Reversing cause and effect is no trouble for quantum computers

Reversing cause and effect is no trouble for quantum computers
Research published in Physical Review X shows that quantum computers can more easily model the reversal of cause and effect -- like following a movie played backwards -- than classical computers. The finding from researchers in Singapore, the United States and Europe may have implications for explaining how we perceive time. Credit: Aki Honda/Centre for Quantum Technologies, National University of Singapore

Watch a movie backwards and you'll likely get confused—but a quantum computer wouldn't. That's the conclusion of researcher Mile Gu at the Centre for Quantum Technologies (CQT) at the National University of Singapore and Nanyang Technological University and collaborators.

In research published 18 July in Physical Review X, the international team shows that a computer is less in thrall to the arrow of time than a classical computer. In some cases, it's as if the quantum computer doesn't need to distinguish between cause and effect at all.

The new work is inspired by an influential discovery made almost 10 years ago by complexity scientists James Crutchfield and John Mahoney at the University of California, Davis. They showed that many statistical data sequences will have a built-in arrow of time. An observer who sees the data played from beginning to end, like the frames of a movie, can model what comes next using only a modest amount of memory about what occurred before. An observer who tries to model the system in reverse has a much harder task—potentially needing to track orders of magnitude more information.

This discovery came to be known as causal asymmetry. It seems intuitive—after all, modeling a system when time is running backwards is like trying to infer a cause from an effect. We are used to finding that more difficult than predicting an effect from a cause. In everyday life, understanding what will happen next is easier if you know what just happened, and what happened before that.

However, researchers are always intrigued to discover asymmetries that are linked to time-ordering. This is because the fundamental laws of physics are ambivalent about whether time moves forward or in reverse. "When the physics does not impose any direction on time, where does causal asymmetry—the memory overhead needed to reverse cause and effect—come from?" asks Gu.

The first studies of causal asymmetry used models with classical physics to generate predictions. Crutchfield and Mahoney teamed up with Gu and collaborators Jayne Thompson, Andrew Garner and Vlatko Vedral at CQT to find out whether quantum mechanics changes the situation.

They found that it did. Models that use quantum physics, the team prove, can entirely mitigate the memory overhead. A quantum model forced to emulate the process in reverse-time will always outperform a classical model emulating the process in forward-time.

The work has some profound implications. "The most exciting thing for us is the possible connection with the arrow of time," says Thompson, first author on the work. "If causal asymmetry is only found in classical models, it suggests our perception of cause and effect, and thus time, can emerge from enforcing a classical explanation on events in a fundamentally quantum world," she says.

Next, the team wants to understand how this connects to other ideas of time. "Every community has their own arrow of time, and everybody wants to explain where they come from," says Vedral. Crutchfield and Mahoney called causal asymmetry an example of 's "barbed arrow."

Most iconic is the thermodynamic arrow. It comes from the idea that disorder, or entropy, will always increase—a little here and there, in everything that happens, until the universe ends as one big, hot mess. While causal asymmetry is not the same as the thermodynamic , they could be interrelated. Classical models that track more information also generate more disorder. "This hints that causal can have entropic consequence," says Thompson.

The results may also have practical value. Doing away with the classical overhead for reversing cause and effect could help quantum simulation. "Like a movie playing in reverse, sometimes we may be required to make sense of things that are presented in an order that is intrinsically difficult to . In such cases, quantum methods could prove vastly more efficient than their classical counterparts," says Gu.

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More information: Jayne Thompson et al, Causal Asymmetry in a Quantum World, Physical Review X (2018). DOI: 10.1103/PhysRevX.8.031013
Journal information: Physical Review X

Citation: Reversing cause and effect is no trouble for quantum computers (2018, July 20) retrieved 21 August 2019 from
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Jul 20, 2018
Let us apply the concept in this article to quantum entanglement: When the classical arrow of time states that an action made to one of two entangled particles "instantly" affects the other no matter how far apart they currently are, we are imposing classical mechanics on a quantum world. We might be better informed by saying that when we take an action on one of two entangled particles, their time arrow runs back to the instant of entanglement when the particles were in contact with each other(entanglement creation instant) and then moves forward in time until we measure the second particle ignorantly believing that the action was instant. I brought this up several times over the years in these columns, and have been roundly criticized. I stand my ground and now feel vindicated.

Jul 21, 2018
Winthrom, your idea is equivalent to an interpretation of QM, it is not new theory. It has no predictive power.
In QM two particles interacting instantly over a distance can be equivalent to the two particles interacting while separated in time.
The exchange of quantum information between the particles will be the same and the results of any experiments done also.
You will not be able to tell whether the exchange of quantum information took place by "jumping" space or "jumping" time.

Jul 21, 2018
IOW Winthron, a Quantum theory based on non-local interactions will be mathematically equivalent to one based on retro-causal interactions. They will give the same results for experiments.

Jul 21, 2018

Your interpretation of what I said is that there is a theory in QM concerning the equivalence and unprovability of that equivalence. you thus ask me to prove the negative of your assertion. The assertion in this article is that QM permits + and - time whereas classical physics is enslaved because "causal asymmetry is only found in classical models".

I propose an experiment: create two pairs of entangled particles, each pair not entangled with each other, but in such close proximity and relationship that any change in one pair will affect the other pair. Send 1/2 of each pair to remote location A and the other 1/2 to remote location B. At location A read the state of one of the particles. Next read the entangled pair to this particle at B. Finally, read the state of the untouched second pair particle at B. If the state of the untouched is affected by the touched particles, backward QM time travel is proven. Do this many times to generate statistical evidence. QED

Jul 22, 2018
In QM two particles interacting instantly over a distance can be equivalent to the two particles interacting while separated in time.

And in QM interpretations that reject "realism", the particle in question doesn't exist except as an interpretation of information, so when two particles are measured to exist, they only exist to their respective measurers, and when the measurers measure each other to see whether they found a particle and in what state, the apparent entanglement appears as an artifact of that interaction. It's meaningless to consider the two measurers separately because they are not real to each other until they interact.

So there's not even the need to invoke the arrow of time, or cause and effect, because your "past" depends only on what information about reality you happen to have.

What information you contain defines your past equally well as it determines your future, and if the information changes then both past and future change.

Jul 22, 2018
The above leads to a situation where things separated in space and time can have different realities - a sort of multiverse - where interactions between the points happen at the speed of light and "synchronize" reality because they end up containing roughly the same information about the universe further away, so people on earth feel approximately the same reality, whereas an alien race very far away in the distant universe may feel an entirely different universe because they've not yet interacted with our information.

Or to paraphrase the Mach's priciple: "local physical laws are determined by the large-scale structure of the universe"

Jul 22, 2018

There seem to be some confusion here. The article is about how classical models impose causal asymmetry while quantum models does not. Quantum field theory guarantee causality by obeying special relativity at observations - no communication theorem - whether or not entanglement meanwhile has been present. As Eikka notes.

Also, you try to hide entanglement in your example by renaming it as "touching", I believe.

Mach's principle has been rejected by general relativity AFAIU, centrifugal acceleration is not intrinsic but a resultant of applied forces.

And I don't think your relative past/multiverse works, since the observed past is within a light cone by definition. In other words, reality is real.

Jul 22, 2018
@torbjorn_b_g_larsson: Quantum Field Theory is an Effective Field Theory, the "real" theory is Quantum Mechanics. Believers in QFT are fighting an end-game. Also the "No Communication Theorem" is "actually" an ansatz to QFT preventing instantaneous communication. But we absolutely know proto-Quantum Computers work and do break the concept of ultimate locality. The Universe is in fact an open "wave-guide", As at the Big Bang, "everywhere" is right "here" and "now" due to "non-locality". The assumption of "point particles" is at fault, As Nikola Tesla (1932) said "There is no energy in matter except that absorbed from the medium." And Heinrich Hertz (1893) "In the sense of our theory we more correctly represent the phenomenon by saying that fundamentally the waves which are being developed do not owe their formation solely to processes at within the origin, but arise out of the conditions of the whole surrounding space." "Matter points" are vestigial "EM Black Holes" and contain "0".

Jul 22, 2018
Electromagnetic "Black Holes" as "Wormholes", are an indisputable form of the wave property matter always had, share the fact that their "Beckenstein Bound" stores all info on prior event states within it as a retro-causal advanced wave communicating through the "hole" to all parts in the non-causal Universe. In electromagnetic theory the near field is describable in terms of two separable components, The Radiation Field:Far field propagating energy, and The Inhomogeneous Static Field:Fisher Information stresses inside the "Wormhole" of "0" energy. The auger reaction (and reverse) is quantized at energy of particle creation which "punches a hole" in spacetime, these are the "quantized electrons" and their wormhole connected antiparticles (positrons). You have been told that propagating waves are due to accelerating static charges. This is incorrect. There is no energy 'in' the wormhole, it is a spacetime stressor only, it's energy and connection is entangled in the Universe around it.

Jul 22, 2018
"Finally, read the state of the untouched second pair particle at B" by "untouched" I mean "currently unobserved" so please read: Finally, read the state of the currently unobserved second pair particle at B.
Regards Mach's theorem, I agree with you.

My idea is that Einsteins definition of locality can exist for entanglement, but at the quantum level only. This functions for entangled pairs if and only if a negative time arrow also exists for the Quantum particles involved.To be sure, it is a very restricted and specific negative time arrow. Also, the "No Communication Theorem" is not violated because backtracking to the local state from the observation (in time and space forward) in non local space permits the transfer of states locally before the non locality takes place.

Jul 23, 2018
@winthrom: I concur with Wheeler-Feynman Absorber Theory Interpretation of Antiparticles. It is in keeping with Acausal Time interpretation and for CPT Symmetry, "negative time" plus "antiparticles" entirely equivalent to near-field real entangled "particles" down the connecting "wormhole". Some people mistakenly misinterpret length contraction and time dilation as being physical deformation eg. "Black Hole" spaghettification. It's a "Looking Glass", Mirror Universe effect... light approaching from different directions as caustics. No way to tell "moving" frames from "stationary" frames, they are just the way you transform SR coordinates from one frame to the other using relative velocity ditto "relativistic mass". These "optics" due to energy stored in the global spacetime vacuum non-locality of the particle wavefunction. Spacetime interferences is quantum holography. General principle is nothing travels faster than light and nothing "compresses" lengths or creates mass due to motion.

Jul 23, 2018
Mach's principle has been rejected by general relativity

It's more like, Mach's principle helped to formulate the general relativity.


This concept was a guiding factor in Einstein's development of the general theory of relativity. Einstein realized that the overall distribution of matter would determine the metric tensor, which tells you which frame is rotationally stationary.

The issue turns up in relativity, because we assume that masses have an intrinsic property called inertia, but if we imagine an object in space alone, we can't measure any inertia or motion. We need other objects around before these properties start to "emerge" and make a difference to the reality we percieve, and therefore this property we call inertia isn't a property of our object but of the whole system.

Jul 23, 2018

And I don't think your relative past/multiverse works, since the observed past is within a light cone by definition. In other words, reality is real.

That wasn't the point.

What you know OF reality is real to YOU. For example, we don't feel the gravity of the sun, but the gravity of the sun that was 8 minutes ago, and the actual sun is not there anymore. Likewise, the sun doesn't feel the effects of its gravity on us until 16 minutes later, so it sees us in a completely different place than we actually are.

This gives rise to two different realities, the sun's reality and our reality. The point I'm proposing is just to extend the same concept for basically everything: what you know of the reality is what reality is to you - there is no single reality that applies to everyone and everything, although over short distances the different realities look very much alike.

Jul 23, 2018
The intellectual issue is that when we theorize about these things, we fall victims of thinking from "God's point of view" in applying this knowledge that different points in time and space don't have the same view of the surrounding universe - they contain different information about the overall state of the whole.

If we understand that this local information defines what is here and now, in the QM sense of wavefunctions adding up and whatnot, i.e. particles aren't real of themselves but arise as the sum of information present at that point, then it stands to reason that people can measure things like entanglement all day long without any weird non-local spooky action at a distance phenomena going on, because at the end of the day when they come together to compare measurements, their information combines and THAT is when the correlations appear.

Jul 23, 2018
@winthrom thus ask me to prove the negative of your assertion.

You misunderstand, I'm not asking you to prove anything at all.
I'm telling you that when constructing a quantum field theory you can decide to get rid of non-locality by including retrocausality.
Since experimentally, all you can do is measure the properties of particles and since non-local theories and retrocausal theories give the same predictions for particle properties it is not possible to distinguish between the two theories experimentally (not unless some new physics is discovered).
You are far from the first person to think of this, it has been done, google it.

Jul 23, 2018
The question of reality is confused by semantics about what reality means, because we can take various points of view that are physically impossible to take (like the "God's point of view") and consider that reality, while forgetting that we aren't "God". The information we're thinking about isn't actually accessible to us.

What I'm saying is, for any observer in space and time, reality is what is percieved of it. If something doesn't make a DIFFERENCE in your state of being, it isn't real TO YOU.

Two particles separated in space or time do not interact directly because that is impossible (non-local), but via the information transmitted between them, and the information about each other is what is real to the particles, not the other particle as it "is". This is basic relativity.

Now just add QM and consider that the particles aren't real, but made of information themselves and can change with the changing information present at their point of space and time.

Jul 23, 2018
If the universe's dark matter is a dimensional placeholder and matter is merely an after effect, then this sort of reversed modelling of the effect of matter, might be able to define dark matter.

Jul 23, 2018
@KBK:The connecting "Wormhole" at center of matter-antimatter particles is topologically identical to a smooth intersection of a "thin" smoke-ring 2D "shell" and a 2D plane (like in Kaluza-Klein Theory). In two dimensions the pair behave like a Falaco Soliton in water embedded in the two dimensional surface of a "Superconducting Pooliverse" (equivalent to a "spacetime surface" in 2D, the 3rd Dim is "hidden under the surface" and is called "the bulk"), a "Cosmic String" connects the "points" Electron→Positron, the core of the donut surface translate to a degenerate electromagnetic vortex space with a "still 0 point" where the particles are "conventionally placed". But in this "Pooliverse" the spin simply increases towards the center until space (but not time) is cavitated into a "EM wormhole defect". With "Black Hole" Fundamental Particles (quarks and electrons) the energy content of the vortex, belongs in the outer donut space away from nodal string, thru a spin thru all space.

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