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MQI

Seminar

Quantum correlations in Newtonian space and time: faster than light communication or nonlocality

Gisin, N (Université de Genève)
Wednesday 16 October 2013, 09:00-10:00

Seminar Room 1, Newton Institute

Abstract

Using a theorem that allows one to assert the presence of nonlocal correlations between parties that are never measured in the same run of an experiment (using marginal), we study the following question: Experimental violations of Bell inequalities using space-like separated measurements precludes the explanation of quantum correlations through causal influences propagating at subluminal speed. Yet, “everything looks as if the two parties somehow communicate behind the scene”. We investigate the assumption that they do so at a speed faster than light, though finite. Such an assumption doesn’t respect the spirit of Einstein relativity. However, it is not crystal clear that such “communication behind the scene” would contradict relativity. Indeed, one could imagine that this communication remains for ever hidden to humans, i.e. that it could not be controlled by humans, only Nature exploits it to produce correlations that can’t be explained by usual common causes. To define faster than light hidden communication requires a universal privileged reference frame in which this faster than light speed is defined. Again, such a universal privilege d frame is not in the spirit of relativity, but it is also clearly not in contradiction: for example the reference frame in which the cosmic microwave background radiation is isotropic defines such a privileged frame. Hence, a priori, a hidden communication explanation is not more surprising than nonlocality. We prove that for any finite speed, such models predict correlations that can be exploited for faster-than-light communication. This superluminal communication doesn’t require access to any hidden physical quantities, but only the manipulation of measurement devices at the level of our present-day description of quantum experiments. Consequently, all possible explanations of quantum correlations that satisfy the principle of continuity, which states that everything propagates gradually and continuously through space and time, or in other words, all combination of local common causes and direct causes that reproduce quantum correlations, lead to faster than light communication, which can be exploited by humans, at least in principle. Accordingly, either there is superluminal communication or the conclusion that Nature is nonlocal (i.e. discontinuous) is unavoidable.

1. Jean-Daniel Bancal, Stefano Pironio, Antonio Acin, Yeong-Cherng Liang, Valerio Scarani and Nicolas Gisin, Quantum non-locality based on finite-speed causal influences leads to superluminal signalling, Nature Physics, 8, 867-70, 2012. 2. N. Gisin, arXiv:1210.7308 3. Tomer Jack Barnea, Jean-Daniel Bancal, Yeong-Cherng Liang, Nicolas Gisin, A tripartite quantum state violating the hidden influence constraints, quant-ph/1304.1812, PRA in press.

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Recent Comments

From
In my comment of 13 Feb 2014, I stated that in the circumstances a possible explanation of the result of the attempt to measure the velocity of communication by quantum entanglement was that the communication was in an extra dimension. On further thought, I do not believe that this is likely; if the measurements are made in the space-time of special relativity, then there will be a start time and an end time to the velocity measurement regardless of the dimension of communication. The idea of an asymmetric distribution of velocities still holds. There will be one frame in which infinite velocity is theoretically possible in all directions, i.e. a symmetric distribution of possible velocities. In all other frames, the distribution will be asymmetric along the direction of motion, although it would be difficult to distinguish frames close to the symmetric frame. I am not aware of any measurements being made in the polar direction, but it would be extremely unfortunate if the equatorial direction was at right angles to earth's motion relative to absolute space. Note that earth's motion in the solar system is negligible compared to that considered likely
relative to space. If the earth carried local space along with it, then a symmetric distribution would be possible, but this not an explanation to be given lightly. A more likely explanation would be that the measurements are incorrect, but this also cannot be made lightly. Further investigation may reveal all.
From
According to my investigation of velocities faster than light, their distribution is asymmetrical. If we consider a frame in which, ideally, all stars are evenly distributed in space, or in which the cosmic background is evenly distributed, we may take that frame as a basic reference frame, Let us assume that in x-t frame S all velocities are possible between minus infinity and plus infinity relative to the origin.No past-directed velocities are possible since they would allow violation of causality. Now consider another frame S1 moving along the x-axis relative to S. If th clocks in S are all set to zero, then the clocks in S1 must be set to read past times in the forward direction, and future times in the backward direction; This means that in S1 both past and future-directed velocities are possible in the forward direction, but only future-directed velocities are possible in the backward direction. Hence, the asymmetry. If this reasoning is correct, then the velocity of communication by quantum entanglement should be dependent on direction. Furthermore, the ratio of forward and backward velocities would be unique to the reference frame, thus enabling the direction of earth's motion w.r.t. absolute space space (the cosmic frame) to be determined and, possibly, the actual velocity. I was very disappointed to find that no velocity differences had been detected. If the measurements are correct, there is the possibility that they have been made at right to the earth's motion, or that my reasoning is incorrect, or that the communication is in extra dimensions.
From
I do not understand why supra-light velocities are regarded as so mysterious. Theoretically, they are measurable using the synchronised clocks of Special Relativity. Whereas, mathematically, one reference frame moving with constant velocity is as good as another, in the real world we can distinguish what we may call a cosmic frame in which the stars are evenly distributed in all directions. When we move relative to this frame the distribution becomes asymmetric; there are more stars in the space ahead of us than behind us, also they appear more energetic, i.e. blueshifted, As we go faster, towards light speed, the universe tends to become concentrated in a cone ahead of us.

If we now consider time t in the cosmic frame S, say, we can set t = 0 at the origin of an x-t frame , and at all points along the x-axis. If we now have a frame S1, say, moving in the x-direction relative to the cosmic frame, then when the origins coincide, all clocks in S1 Have to be synchronised to past times in the forward direction relative to the origin, and to future times in the backward direction. If we allow all velocities between +
From
A very interesting presentation Professor Nicolas Gisin gave but I missed the next steps part.
Theory without practical conclusions is the bread without the beef.
It was interesting to see that the by now quite convoluted language in the domain of quantum communication made it difficult for the audience to follow in parts.
The whole topic would benefit from a more direct language: After all communicating ideas is all about making them understandable to others in the most effective way, for them to build on and further and enrich the core ideas.

But when Gisin noted that he would not recommend that any junior physicist should speculate about faster than light communication in public he hit on the core problem of front line science nowadays.
Scientists much rather speak about dark energy, dark matter, hidden influences and spooky action at a distance than using more practical and reality related terms.
So using the S word (supraluminal communication) is very dangerous for a genuine scientist nowadays whereas spooky action at a distance or the universe is fundamentally not-real goes perfectly conform with mainstream teachings.
I wonder how Galileo would describe this self-imposed shackling - he who was threatened with the stake - while we might be threatened with a smaller budget.

Gisin put it very nicely when he said that radio communication is millions of times faster than sound communication and why should there not be the next level of quantum communication another n orders higher than radio communication in 4D space.
So the summary could be read as:
The collapse of the wavefunction, according to current knowledge, happens far faster than the speed of light and may be instantaneous.
The entanglement and collapse of particles happens outside of our well known 4D spacetime configuration. (hidden influence ; behind the scenes)
The 3 and 4 partite theoretical proof of supraluminal quantum communication should be subjected to a practical experiment. However the "ugly" (whatever that means) quantum measurement methods must be purified beforehand.
No causal paradox (grandfather paradox) will happen if communication only happens in a /few select reference frame(s).
We are still in search of a story of how and why nature works like this.

The stories:
Gisin quite well pointed out that stories are important to set the stage for physics to hypothesize, theorize and eventually experiment.

Luckily do have many possible contender stories for this quantum communication effect ( hitherto hidden influence)

eg.We have the Randall-Sundrum (RS) model with large higher dimensional bulk space dimensions, where gravity unfolds from a neighbouring gravity brane.

And we also have the string theory models where particles in 4D space are merely the image of looped or closed strings connected to our 4D brane surface.

So just like an electron and a photon can decouple (electron emits energy in form of photon) so can two photons be coupled (entangled).
The infinite length of photon strings makes it easy to explain why coupled (entangled) photons know of each other's position in 4D space (on the world sheet, brane) and thus collapsing their wavefunction is nothing but decoupling these strings.
Superimposed rotations (polarizations, vibration modes) of the photon-strings then become defined for each photon.
As for the speed it is quite relative and depends on mass/gravity. If eg. the bulk space of the RS model is indeed a gravity dominated space then any signalling can occur quite leisurely in bulk-space-time whereas it would seem many orders faster in our 4D space.
Where the end-points of the string attached to 4D-space travel at light speed the disentanglement (collapse) signal would be conveyed much faster, from our point of view.

There are a variety of stories one can think of to explain the propagation of the signal from the measured photon to the distant entangled photon.

But how important is the story really. Or do we need the full story before we can make use of it. After all knowledge without application is only half the rent.
Had our brethren cavemen really not dared to make use of the (magic, holy) fire phenomenon until there was a thorough explanation of how things can be made to burn we would still all sit in a cold dark cave.
So what is more important ? How it works or whether we can make it work?
Eventually the Nobel prize is only granted when the theoretical or hypothesized effect is actually measured or produced.
The story and the application (practical experiment first) and the hypothesize are all intertwined and need to evolve iteratively.

Next steps:

Well we all want to know, whether we can make use of this supraluminal quantum communication model or not.
What then will be the next steps?
How to purify the "ugly" 3-partite states?
How to set up the experiment?

We all know other than in quantum mechanics there are only two possible outcomes: either it can be done or not.

With less fear of formulating new hypotheses and a less convoluted (Galilean) ways of realizing things we would all know sooner and could move on to the next quests.

I am convinced that Gisin is on to something fundamental as it is a logical consequence to past discoveries of how nature and the universe work.

From a philosophical point of view it is extremely unlikely that after barely 2500 years of scientific-successes it should all end after Einstein and there will be no more to discover until the end of time.
This is not how nature presents itself. Quite the opposite- it is far more likely that the best and interesting is yet to come - always.

So we should encourage young students to ask the blasphemic questions and to "torture" the established science community with daring and shocking assumptions and hypotheses.

Because the only regret we should have at the end of our lives, is not to have dared asking questions and searching for knowledge in the most unconventional ways and places.

regards

Anthony

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