Physics team entangles photons that never coexisted in time

May 28, 2013 by Bob Yirka report
Time line diagram. (I) Birth of photons 1 and 2, (II) detection of photon 1, (III) birth of photons 3 and 4, (IV) Bell projection of photons 2 and 3, (V) detection of photon 4. Credit: Phys. Rev. Lett. 110, 210403 (2013) DOI: 10.1103/PhysRevLett.110.210403

(Phys.org) —Researchers at the Hebrew University of Jerusalem have succeeded in causing entanglement swapping between photons that never coexisted in time. In their paper published in the journal Physical Review Letters, the team explains how their experiment proves true an entanglement phenomenon first described by researchers last year at the University of Erlangen-Nuremberg.

The idea seems not just counterintuitive, but impossible—that could be entangled that never existed at the same time—but that's just what the team in Germany, led by Joachim von Zanthier, suggested. In this new effort, the team in Israel, led by Hagai Eisenberg, has proven it's possible by actually doing it.

is, of course, where the quantum states of two particles are linked—what happens to one happens to the other regardless of the distance between them. This new work shows that they can be linked via time as well.

To prove it, the researchers first used a laser to cause entanglement between a pair of photons, P1, P2. They then measured the of P1, which was immediately followed by the entangling of another pair of photons, P3, P4. This was followed by measuring P2 and P3 simultaneously and causing them to become entangled with one another—a process known as projective measurement. Then, P4 was measured. Measuring P1 caused its demise of course—before P4 was born—but the measurement of P4 showed that it had become entangled with P1 nevertheless, if only for a very short period of time.

The researchers suggest that the outcome of their experiment shows that entanglement is not a truly physical property, at least not in a tangible sense. To say that two photons are entangled, they write, doesn't mean they have to exist at the same time. It shows that quantum events don't always have a parallel in the observable world.

Being able to entangle particles that don't exist at the same time opens up the door to new for building ultra-secure networks—communications could occur between physical locations, for example, that never actually sent an encrypted key directly to one another. It could also perhaps lead to new developments by researchers hoping to create a true quantum computer.

Explore further: 'Cavity protection effect' helps to conserve quantum information

More information: Entanglement Swapping between Photons that have Never Coexisted, Phys. Rev. Lett. 110, 210403 (2013) DOI: 10.1103/PhysRevLett.110.210403

Abstract
The role of the timing and order of quantum measurements is not just a fundamental question of quantum mechanics, but also a puzzling one. Any part of a quantum system that has finished evolving can be measured immediately or saved for later, without affecting the final results, regardless of the continued evolution of the rest of the system. In addition, the nonlocality of quantum mechanics, as manifested by entanglement, does not apply only to particles with spacelike separation, but also to particles with timelike separation. In order to demonstrate these principles, we generated and fully characterized an entangled pair of photons that have never coexisted. Using entanglement swapping between two temporally separated photon pairs, we entangle one photon from the first pair with another photon from the second pair. The first photon was detected even before the other was created. The observed two-photon state demonstrates that entanglement can be shared between timelike separated quantum systems.

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antialias_physorg
3.3 / 5 (7) May 28, 2013
communications could occur between physical locations, for example, that never actually sent an encrypted key directly to one another.

Still means you have to transmit the key somehow, doesn't it? Be it via other, entangled intermediaries or directly.

But the entanglement 'in time' with the retroactive fixing of the entangled photon P1 sounds freaky. Interesting stuff.
hemitite
1 / 5 (4) May 28, 2013
"...communications could occur between physical locations, for example, that never actually sent an encrypted key directly to one another."

Could this be a way to eavesdrop?
Noumenon
1.4 / 5 (51) May 28, 2013
The researchers suggest that the outcome of their experiment [entanglement between photons not coexisting in time] shows that entanglement is not a truly physical property, ...


I think they have this backwards. It shows that time is not a physical property.
Erik
1 / 5 (3) May 28, 2013
Cramer's Transactional Interpretation of QM has standing waves in time between events (from QM waves that travel both forward & backwards in time). In this experiment, it looks like the P1P2 standing time wave overlaps the P3P4 standing time wave. That would provide a path to link the entanglements.
cmn
1 / 5 (3) May 28, 2013
So, if we collapse a particles field here and now, is it plausible that we've also collapsed the field of a particle yet to exist? And, can this happen bilaterally? i.e. If I collapse a particle's field now, if it's entangled with a particle that existed sometime in the past would that past particle's field have collapsed as well?

Is this evidence of a holographic space-time?
Ober
3.5 / 5 (8) May 28, 2013
Maybe I'm not thinking right, but hasn't P2 simply passed information regarding P1 to P4?? So despite P1 being destroyed, P2 still contained info about P1. By entangling P2 and P3 and considering P3 and P4 were already entangled, then we have a direct route for information to be transmitted. Isn't this conservation of information? I see this experiment as an information relay. It's not as if P1's information disappeared, and reappeared later in time with no intermediate steps in time. It was always there, just passed from entity to entity to entity via entanglement.
A question I've asked before, and didn't get an answer to. If a quantum entity's wave function is collapsed via measurement, does it remain collapsed for ever, (if isolated) or does it return to a non collapsed state at some point? I think it has to return to non collapsed, or else the universe would have totally collapsed by now. Or is this non collapsed state only available to newly created quantum unmeasured entities?
antialias_physorg
3.5 / 5 (6) May 28, 2013
It's not as if P1's information disappeared, and reappeared later in time

You're right. But the freaky thing is that you can determine whether the state of P1 is in superposition or not AFTER P1 has ceased to be (by measuring P4). And that's totally out of this world.
(Note that this does not constitute information transmission...otherwise we'd just have invented time machines)
grondilu
3 / 5 (2) May 28, 2013
Well, quantum correlations transcend time. There was a paper in nature communication about something like that. I'd lie if I'd tell I understand it all, but from the gist of it I get the idea that it is related:

http://www.nature...076.html
ValeriaT
1 / 5 (14) May 28, 2013
At the water surface it's quite natural, when soliton disappears temporarily (it's commented loudly at the end of video). The neutrino, mezon or neutron oscillations are of the same category.
ValeriaT
1 / 5 (20) May 28, 2013
Is this evidence of a holographic space-time?
Nope, it's the evidence of dense aether model. Anyway, it's a hyperdimensional effect - but I'm not sure, if some hologram behaves in such a way (hologram is stationary model) Holographic model is very counterintuitive description of phenomena, which already have direct mechanical analogies from common life. It's typical "explanation" of mainstream physics, which just brings more questions than answers. I can imagine, why/how the holographic model works with water surface analogies, but the opposite way is virtually unfeasible for me.
beleg
1 / 5 (5) May 28, 2013
"...provide a path to link the entanglements...- Erik

Whatever " a path" or "link" is or means, entanglement does not give a hint to the nature of a physical way to accomplish this.
ValeriaT
1 / 5 (19) May 28, 2013
entanglement does not give a hint to the nature of a physical way to accomplish this.
But the water surface model does. At the elastic water surface the surface solitons can be connected via underwater "density" waves, not just surface waves only. After all, we have water surface analogies of entanglement developed already. Here you can see pair of "entangled" droplets jumping above water surface in phase. After all, the tunneling of Cooper pair across Josephson barrier is a nice example of how the entangled pair can become temporarily "invisible" for material obstacles and/or objects - not just observer.
Noumenon
1.4 / 5 (49) May 28, 2013
A question I've asked before, and didn't get an answer to. If a quantum entity's wave function is collapsed via measurement, does it remain collapsed for ever, (if isolated) or does it return to a non collapsed state at some point?


The term 'collapsed', just means it takes on a specific observable value in the given representation (projected onto a basis in Hilbert space). It then continues to evolve again via the Schrödinger equation. If one measures again, the same result will be expected, but one must measure quick enough, because the 'wave-function' will spread.
ValeriaT
1 / 5 (14) May 28, 2013
If a quantum entity's wave function is collapsed via measurement, does it remain collapsed for ever
As I noted above, the entanglement means, that the quantum waves of members inside the entangled pair become phase dependent. If two objects undulate with the same frequency, their undulations effectively disappear for each other.
ValeriaT
1 / 5 (15) May 28, 2013
Try to imagine, you're a sailor, who is staying at night on the end of floating wharf, to which some boat is attached. Because night sea is stormy, everything (both sailor, wharf and boat) are wobbling up and down, but in different phases. From the perspective of sailor this boat sways randomly.

The observation of quantum particle is analogous to situation, when the sailor touches the boat for a moment, thus exchanging some momentum with it. What will happen, after then? The wharf and boat will begin to oscillate at phase. It means, the sailor will keep his relative position with respect to boat, so he cannot detect any boat wobbling anymore, because he moves by the same way. We can say, the wave motion/function of boat has collapsed from local perspective of that sailor. It still remains undulating from perspective of another observer, though - this is the moment, when the Everett's "many worlds" concept takes place in quantum mechanics. It's conceptually very simple stuff.
ValeriaT
1 / 5 (13) May 28, 2013
The entanglement can be quite persistent, but due the process of so-called decoherence its keeping for infinite time would require the infinite energy in accordance to uncertainty principle. It can be maintained for infinite time with finite energy if we renew the entanglement periodically with process of observation. This is the principle of so-called "weak measurement".

Illustratively speaking, without supervision your sheep will always find its way across fence soon or later. But it you will watch it periodically, its keeping at place will not be so demanding - it just must be done sufficiently often.
Ober
1 / 5 (3) May 28, 2013

You're right. But the freaky thing is that you can determine whether the state of P1 is in superposition or not AFTER P1 has ceased to be (by measuring P4). And that's totally out of this world.
(Note that this does not constitute information transmission...otherwise we'd just have invented time machines)


But this would be another verification of conservation of information, right?
Why does it not constitute information transmission? The information is relayed via the intermediates is it not? Is that not transmission?

@Noumenon, Thank you!! I wonder what the rate of spread IS for a given wave function? Seems I really must do some research in this area, I have too many questions.
Noumenon
1.3 / 5 (48) May 28, 2013
It still remains undulating from perspective of another observer, though - this is the moment, when the Everett's "many worlds" concept takes place in quantum mechanics. It's conceptually very simple stuff.


Except that the 'many worlds' interpretation is merely a means of sweeping the rest of the linearly superposed 'waves' under the rug, and in any case is not observable.

Staying within THIS world, the incongruity between the 'wave-function evolution' and its 'state reduction' upon measurement, can be explained by the forcing of the underlying reality to take on a conceptual form (an observable value), dictated by experimental apparatus on the macro scale.

dav_daddy
1 / 5 (11) May 28, 2013
It could be evidence of a 5th dimension, or actually a 2nd dimension of time to be more accurate.

I've read a couple books on quantum physics that required the 3 spacial dimensions, plus 1 dimension of time running in either direction. I can't recall the name of either off hand but 1 incorporated a lot of Mach's principals.
ValeriaT
1.4 / 5 (11) May 28, 2013
I wonder what the rate of spread IS for a given wave function
It corresponds the speed of Schrodinger probability wave spreading of free particle.
antialias_physorg
3.7 / 5 (6) May 28, 2013
But this would be another verification of conservation of information, right?

Yes.

Why does it not constitute information transmission?

That's a bit tricky and hinges on how information is defined. For information you need an encoding - which means you must have a priori knowledge of the state you encode something in.

Information is rendered when there is a correlation between a priori knowledge (the encoding) and a posteriori knowledge (the reading out). The better the correlation the more information is rendered.

With entanglement you are missing the a priori knowledge (because if you were to set the value of the first photon then that would constitute a measurement you wouldn't get entanglement at all). So it's not information transmision.

It's, I find, a rather subtle point - but crucial to the application of information theory (and the idea of causality in general)
ValeriaT
1 / 5 (12) May 28, 2013
Except that the 'many worlds' interpretation is merely a means of sweeping the rest of the linearly superposed 'waves' under the rug, and in any case is not observable
Of course, these "worlds" are "connected" mutually. But for example their clocks will differ for ever, if they will move mutually.
beleg
1 / 5 (5) May 28, 2013
"...there does not exist a perfectly uniform probability density distribution over an infinite surface."
In my simplistic world all I need is a "surface" (geometry) in which the ability of photons to entangle is provided for from that surface locally and non-locally.

http://mathpages....7-07.htm

Of course classical information theory is not a physical theory so information transfer is never considered under physical constrictions.
(Note transfer =/= transmission)
hemitite
1 / 5 (4) May 28, 2013
So there must be a world in the "many worlds" mega-multivers where the many worlds theory doesn't hold...
jdbertron
1 / 5 (5) May 28, 2013
Either the abstract is very poorly written or there is something wrong with that research. It just sounds like a domino effect using entanglement. Sure you can say the last domino was 'entangled' with the first one, but that's nothing new.
LarryD
not rated yet May 28, 2013
Whoa there, wait a minute...'This was followed by measuring P2 and P3 simultaneously and causing them to become entangled with one another—a process known as projective measurement.'
If one invokes RQFT and the Gaussian arn't we talking about Simultaneity but only if higher dimensions are considered?

antialias_physorg
3.7 / 5 (6) May 29, 2013
Sure you can say the last domino was 'entangled' with the first one, but that's nothing new.

That's not the main thing here. What they have demonstrated is 'spooky action at a distance' - not only in space, but also back in time (Woha! ...and I don't use exlamation marks lightly.)
This is some hea-vy shit.
Moebius
1 / 5 (12) May 29, 2013
They should build a quantum receiver first and see if they get any messages from the future.
beleg
1 / 5 (6) May 29, 2013
Geometers must be a hapless lot. The first geometer to provide a geometry to account for observations from GR as well as from QM will have earn the recognition that the person who invented the wheel received.
Anonymous recognition. How ironic.
gwrede
1 / 5 (9) May 29, 2013
With entanglement you are missing the a priori knowledge (because if you were to set the value of the first photon then that would constitute a measurement you wouldn't get entanglement at all). So it's not information transmision.

It's, I find, a rather subtle point - but crucial to the application of information theory (and the idea of causality in general)
I think you can transmit by putting a gate between II and III which is only open if Photon 1 is what we want. If it isn't then we create pairs until Photon 1 is what we want. Thus a recipient at V does get our message.
Noumenon
1.5 / 5 (50) May 29, 2013
With entanglement you are missing the a priori knowledge (because if you were to set the value of the first photon then that would constitute a measurement you wouldn't get entanglement at all). So it's not information transmision.

It's, I find, a rather subtle point - but crucial to the application of information theory (and the idea of causality in general)
I think you can transmit by putting a gate between II and III which is only open if Photon 1 is what we want. If it isn't then we create pairs until Photon 1 is what we want. Thus a recipient at V does get our message.


Your gate (which would be like a Stern-Gerlach apparatus) constitutes a measurement. IOW, how would you know 'it's the one you want', unless you measure it.
jibbles
not rated yet May 29, 2013
would this be a good way to extend (indefinitely) entanglement for quantum computing?
jibbles
not rated yet May 29, 2013
Would somebody (other than ValeriaT or natello) please explain how exactly P2 and P3 are measured simultaneously and become entangled? What is the "Bell projection" at IV in the diagram? Don't they become entangled first, and are subsequently measured?
beleg
1 / 5 (7) May 30, 2013
Science presently does not have a physical process for entanglement itself.
The process of measurement is physical. Science holds measurement as well as an innate property of nature suspect to the result.
Bell's role is to insure the exclusion of hidden variables.
Requiem
1.7 / 5 (11) May 30, 2013
I'm so sick of hearing about how dense aether model applies to every article on this site.

Seriously, what is wrong with your life? Who spends their life sitting on a mainstream physics website and blabs about some theory, crackpot or not? What is WRONG with you?

Do you even have any friends? If so, have you ever noticed that they all get that look like "humor the poor mentally challenged guy who I'm hanging out with so he doesn't commit suicide or something" every time you start talking about how the foam on your pint or the way that dart flew through the air confirms dense aether model?

Seek help... Seriously.
AmritSorli
1.8 / 5 (10) Jun 01, 2013
This experiment proves that timeless universal space is a medium of quantum entanglement, regardles numerical order of particles creation. Time s a numerical order of change has only a mathematical existence.....yours amrit
StarGazer2011
1 / 5 (8) Jun 01, 2013
can this be explained as a split coin?
dalmuti
not rated yet Jun 02, 2013
So does this exhibit temporal determinism? I.e., if i understand correctly, you could create a particle, measure it, and via a chain of entanglement, know that a particle that does not exist yet, will (very likely) be created (at a specific time?) in the future, and if created, will have a specific property, with 100% certainty.
Stevepidge
1 / 5 (8) Jun 02, 2013
So, if we collapse a particles field here and now, is it plausible that we've also collapsed the field of a particle yet to exist? And, can this happen bilaterally? i.e. If I collapse a particle's field now, if it's entangled with a particle that existed sometime in the past would that past particle's field have collapsed as well?

Is this evidence of a holographic space-time?
Sounds like evidence for reincarnation to me. Bearing non-local consciousness is true.