kill his grandfather, and so on. Some theorists 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, an LSU assistant professor with a joint appointment in the Department of Physics and Astronomy and with the Center for Computation and Technology, or CCT.

Deutsch solved the Grandfather paradox originally using a slight change to quantum theory, proposing that you could change the past as long as you did so in a self-consistent manner.

"Meaning that, if you kill your grandfather, you do it with only probability one-half," Wilde said. "Then, he's dead with probability one-half, and you are not born with probability one-half, but the opposite is a fair chance. You could have existed with probability one-half to go back and kill your grandfather."

But the Grandfather paradox is not the only complication with time travel. Another problem is the no-cloning theorem, or the no "subatomic Xerox-machine" theorem, known since 1982. 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 the Uncertainty Principle, it is thus impossible to have a subatomic Xerox-machine that would take one particle and spit out two with the same position and momentum – because then you would know too much about both particles at once.

"We can always look at a paper, and then copy the words on it. That's what we call copying classical data," Wilde said. "But you can't arbitrarily copy quantum data, unless it takes the special form of classical data. This no-cloning theorem is a fundamental part of quantum mechanics – it helps us reason how to process quantum data. If you can't copy data, then you have to think of everything in a very different way."

But what if a Deutschian closed timelike curve did allow for copying of quantum data to many different points in space? According to Wilde, Deutsch suggested in his late 20th century paper that it should be possible to violate the fundamental no-cloning theorem of quantum mechanics. Now, Wilde and collaborators at the University of Southern California and the Autonomous University of Barcelona have advanced Deutsch's 1991 work with a recent paper in Physical Review Letters. 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," Wilde said. "To the best of our knowledge, these time loops are not ruled out by the laws of physics. But there are strange consequences for processing if their behavior is dictated by Deutsch's model."

A single looping path back in time, a time spiral of sorts, behaving according to Deutsch's model, for example, 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 need to maintain self-consistency as it looped back in time.

"In some sense, this already allows for copying of the particle's data at many different points in space," Wilde said, "because you are sending the particle back many times. It's like you have multiple versions of the particle available at the same time. You can then attempt to read out more copies of the particle, but the thing is, if you try to do so as the particle loops back in time, then you change the past."

To be consistent with Deutsch's model, which holds that you can only change the past as long as you can do it in a self-consistent manner, Wilde and colleagues 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," Wilde said. "It just worked."

However, there is still some controversy over interpretations of the new approach, Wilde said. In one instance, the new approach may actually point to problems in Deutsch's original closed timelike curve model.

"If quantum mechanics gets modified in such a way that we've never observed should happen, it may be evidence that we should question Deutsch's model," Wilde said. "We really believe that quantum mechanics is true, at this point. And most people believe in a principle called Unitarity in quantum mechanics. But with our new model, we've shown that you can essentially violate something that is a direct consequence of Unitarity. To me, this is an indication that something weird is going on with Deutsch's model. However, there might be some way of modifying the model in such a way that we don't violate the no-cloning theorem."

Other researchers argue that Wilde's approach wouldn't actually allow for copying quantum data from an unknown particle state entering the time loop because nature would already "know" what the particle looked like, as it had traveled back in time many times before.

But whether or not the no-cloning theorem can truly be violated as Wilde's new approach suggests, the consequences of being able to copy quantum data from the past are significant. Systems for secure Internet communications, for example, will likely soon rely on quantum security protocols that could be broken or "hacked" if Wilde's looping time travel methods were correct.

"If an adversary, if a malicious person, were to have access to these time loops, then they could break the security of quantum key distribution," Wilde said. "That's one way of interpreting it. But it's a very strong practical implication because the big push of quantum communication is this secure way of communicating. We believe that this is the strongest form of encryption that is out there because it's based on physical principles."

Today, when you log into your Gmail or Facebook, your password and information encryption is not based on physical principles of quantum mechanical security, but rather on the computational assumption that it is very difficult for "hackers" to factor mathematical products of prime numbers, for example. But physicists and computer scientists are working on securing critical and sensitive communications using the principles of . Such encryption is believed to be unbreakable – that is, as long as hackers don't have access to Wilde's looping closed timelike curves.

"This ability to copy quantum information freely would turn quantum theory into an effectively classical theory in which, for example, classical data thought to be secured by quantum cryptography would no longer be safe," Wilde said. "It seems like there should be a revision to Deutsch's model which would simultaneously resolve the various paradoxes but not lead to such striking consequences for quantum information processing. However, no one yet has offered a model that meets these two requirements. This is the subject of open research."

Explore further: The importance of three-way atom interactions in maintaining coherence

More information: DOI: 10.1103/PhysRevLett.111.190401

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Mimath224
1.6 / 5 (7) Dec 06, 2013
I like the first paragraph;
'...But problem of time travel is even more complicated than one might think....'
Really!
David Deutsch, I understand, also favors the 'many-worlds' theory. From his p(in) and p(loop) equation
'...The self-consistency of this equation proves the self-consistency of time travel in quantum physics...'
which I suggest is another 'loop'.
betterexists
1.4 / 5 (8) Dec 06, 2013
What you get on a Silver nitrate Film is a Photo.

Does it not mean that several other compounds TOO captured the past in themselves?

Particularly, What did the Brains of those long lasting Turtles capture until they died?
Can't it be put on a Super Computer some how as a Video?
eachus
not rated yet Dec 06, 2013
If you can built a time machine, it is possible to use it as an oracle in a computing system. As a simple example, solve an NP-complete problem by getting the answer from the future, then validating it and sending it back. The system is self-consistent, and answers only take as long as the verifying step.

The same logic applies to quantum key distribution. You can't intercept the key in transit, but once a message is intercepted, you don't need the key to read it, just the oracle.

So worrying about the effects of time travel on quantum key distribution is silly. What you need is a way to use time travel to send messages that can't be intercepted.
PacRim Jim
1.5 / 5 (8) Dec 07, 2013
"...one can measure either the position of a particle or its momentum, but not both with unlimited accuracy..."
What is unlimited accuracy? Surely distances below the Planck length are immeasurable.
Mimath224
1 / 5 (6) Dec 07, 2013
@eachus
If you can built a time machine, it is possible to use it as an oracle in a computing system. As a simple example, solve an NP-complete problem by getting the answer from the future, then validating it and sending it back. The system is self-consistent, and answers only take as long as the verifying step...

First one has to be sure the/a future exists! It is somewhat 'easier' to consider past events because they have happened but to consider that future events have also 'already happened' so that a time machine can collect info and send it back, is something else. If future events already exist it means the world line is already mapped out and we are simply treading the path.
If the future exists on the QM level only would that not imply did have a hidden variable?
Mayday
2 / 5 (4) Dec 07, 2013
Why do so many time travel discussions ignore the fact that in addition to traveling through time, our planet is also traveling very quickly through space? Time travel as most people apparently envision it would require extremely precise travel across vast distances of space in addition to traveling across time. Miss the mark by just a little, say a few miles up or down, and you are done. Best bet would be to go in some sort of spacecraft and then attempt to locate Earth. Best of luck.
thingumbobesquire
1.7 / 5 (6) Dec 07, 2013
The real paradox is why anyone is paying these loonies to purvey such risible psychobabble.
KBK
1 / 5 (7) Dec 07, 2013
When one goes back to the past, entanglement begins and probability curves begin to reduce to zero, chance slowly becomes inevitability and localized loops solve themselves, as isolated quantum temporal wormholes.

Which either conclude, ie. fall into disarray, or perpetuate-stabilize, but they are relative and localized, which means they are out of the standard linear unidirectional flowing probability continuum... and will not be encountered by the rest of the 'standard' universe model.

So, one can make such a past-now time connection... but as it resolves itself into existence, it will be localized and separate from unidirectional probability. Proof of it's existence is verifiable from within the loop, not external. the more real, the more isolated, the less connected to 'regular' (as we generally know it) flow.
Zephir_fan
Dec 07, 2013
This comment has been removed by a moderator.
Mayday
1.6 / 5 (5) Dec 07, 2013
A quick search suggests that if you went back in time just one second, your would end up at a spot roughly 369 kilometers from where you began. But in which direction? The odds of your arrival point being anywhere near the surface of the Earth would be quite low. How does the brave time traveler propose moving their position through space simultaneously to account for the movement of the planet through the universe as they "travel" away from the local flow of time? Whatever they have in mind, this amazing transport technology might be at least as valuable as their time travel ideas. Why do they keep it secret?
vjagpal
1 / 5 (5) Dec 07, 2013
I always have difficulty understanding concept behind time travel because I think of time as a concept to access and arrange information. To me talking to traveling time us like talking about traveling from English language to Spanish language.
met a more fishes
1 / 5 (5) Dec 08, 2013
You can't travel into the past. Time is a sequence of events that cease existing as soon as they are over, there's no record of the arrangement of all the particles in the universe at all moments of history just waiting some where to be perused, that's ridiculous. You can travel into the future in a sense but only by traveling across distances so vast they operate on a different time scale. It may be possible for two pints in timeto be the same Point in time though, at least for some aspects of some particles. IMO, obviously.
Moebius
1 / 5 (2) Dec 08, 2013
This is nothing more complicated than the chicken and egg question and the answer is just as simple, there is no paradox.

If you go back in time obviously you can change the past, you're there. Your father will be there and if you happen to also be someone who hates your father you will have no problem with doing the experiment. What will happen is you will create a new timeline where your father no longer exists at the time of your conception and a duplicate of you won't be born.

The real question which can never be answered is this. Is the new timeline the only one or does the old timeline also exist in parallel with you and your father. There is no paradox either way. The problem with it is that both possibilities require an infinite number of parallel versions of our universe existing simultaneously. That is the real reason why it's impossible.
Moebius
1 / 5 (2) Dec 08, 2013
Here is how a time machine could destroy the earth.

Assume time travel to the past exists. Make a permanent granite monument with a party invitation for all time travelers and a date (any famous date will attract time travelers). This could cause an infinite number of time travelers to try to appear at the same time in the same place.
virtuality
1 / 5 (1) Dec 09, 2013
If Wilde's theory is correct could I go back to the past for something less sinister than killing my grandfather - maybe find some of those socks I have lost over the years? Or would I simply be creating a copy of myself sitting with the same headache?
kochevnik
1 / 5 (1) Dec 09, 2013
A quick search suggests that if you went back in time just one second, your would end up at a spot roughly 369 kilometers from where you began. But in which direction? The odds of your arrival point being anywhere near the surface of the Earth would be quite low. How does the brave time traveler propose moving their position through space simultaneously to account for the movement of the planet through the universe as they "travel" away from the local flow of time? Whatever they have in mind, this amazing transport technology might be at least as valuable as their time travel ideas. Why do they keep it secret?

Not a problem because you are not moving only in space, but in spacetime. A component of your travel is in time, which is orthogonal to the three space vectors. A movement backward in time will yield a corresponding movement in space
kochevnik
1 / 5 (1) Dec 09, 2013
@Moebius There is no paradox either way. The problem with it is that both possibilities require an infinite number of parallel versions of our universe existing simultaneously. That is the real reason why it's impossible.
Not impossible. The multiverse is clearly observable in quantum mechanics. By making observations you steer your way through the multiverses. Going backward in time involves observations just as going forward. The only difference is that the universe began in a highly ordered state, but the future will be a completely randomized state. So going backward in time gives you fewer configurations of particles, as the past is more ordered. For this reason the future gives you more choices, which makes it more probable
Mimath224
1 / 5 (2) Dec 09, 2013
@kochevnik, entropy must be involved! Entropy could be the principle that determines which part of a 'multiverse' survives. Indeed, it might be a 'stronger' former of entropy that will deny TT in the end
PoojaKashyap
Dec 10, 2013
This comment has been removed by a moderator.
eachus
3 / 5 (2) Dec 12, 2013
...It is somewhat 'easier' to consider past events because they have happened but to consider that future events have also 'already happened' so that a time machine can collect info and send it back, is something else. If future events already exist it means the world line is already mapped out and we are simply treading the path.
If the future exists on the QM level only would that not imply did have a hidden variable?


No hidden variables. Enough work has been done on quantum computing and superposition of states to know that the superposed states are real. Not only have experiments been done where one atom was in two places at once, it was possible to photograph the particle emitting photons in both places.

Same thing goes with your argument about the existence of the future. We don't care whether or not the future corresponding to the answer we need from the oracle ever really happens. If that future was possible, the QM oracle will provide the correct answer.
Mimath224
1 / 5 (2) Dec 13, 2013
@eachus As far as hidden variables are concerned...my question was really suggesting the opposite No hv. With 'future' you imply 'pssibilities' ('...was possible...') so if the fututre is structured with 'possibilities' (could be infinite) a 'QM oracle' surely would have to explore all before any answer (let alone correct) could be given.

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