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[Ed. note: This article was recently published (July 6, 2019) on the Science Alert web site. As a footnote on the Science Alert story notes: "This article was originally published at Aeon and has been republished under Creative Commons." Viewing the source HTML at Aeon, I discovered it was originally published 02-Feb-2018. Though the material is somewhat dated, it was the first I'd heard of this and thought it sufficiently interesting to share with the SoylentNews community. --martyb]
Entanglement of particles, i.e. quantum nonlocality, is routinely demonstrated in particles separated by space.
But space and time are related, leading to a team of physicists demonstrating that quantum entanglement can occur across time with particles that shared no concurrent existence.
Just when you thought quantum mechanics couldn't get any weirder, a team of physicists at the Hebrew University of Jerusalem reported in 2013 that they had successfully entangled photons that never coexisted.
Previous experiments involving a technique called 'entanglement swapping' had already showed quantum correlations across time, by delaying the measurement of one of the coexisting entangled particles; but Eli Megidish and his collaborators were the first to show entanglement between photons whose lifespans did not overlap at all.
One might be curious how a measurement done on one particle might be instantly reflected on another that doesn't exist yet, so here is how this was accomplished:
First, they created an entangled pair of photons, '1-2' (step I in the diagram below). Soon after, they measured the polarisation of photon 1 (a property describing the direction of light's oscillation) – thus 'killing' it (step II).
Photon 2 was sent on a wild goose chase while a new entangled pair, '3-4', was created (step III). Photon 3 was then measured along with the itinerant photon 2 in such a way that the entanglement relation was 'swapped' from the old pairs ('1-2' and '3-4') onto the new '2-3' combo (step IV).
Some time later (step V), the polarisation of the lone survivor, photon 4, is measured, and the results are compared with those of the long-dead photon 1 (back at step II).
The upshot? The data revealed the existence of quantum correlations between 'temporally nonlocal' photons 1 and 4. That is, entanglement can occur across two quantum systems that never coexisted.
The physicist's speculation on what this means is somewhat reminiscent of a cat in a box:
Perhaps the measurement of photon 1's polarisation at step II somehow steers the future polarisation of 4, or the measurement of photon 4's polarisation at step V somehow rewrites the past polarisation state of photon 1.
For this to begin to make sense, recall that simultaneity is not the absolute Newtonian property you perceive, but per Einstein
a relative one. There is no single timekeeper for the Universe; precisely when something is occurring depends on your precise location relative to what you are observing, known as your frame of reference.
So the key to avoiding strange causal behaviour (steering the future or rewriting the past) in instances of temporal separation is to accept that calling events 'simultaneous' carries little metaphysical weight.
It is only a frame-specific property, a choice among many alternative but equally viable ones – a matter of convention, or record-keeping.
The lesson carries over directly to both spatial and temporal quantum nonlocality.
Hopefully the temporal entanglement of entire objects is next. Imagine checking out the final episode of a show on your entangled TV, realizing it is terrible, and avoiding the entire series which the studios don't even make because nobody watched it...
Journal Reference
E. Megidish, et al. Entanglement Swapping between Photons that have Never Coexisted Phys. Rev. Lett. 110, 210403 DOI:10.1103/PhysRevLett.110.210403
(Score: 3, Interesting) by JoeMerchant on Saturday July 06 2019, @08:57PM (12 children)
I've never gotten a sufficiently satisfying explanation from a physicist as to how there is a difference between "presently unmeasured, indeterminate" state and "presently unknown, determinate" state.
They prattle on about statistical this and that, again unconvincingly, and either flash-bang-dazzle you Bill Nye/NdGT style with half of an explanation that, well, obviously this is incomplete but we don't want to bore our audience with details; or they descend into nearly incomprehensible gibberish, that smells strongly of circular reasoning, until you simply have pity on their poor presentation skills - tell them "yes, I see it now" and let them retreat to their lair.
As for this particular presentation: A) it would seem that if you can imprint a state from one (large statistically measured set of) photon(s) onto another, of course you can do this 1-2 pause 2-3-4 thing, and B) sure, photon 4 never "met" photon 1, but... that doesn't mean they didn't "coexist in time" - maybe not as free range photons, but their energy certainly coexisted before, during, and after the experiment.
So, kudos for yet another demonstration of entanglement - until the difference between "indeterminate" and "unmeasured" is more clearly demonstrated, it all reeks like the emperor's new robes after having been stored in a box with a dead cat.
🌻🌻 [google.com]
(Score: 5, Informative) by sshelton76 on Saturday July 06 2019, @10:42PM (8 children)
This is because you are lacking a lay understanding of QM.
The no cloning theroem states that copying quantum states is impossible. They must be entangled and that entanglement happens at creation.
Indeterminate and unmeasured is already well defined by Bell's inequalities and the EPR paradox.
Here is a simple way to think about it.
Imagine I have 2 gloves, a left hand glove and a right hand glove.
I ask someone to put each glove in a suitcase without telling me which case contains which glove.
We now mail one of the suitcases to Antarctica, while I take the second suitcase with me on vacation in the Bahamas.
Once I arrive in the Bahamas I open my suitcase. I see it is the left glove, and I now know for certain that the suitcase that went to Antarctica is the right glove even though it hasn't been opened yet.
This is an example of unmeasured. The state was set from the beginning, it existed independent of my knowledge.
Indeterminate is different. In quantum theory, both gloves are made of a material that is linked. They are co-equal and statistically each has a 50/50 possibility of being left or right. Yet if I observe one of them, then wave function will collapse for the pair of them, meaning if I see the right the other will become the left. Ergo, a left or right glove cannot be put into either suitcase because the glove is neither left nor right yet. It is an probability wave with 50/50 odds of being left or right and 100% odds of being one of them and 100% odds of being the opposite of the partiner.
When I arrive, I open the suitcase and the wave function collapses and the quantum glove becomes a left glove, and now I know that the glove in Antarctica is now a right glove.
Bell's inequality is how we test which of these scenarios is true. It proves that the glove is neither left nor right handed until it is observed. It does this by entangling tons of particles and passing them through detectors rapidly looking for statistical anomalies. This is because the wave function will sometimes collapse due to environmental issues encountered before the filter. Ergo a 50/50 polarizer will not in fact give exactly 50/50 results if EPR is true.
(Score: 2) by shortscreen on Sunday July 07 2019, @02:14AM (3 children)
Before opening the suitcase you didn't know it was, or would be, a left glove. But after opening it, why is it not safe to assume that it had been a left glove all along, since it originally went into the suitcase? Saying that it's both left and right at the same time (or that the cat is both alive and dead) implies that this is not a safe assumption.
It's not clear why it needs to be described as a superposition of states instead of merely an unknown/unpredictable state.
(Score: 2) by AthanasiusKircher on Sunday July 07 2019, @04:03AM
Well, I think the analogy breaks down here a bit, but (someone correct me if this sounds off -- I'm trying to work within the parameters of this analogy) I think one way of answering your question is that the special "indeterminate glove" material behaves differently than either a left or right glove would behave. If you did tests on the suitcases, you'd notice that it behaves as if it contains the indeterminate material, NOT the same behavior you'd get if the suitcase contains a specific left glove or whatever.
But then when you open the suitcase, it turns out that it is a left glove. Yes, it's weird. But that's the quantum world.
(Score: 0) by Anonymous Coward on Sunday July 07 2019, @02:18PM (1 child)
Try reading this for a full explanation:
https://www.lesswrong.com/posts/hc9Eg6erp6hk9bWhn/the-quantum-physics-sequence [lesswrong.com]
But there are real reason to consider a superposition, backed by experimental results.
(Score: 2) by shortscreen on Monday July 08 2019, @05:02PM
I wasn't sure where to begin but found this section to be helpful https://www.lesswrong.com/posts/5vZD32EynD9n94dhr/configurations-and-amplitude [lesswrong.com]
The third "experiment" reminded me of the double-slit experiment, which is something I have a tendency to forget about.
My takeaway is that any explanations relying a concept of a photon as a particular thing with particular properties (like a billiard ball) simply can't make sense because it's just not an adequate analogy. In this case it seems to be more like a beam, although there may be other situations in which that would also not be a suitable analogy.
In any case, I see an opportunity for a Lolo-like puzzle video game based on quantum mechanics ^____________^
(Score: 1, Informative) by Anonymous Coward on Sunday July 07 2019, @02:16AM (2 children)
Classical analogies like this suffer from missing a key component that really distinguishes quantum entanglement from conventional correlation: the role of the measurement. In the "quantum" case, your gloves might also have a "colour", say, red or blue. The scenario would be that you can make only one measurement, handedness or colour. This is where entanglement plays a fundamentally stronger role: your gloves can be created in such a way (entangled) that they will always have opposite results, regardless of which measurement you choose, and yet taken individually the results will be distributed uniformly randomly.
Even this extension to the analogy struggles, though, because in reality a glove can have both a colour and a handedness at the same time. The quantum properties we're talking about don't really exist simultaneously. A photon's polarization, for example, can be described as components along horizontal and vertical axes, say, or alternatively along diagonal axes. But these are just different ways to describe the same property of a photon (i.e., which way it's electric field oscillates). And yet, measuring in either the horizontal/vertical basis OR in the diagonal basis is precisely the sort of thing we'd do to illustrate entanglement. Each polarized photon of an entangled pair will be measured to have an apparently random direction, yet their directions will always be opposite each other if they are both measured in the same basis, regardless of what basis that is, and even if that basis is chosen after the photons were actually created.
(Score: 2) by Rupert Pupnick on Tuesday July 09 2019, @04:16PM (1 child)
So the fundamental limitation of Quantum Teleportation [tm], is that you don’t know what you’re sending, only that it’s the opposite, in some sense, of the piece that remains at the transmitter once either side has collapsed the wave function (e.g. measured at the receive side)?
Is this useful?
(Score: 2) by maxwell demon on Saturday July 13 2019, @06:19AM
No, quantum teleportation is a specific protocol that allows to send arbitrary given quantum states by sending classical information (which before was not thought to be possible because you cannot completely measure the quantum state). It uses quantum entanglement, but consists of more than just measuring the parts of the known shared entangled state (in particular, it obviously involves the original quantum state to be sent, and most importantly, it involves actually sending information).
The Tao of math: The numbers you can count are not the real numbers.
(Score: 2) by hendrikboom on Sunday July 07 2019, @02:20PM
Another way of looking at the state collapse is that the observer becomes entangled with the observed system.
(Score: 3, Interesting) by maxwell demon on Sunday July 07 2019, @09:40AM (2 children)
Well, maybe the following helps you (I decided to put it into a story, too prevent it getting too dry).
Harry Potter and the Quantum of Magic
One day in Hogwarts, Harry got a mysterious small package. It contained a small ball of a substance that he couldn't recognize, and a letter:
Harry couldn't see the meaning of all this, but he did as he was asked to. In the following time, he got more packages, sometimes up to ten per day. Harry recorded all the results, as instructed, but he could not see any pattern. When he threw it in the fire, half of the time the flames turned red, and half of the times they turned blue. When he dissolved it in water, half of the time the water turned red, and half of the time it turned blue. It seemed completely random. Well, he would have to wait for the meeting with Dumbledore.
Finally, half a year and thousands of packages later, Dumbledore finally asked to meet him. When arriving at the moving stairs, he was surprised to find Hermione and Ron there. Were they also part of the secret experiment? But Harry decided not to ask, so that he didn't accidentally reveal something that he shouldn't.
However this mystery was solved by Dumbledore right after reaching his office: “I greet you three. You all have gotten packages from me, and I hope you all followed my instructions exactly, and brought your records with you.”
They confirmed it and handed over their records.
“Let's first check whether you really did your decisions randomly and independently.” Dumbledore run a few statistical tests on the lists of “fire” and “water”. “Well, seems so. So let's get to the interesting part. But first, I guess I owe you an explanation.”
Harry, Hermione and Ron almost couldn't bear the suspense.
“I think I've found the quantum of magic, the fundamental building block of which all magic is made. I don't have to tell you that this is an important discovery. But I had to run a test first to be absolutely sure. The quantum of magic shows quite particular behaviour, and that's what I looked for. Each of you got a series of packages with balls, each containing exactly one quantum of magic. It was packaged in a safe way so that only a few magic effects could happen, in particular colouring of flames or water. I'm really grateful that no one tried to do further examinations; an uncontrolled release of pure general magic, even if it was just a quantum, could have had terrible consequences. Had the experiment not been that important, I never would have risked that. But let's now look at the results. First, only consider the records of all those packages where exactly one of you threw the ball into the fire. I'll ask you to examine the combined records of the colours for this list. I want you to do that yourself, so that my expectations don't affect my results.”
Harry, Hermione and Ron looked at the results. They noticed a peculiar fact: In all those records, there had either been three blue, or one blue and two red colours. Sometimes the single blue record was from the ball thrown into the fire, sometimes it was from one of the balls dissolved in water. It didn't seem to matter who throw the ball into the fire.
“Great. Just how I expected. Now I'll ask you to do the same with all the records where all three of you threw the ball into fire.”
Hermione intervened: “There's no need to do that, the result is already clear!”
“How so?” wondered Harry.
Hermione explained: “Look, the results could not have been randomly decided at the time we burned or dissolved the balls, as then sometimes we would have gotten an odd number of red on the one-fire list. We shielded our rooms from outside magic influences, so the results from one could not have affected the results of the other. This means that the results had to be predetermined at the time we received the balls.”
“Sounds reasonable.” said Harry. “But even if the results were determined, that doesn't mean we know them.”
“In this case, we do, because we know that when only one of us threw the ball into the fire, there was always an even number of red effects. Now with each triple of balls, we could only made on decision each, but if the results were predetermined, we also can reason about what we would have gotten had we made another decision. Indeed, we can pretend we could just have made copies of those balls, and then performed the different experiments on those copies.”
“Well, I can understand that.” confirmed Harry. “But what does it tell us about the three-fire case?”
“Look, say we had each made two additional copies of those balls, and made the one-fire experiment with each of the three triples, only that in each case another one of us had thrown the ball into the fire. Since we have established that each such experiment gets an even number of red results, then this triple experiment should get them as well.”
“Sure.” Harry answered. “But I still don't understand how that should help us determine the three-fire case.”
Hermione continued: “Obviously, with the result predetermined, dissolving the copy of the same ball into water would have gotten the same colour twice. Buteach of us would have resolved two copies of his balls in water, thus getting either two red or two blue results. This means the number of red results from dissolving in water would automatically have been even. Thus to get an even number of red results overall, we would have needed an even number of the balls thrown into fire.”
“I think I understand now” Harry replied. “Since in this hypothetical experiment the balls dissolved in water don't matter in determining whether an even number of red colours was observed, doing that part of the experiment could have been omitted. But in this hypothetical experiment, each of one threw exactly one ball into the fire, so that's an experiment we could have performed without the ability to copy the ball. Indeed, it's an experiment we have performed many times. And it is exactly the experiment whose results Dumbledore wanted us to determine. So the result is that again, there is an even number of red results. Brilliant!”
Now Ron intervened: “But wait a moment. All Hermione showed is that for those cases where we threw just one ball into the fire, we would have gotten that result if we had thrown all thre balls into the fire. But those were not the ones where we actually did that.”
Hermione responded: “But at the moment we received the balls, we didn't yet know whether we would throw the balls into the fire or dissolve them in water. So to achieve that perfect correlation in the cases where we ended up throwing just one of them into water, it also had to be true in the cases where we ended up throwing all three balls into the fire.”
Dumbledore insisted: “Please, do the examination anyway.”
Reluctantly they did so, but already the first result surprised them: “Three red flames!” Hermione shouted out. “One of you must have gotten the record wrong!”
But further examination showed that each of the three-fire records had either one or three red flames. That could no longer be explained with someone doing wrong records. It would not make sense that anyone made perfect records when only one of them chose the fire, but consistently failed to do so when all three were doing that decision. The records had to be considered trustworthy.
“There's only one explanation.” Hermione claimed and turned to Dumbledore: “You somehow managed to circumvent all our protection magic. That of course would make sense, as that ability would be very useful to fight the one who should not be named.”
“It indeed would be very useful,” Dumbledore replied, “but unfortunately I don't know a way to do that. No, I didn't mess with your protection magic. The point is, quantum magic doesn't work the way you assumed.”
Hermione protested: “But my logic was water-tight. If the result is predetermined, that implies that we should have seen an even number of red flames in the three-fire cases.”
“That's true.” Dumbledore confirmed.
“And if the colour was just chosen completely random, we should have seen both even and odd number of red flames.”
“That's also true.”
“But if it is not predetermination, and not local randomness, then there must be an outside influence!”
“And that's where you're wrong. It turns out that quantum magic has another way to cause such correlations. A way that allows to predetermine the correlations without predetermining the results.”
Hermione gasped. “But how is that possible?”
Dumbledore shrugged. “I have no idea. All I can say it that it works, and it works predictably. I call it entangled magic.”
The Tao of math: The numbers you can count are not the real numbers.
(Score: 2) by Rupert Pupnick on Tuesday July 09 2019, @03:58PM (1 child)
I really appreciate this effort post, but please allow me to condense:
Each set of three suitcases represents a single quantum to be measured in a random way by independent observers using either fire or water. The fact that the observers are different people in different places illustrates the idea that a single quantum can be distributed in an arbitrarily large space. It seems that times at which each person opens their suitcase doesn’t matter.
When only one ball is thrown in the fire, each set of three suitcases results in either zero red or two red. The single ball that lands in the fire could be blue or red.
When all three balls are thrown in the fire, you instead get only one red or three red.
Any single three suitcase outcome cannot be predicted, but certain results are always excluded, depending on how the measurement is performed.
You can therefore conclude that in some cases, you can predict what the outcome must be for a given test (fire or water) on a single suitcase once you have tested the other two in an appropriate way.
Is that a fair summary?
(Score: 2) by maxwell demon on Saturday July 13 2019, @05:55AM
No, it is not a fair summary, because it omits the most important part: The assumption that the result is pre-determined but unknown gives a prediction of measurement results that contradict what actually is measured. From the one-fire results you predict what you should see in the three-fire case, but what actually is measured is exactly the opposite. This is why we know that the assumption of predetermination is wrong. But as you correctly stated, we can in some cases predict one of the results from knowing the others, therefore those results cannot be locally decided at random either.
There's also one misunderstanding in your summary: The three balls don't share a quantum but a quantum state. If they shared a quantum, then at most one of them would show an effect at all (because the moment you measure it, the quantum would “decide” in which of the balls it is found). Strictly speaking, the story doesn't cover that point at all, but I see how you could get the false impression.
Also a remark: This is not the same as the usual Bell test (the scenario I've used is known as GHZ paradox or Mermin paradox). The Bell inequality has the advantage that it only involves two systems (and thus is experimentally easier), but has the disadvantage that the contradiction is only in probabilities (which for many people are harder to understand), while in the GHZ paradox, at least if you assume no measurement errors, you get the “wrong” result every single time.
The Tao of math: The numbers you can count are not the real numbers.