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posted by martyb on Saturday July 06 2019, @05:23PM   Printer-friendly
from the spoonful-of-relativity dept.

[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


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  • (Score: 5, Informative) by sshelton76 on Saturday July 06 2019, @10:42PM (8 children)

    by sshelton76 (7978) on Saturday July 06 2019, @10:42PM (#863957)

    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.

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  • (Score: 2) by shortscreen on Sunday July 07 2019, @02:14AM (3 children)

    by shortscreen (2252) on Sunday July 07 2019, @02:14AM (#864001) Journal

    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.

    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

      by AthanasiusKircher (5291) on Sunday July 07 2019, @04:03AM (#864022) Journal

      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)

      by Anonymous Coward on Sunday July 07 2019, @02:18PM (#864108)

      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

        by shortscreen (2252) on Monday July 08 2019, @05:02PM (#864569) Journal

        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)

    by Anonymous Coward on Sunday July 07 2019, @02:16AM (#864002)

    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)

      by Rupert Pupnick (7277) on Tuesday July 09 2019, @04:16PM (#865031) Journal

      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

        by maxwell demon (1608) on Saturday July 13 2019, @06:19AM (#866512) Journal

        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)?

        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

    by hendrikboom (1125) Subscriber Badge on Sunday July 07 2019, @02:20PM (#864109) Homepage Journal

    Another way of looking at the state collapse is that the observer becomes entangled with the observed system.