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Science

Collapse of Quantum Wavefunction Captured In Slow Motion 242

Posted by samzenpus
from the look-at-this dept.
ananyo writes "It is the most fundamental, and yet also the strangest postulate of the theory of quantum mechanics: the idea that a quantum system will catastrophically collapse from a blend of several possible quantum states to just one the moment it is measured by an experimentalist. Researchers have now been able to capture that collapse through the use of weak measurements — indirect probes of quantum systems that tweak a wavefunction slightly while providing partial information about its state, avoiding a sudden collapse. Atomic and solid-state physicist Kater Murch of the University of California, Berkeley, and his colleagues performed a series of weak measurements on a superconducting circuit that was in a superposition — a combination of two quantum states. They did this by monitoring microwaves that had passed through a box containing the circuit, based on the fact that the circuit's electrical oscillations alter the state of the microwaves as they pass through the box. Over a couple of microseconds, those weak measurements captured snapshots of the state of the circuit as it gradually changed from a superposition to just one of the states within that superposition — as if charting the collapse of a quantum wavefunction in slow motion."
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Collapse of Quantum Wavefunction Captured In Slow Motion

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  • No video in the link (Score:4, Informative)

    by Pikoro (844299) <init AT init DOT sh> on Thursday October 10, 2013 @06:42AM (#45089521) Homepage Journal

    So don't bother unless you want to read a dry paper.

    • Re: (Score:2, Funny)

      by Anonymous Coward

      Everyone knows that when you try to capture a quantum state on video, all you end up with is cats. How else do you explain all the cat videos on Youtube?

    • Video? When I read "captured" I thought they meant "in captivity", like they had the Quantum Critter in a box with the cat or a microscopic cage or something like that.

    • by r1348 (2567295) on Thursday October 10, 2013 @07:25AM (#45089671)

      Did you really expect to see a video of a quantum wavefunction?

    • by Dunbal (464142) *
      Sure there was video in the link! Oh, wait... you must have gotten the "other" version. Well, quantum stuff can suck sometimes but hey, at least you're not a cat!
    • by PopeRatzo (965947)

      So what you're saying is, "pictures or GTFO"?
      '
      I'm with that.

    • Uh.. this is slashdot, you DO NOT observe the article anyway, or strange things may occur.

      Speaking of strange, why do we keep calling stuff like this "strange"? at a microscopic level matter behave differently, ok. Had we mostly experience of quantum states, we would classify classical mechanic strange.
      It is what it is. Model it with known concepts if you can, but don't try to fit everything in existing categories.

      • Speaking of strange, why do we keep calling stuff like this "strange"?

        Because it is strange. I am an engineer, not a physicist, but I took plenty of physics courses in college, and have kept up on progress in QM. This experiment is not the result that I would have expected. I had always understood that it is either one state or the other, all or none, and the collapse was instantaneous with any measurement. To now learn that none of that is true, that the collapse can be "partial", and that intermediate measurements can be made, is very strange indeed. Fascinating

      • by fatphil (181876)
        If you observe the paper, you change it?

        And thus wikipedia and blogs were invented.
    • by fatphil (181876)
      So is the paper like a flip book?
    • by Prune (557140)
      While a video would satisfy an instinctual curiosity of "what it looks like", it would, in this case, provide essentially no insight. Since when was the job of science entertainment--or even the job of Slashdot, for that matter? Calling it "dry" is not a valid criticism for a scientific subject.
  • I thought that this "waveform collapse" stuff was an illusion on our part, signifying a certain lack of understanding.
    • There's a certain amount of presumption with the whole superposition of states and then finding only one state upon measurement. However, it's not exactly easy to measure the superposition of states when any such measurement causes it to "collapse" into one particular state.

      Some people prefer the many-worlds explanation where we "select" a reality by performing the measurement. That would imply that the waveform doesn't collapse as such, but the difference is only in interpretation and is not measurable.
      • What is the consensus on the metaphysical messiness of the many worlds interpretation. I would argue it is the very opposite of Occam's Razor at work - it invokes and assumes so much (new universes being created every plank time) that can't be explained that it seems little better than saying God chooses which measurement will happen.

        The principle benefit seems to me to be that it renders the interpretation as being outside of science as it is logically not possible to test a many worlds theory. Whereas lo

        • There isn't much consensus on the interpretations of quantum mechanics, so as long as the maths works out, then it's really just a matter of preference.

          A lot of people have problems with the many worlds interpretation due to the huge number of versions of the universes (not sure that should be a word) that we end up not being in. However, collapsing a waveform has a similar issue with all the different possible outcomes being "thrown away" when a measurement is made.

          Either way, the results give us the p
  • I know that this question is off topic, but I also know there will be many readers of this story who may be able to answer my question. It's something I'd desperately like an answer to, having posted it around to a few folks with no response. . .My understanding of electromagnetics is that there is a waveform. I'd like to know if it is possible for a directly inverse waveform could coincide with, say, a photon of red or blue light, in such a way that it cancels the waveform out, the same way that an inverse
    • by Yetihehe (971185) on Thursday October 10, 2013 @07:11AM (#45089627)

      Yes. You have this effect in double slit experiment, there are places where waves cancel out and you have dark place. The problem is that it's almost impossible to generate an inverse waveform from source other than the one which generated your photon. Typically it's done by splitting one waveform.

    • Waveform "cancellation" is limited by a number of factors. There is always interference, and it takes as much energy to create the canceling waveform as the original, and unless the "inverted" waveform comes from precisely the same source, and orientation, as the original waveform, it cannot cancel the waveforms out everywhere, Moreover, if the canceling waveform is is being generated based on receiving the original waveform, simple lightspeed limitations preclude being able to completely detect and counte

    • The photon is it's own anti-particle, so in theory you could have a photon meeting it's opposite and thus "cancel out". However, photons can exist in the same state in the same place and thus it's not easy to get a photon to interact with another photon. Typically, you can get an area where you won't detect the photons (think dark areas in double split experiments), but the photon/anti-photons will just pass through each other without interacting.
    • In principle, yes.

      Keep in mind that (in the standard QM theory) the waveform describe by the Schrodinger equation is a probability wave - which is to say that the intensity (amplitude squared) of the wave at any one point in spacetime describes the probability of finding the particle at that point in spacetime.

      So if you have two waves which meet, and they are exactly 180 degree out of phase, then there will be no point in spacetime that has a non-zero probability and so you won't find any particle anywhere

  • Information (Score:5, Interesting)

    by Warbothong (905464) on Thursday October 10, 2013 @07:25AM (#45089665) Homepage

    'Wavefunction collapse' is how the Copenhagen Interpretation 'explains' this phenomenon, but like many of its 'explanations' they don't provide a compelling reason for things to happen this way. Terms like 'measurement' go from a precise QM meaning (eg. matrix multiplication) to a vague, ambiguous meaning like 'a concious observer'. This leads to tenuous extrapolations and conclusions, like the distinguished position of observers, the inclusion of conciousness into the interpretation and all the quantum 'explanations' of consciousness which that has spawned.

    Alternative interpretations are much less mysterious. For example, the Many Worlds Interpretation explains it via information transmission. A measument is anything which transmits information from inside the system to outside the system. When a system is measured, it doesn't 'collapse' into one state; rather, the thing which performed the measurement becomes part of the (now larger) system.

    The Transactional Interpretation explains it as two-way communication between events at different times; a measurement is any event which propagates information back in time and a system is only in multiple states because the event which caused it is awaiting the information from the measurement.

    Schrodinger's cat can be used to point out the difference:

    The Copenhagen Interpretation says that the cat is literally both alive and dead at the same time in the box, then when a concious observer (a human) opens the box, the cat immediately becomes either alive or dead. This is very strange, for example why is a concious observer necessary?

    The Many Worlds Interpretation says that the cat is literally both alive and dead at the same time in the box. Anything which interacts with it, for example photons of light, will become part of the system; ie. the light will literally be both a reflection of a living cat and a reflection of a dead cat. If those photons enter my eye then I will literally be both a human who has seen a living cat and a human who has seen a dead cat. If you talk to me, you will literally be a human who has talked to a human who has seen a living cat and a human who has talked to a human who has seen a dead cat, and so on. This propagation is exactly the flow of information between systems; there is nothing magical about humans, except that we happen to be human. A photon would get the same results as us if it repeated our experiments, with no 'concious observer' involved, except for the fact that photons don't tend to perform experiments (ie. the 'conciousness' part of Copenhagen is an anthropic bias).

    • Re:Information (Score:5, Interesting)

      by alexgieg (948359) <alexgieg@gmail.com> on Thursday October 10, 2013 @08:10AM (#45089865) Homepage

      the 'conciousness' part of Copenhagen is an anthropic bias

      It's worse than that. According to defenders of the Many-Worlds interpretation (of which I consider myself one), Copenhagen's collapse has several problems. Less Wrong's Eliezer Yudkowsky has written an extensive introduction to QM from the perspective of the Many-Worlds Interpretation [lesswrong.com] and as part of the series he's extensively criticized the collapse postulate, summarizing its problems [lesswrong.com] thus:

      If collapse actually worked the way its adherents say it does, it would be:

      1. The only non-linear [lesswrong.com] evolution in all of quantum mechanics.
      2. The only non-unitary [lesswrong.com] evolution in all of quantum mechanics.
      3. The only non-differentiable [lesswrong.com] (in fact, discontinuous) phenomenon in all of quantum mechanics.
      4. The only phenomenon in all of quantum mechanics that is non-local [lesswrong.com] in the configuration space.
      5. The only phenomenon in all of physics that violates CPT symmetry [lesswrong.com].
      6. The only phenomenon in all of physics that violates Liouville's Theorem [lesswrong.com] (has a many-to-one mapping from initial conditions to outcomes).
      7. The only phenomenon in all of physics that is acausal / non-deterministic / inherently random [lesswrong.com].
      8. The only phenomenon in all of physics that is non-local in spacetime and propagates an influence faster than light [lesswrong.com].

      Given the above considerations, whatever the experiment detected is most certainly not collapse.

      • by wytcld (179112)

        The many worlds model's absurdity is right in its name. It's the belief that we have no choice, make no choices, but just randomly find ourselves in a world where certain things have happened, while duplicates of ourselves, at each instant where different things might happen, including our own different actions, find themselves inhabiting each of those many worlds. That's to say, the many worlds model requires that the illusion of choice model is the correct one for human agency. And not in the Newtonian se

        • by Jamu (852752)
          It doesn't contradict evolution. The most popular worlds are still Darwinian.
        • by alexgieg (948359)

          It's the belief that we have no choice, make no choices

          No, it's the "belief" that our choices are physics. No more and, most importantly and fundamental, no less. "Choice" is a function a physical brain executes upon physical information, it's the name we do to our subjective perception (another physical function) of our brain doing a bidirectional search [wikipedia.org] between our set of initial conditions, the things we subjectively call "can", and our set of goals, the things we subjectively call "should". When paths between our several "cans" and "shoulds" are finds, we w

          • by alexgieg (948359)

            Edits: that -> than; finds -> found; gets -> get etc. Lack of editing, the bane of ./ ... :-(

        • And why does contradicting human psychology cause problem in a physics theory?

          I am in no way up on deep QM theory, but it seems to me that a lot of QM goes against our built in psychology. And that is ok. Humans are built to survive WAY above the quantum level. Early homonids didn't need to understand quantum theory to hit an animal with a stick. (or not). Their development was not based on something that by definition they could not observe or recognize.

          As I said, I am not no expert, but if one theory brea

        • by Prune (557140)
          I'm not an aderent to many worlds, but your argument against it flawed. The less fit ones simply occur with a smaller frequency.
      • Re:Information (Score:4, Informative)

        by seibai (1805884) on Thursday October 10, 2013 @12:03PM (#45092687)
        This is bizarrely propagandist for a site that would claim itself to be rational. It's also, ironically enough, wrong.

        5. The only phenomenon in all of physics that violates CPT symmetry.

        Actually, many worlds violates CPT symmetry [wikipedia.org] - worldlines divide only forward in time, not backwards. CPT symmetry requires that there be no physical bias to the direction of time. CPT symmetry is plainly untrue anyway, as we have entropy. Trying to use it as an argument against Copenhagen is disingenuous at best.

        MW shows every sign of being equally wrong with every other interpretation of QM at the moment. The truth is that for many people, it represents a convenient belief. Most of its advocates lack understanding of the effective distinctions between interpretations in any case, which leads to sites and arguments like this. This is particularly bad in followers of Dawkins who argue that MW solves the fine-tuning problem, where half of the problem arises from balance in mathematical entities that QM has no plausible "ratchet" for.

        • I would only point out that humans lack a fundamental understanding of entropy. Given all of the output state required over a timeframe in a locality, such can be reversed... Encryption, or more specifically, Decryption proves this mathematically. If one bit is gone, then the reconstruction can not occur. Further, the world lines do not diverge. They are all merely a single world line which are superpositions of themselves, not completely unlike a hologram. The quantum theory is wrong. It's right the

        • by Prune (557140)
          Try this one http://arxiv.org/abs/quant-ph/0105097 [arxiv.org] http://arxiv.org/abs/quant-ph/0412182 [arxiv.org] Point made about interpretations being equally wrong, but to borrow the phrasing of Animal Farm, some are more equally wrong than others...
        • by alexgieg (948359)

          Actually, many worlds violates CPT symmetry [wikipedia.org] - worldlines divide only forward in time, not backwards.

          Your link provides this objection but also the response: "The splitting is time asymmetric; this observed temporal asymmetry is due to the boundary conditions imposed by the Big Bang". Yudkowsky explains this in a comment in the thread in next paragraph's first link: "If you took one world and extrapolated backward, you'd get many pasts. If you take the many worlds and extrapolate backward, all but one of the resulting pasts will cancel out! Quantum mechanics is time-symmetric." (emphasis mine) So the viola

      • Given the above considerations, whatever the experiment detected is most certainly not collapse.

        On the contrary: It is a collapse... of the Copenhagen interpretation itself. One that has been happening since such interpretation was given; Almost, as the researchers say, in slow motion...

      • by Prune (557140)
        Many worlds is just one way to resolve the problem. There are others one that do effectively, without resorting to consciousness: http://arxiv.org/abs/quant-ph/0105097 [arxiv.org] http://arxiv.org/abs/quant-ph/0412182 [arxiv.org]
    • I just came here to become a person who has seen a video of Schrodinger's cat collapsing. Nothing to see here, move along.
    • by Shavano (2541114)

      Well stated. It's remarkable that so many physicists have adhered to the Copenhagen interpretation for so long, stating it as fact when explaining it to laymen and worse, to students of physics when in fact it is a subject of debate among prominent physicists and has been all along. The interpretation itself makes a lot of untestable claims and for that reason alone it ought to be regarded with great skepticism and no conclusions should be drawn from it.

      I also find it remarkable that it's named for Copenh

    • by omnichad (1198475)

      'Wavefunction collapse' is how the Copenhagen Interpretation 'explains' this phenomenon, but like many of its 'explanations' they don't provide a compelling reason for things to happen this way. Terms like 'measurement' go from a precise QM meaning (eg. matrix multiplication) to a vague, ambiguous meaning like 'a concious observer'. This leads to tenuous extrapolations and conclusions, like the distinguished position of observers, the inclusion of conciousness into the interpretation and all the quantum 'explanations' of consciousness which that has spawned.

      If a tree falls in a forest and no one is around to hear it, does the wavefunction collapse to make a sound?

    • Do you actually know what you're talking about, or have you just read too much science fiction? I don't remember the Copenhagen Interpretation being related to "consciousness", either in the requirement that an observer be "conscious" or in using quantum mechanics to explain consciousness. Are you sure that's not just some whacky interpretation of the Copenhagen Interpretation that people have come up with more recently, due to misunderstanding?

      • Re:Information (Score:4, Interesting)

        by Warbothong (905464) on Thursday October 10, 2013 @10:43AM (#45091615) Homepage

        Do you actually know what you're talking about, or have you just read too much science fiction? I don't remember the Copenhagen Interpretation being related to "consciousness", either in the requirement that an observer be "conscious" or in using quantum mechanics to explain consciousness. Are you sure that's not just some whacky interpretation of the Copenhagen Interpretation that people have come up with more recently, due to misunderstanding?

        Well I've got a Master's degree in Physics, which doesn't make me an expert in all things Physicsy, but the Copenhagen Interpretation isn't exactly cutting edge.

        The Copenhagen Interpretation itself doesn't mention conscious observers, but it makes a vague distinction between measurements (which collapse the wavefunction) and non-measurements (which don't), without actually explaining what the difference is.

        For example, in the classic Young's Slits experiment, light shines on to a surface containing two slits. If we measure which slit a photon passes through, it will appear on the other side as expected. If we don't measure which slit a photon passes through, it will pass through both and cause an interference pattern. But what consitutes a measurement and, more importantly, why does it depend on something 'we' do?

        According to (Heisenberg's formulation of) the theory, everything can be modelled as matrix transformations. A unitary transformation is not a measurement, so it will not collapse the wavefunction. A non-unitary transformation is a measurement and will cause collapse (specifically, the system will collapse to an eigenstate in the basis of the transformation). All well and good *in theory*, but it's difficult to apply to the real world. Is "looking at something" a unitary transformation? Is "poking with a stick" a unitary transformation? It's difficult to tell. What's more, transformations aren't just caused by actions, they depend on the configuration of the whole system. For example, we can collapse a wavefunction by measuring where a particle *isn't*!

        So, in the case of Young's Slits, why is it that when *we* try to interact with a photon its wavefunction collapses (ie. a measurement is made), but it *doesn't* happen when the slits themselves interact with the photons, or electrons in the air absorb and re-emit the photon, etc.? The Copenhagen Interpretation doesn't say anything about this; it makes the distinction and leaves it at that. But if there's a distinction, what is it? What's the only thing that sets apart all of the possible measurement-making things and all of the possible non-measurement-making things? In all seriousness, the answer to that question is consciousness. This has perplexed Physicists for decades, leading to philosophies like "shut up and calculate".

        However, it turns out that this is the wrong question to be asking in the first place! According to Many Worlds, measurement is all about information flow. If information is transmitted between two things, they become part of a QM system with a corresponding wavefunction; it doesn't matter whether those things are electrons, slits, screens or humans.

        An analogy would be to think of the boundary of the system as being like a bubble, expanding to engulf anything which interacts information-theoretically with the contents. What Copenhagen calls 'wavefunction collapse' would be the moment when that bubble engulfs the experimenter. According to Copenhagen, when the bubble hits the experimenter it bursts, causing the entire contents of the bubble to instantly take a fixed configuration (as an aside, since QM systems can be any size, this line of thinking has also lead to the 'faster than light' nonsense, since the bubble must burst instantly everywhere as soon as anyone measures its contents). Accoring to Many Worlds, engulfing an experimenter is just like engulfing an electron, or a table, or a planet; the bubble just keeps growing as long as information keeps flowing.

        *From the point of view of the observer* these inter

        • So, in the case of Young's Slits, why is it that when *we* try to interact with a photon its wavefunction collapses (ie. a measurement is made), but it *doesn't* happen when the slits themselves interact with the photons

          Well I thought the point is that the slits *are* interacting with the photon, which is why you get interference patterns. Interfering with the light in a way that would require it go through one slit or the other will cause it to go through one slit regardless of what you're "trying" to do. It's not about intention or consciousness.

          Personally, I think your notion of "information flow" makes it more weird, spooky, and bound up with consciousness. Because what determines whether a thing is "information" i

    • by ath1901 (1570281)

      I thought the main point of the copenhagen interpretation was the instrumentalist approach. That is, the mathematical description should not be considered a description of reality but just a tool for calculating probabilities of real events. So, these supposed issues with 'conscious observers' do not apply. There is no real wavefunction and no real collapse. It's just a mathematical description.

      But, even if you take a realist approach to the standard QM theory and argue that the wavefunction is real, consci

    • Copenhagen and Many Worlds interpretations are not the only two out there, though they seem to be the only two discussed. I personally prefer objective collapse theory [wikipedia.org].

      Here the wavefunction is an actual physical phenomenon, collapse is not subjective while still being non-deterministic and having no hidden variables. You also don't have to consider both alive and dead cats or many simultaneous worlds you have to take on faith.
    • What I don't get is how the Copenhagenists forget the cat can determine it's own existence simply by meowing.
    • by Prune (557140)
      Here's another good interpretation that avoids mixing up consciousness: http://arxiv.org/abs/quant-ph/0105097 [arxiv.org] http://arxiv.org/abs/quant-ph/0412182 [arxiv.org]
  • by Dialecticus (1433989) on Thursday October 10, 2013 @07:34AM (#45089699)
    Do you realize what this means?! We can finally make a cat that's dead or alive to an arbitrary percentage!
  • by Nightlight3 (248096) on Thursday October 10, 2013 @08:19AM (#45089913)

    This is not the non-local collapse which some QM physicists (mystical school of thought) believe in. Everything in this experiment is local, the two superposed wave components which collapse into one are fully overlapped. Hence it is no more mysterious than your radio antenna collapsing superposed waves of thousands of radio stations striking it, into one component, that of a station you tuned in.

    The real controversy is about existence of non-local collapse i.e. when two components and detectors are "far apart" (at space like distance), so that detection by detector D1 (supposedly) instantly collapses the remote field component causing the remote detector D2 to fail to detect it. Most recent experiment claiming to demonstrate such phenomenon with photon on a beam splitter actually cheated (see discussion here [physicsforums.com]). In that claim they basically tweaked the timings on two coincidence circuits well out of manufacturer's specs so that they could never trigger D1 and D2 simultaneously.

    Non-local collapse, which was never demonstrated empirically, does not follow from the Quantum Field Theory (discussion here [physicsforums.com]) but is merely a hypothesis in the QM "measurement theory", which is the speculative, soft and fuzzy, part of the theory that has been debated among physicists, philosophers and mystics for nearly a century without getting anywhere so far.

    • Thank you! I was wondering about this, because it seemed that knowing whether a non-local collapse had occurred, or not, would allow for FTL information exchange.
    • Are you familiar with the work of Yves Couder [youtube.com]? Macroscopic Newtonian systems exhibiting features of quantum behavior. All this voodoo is probably just a lack of understanding.
  • You know in games like Call of Cthulu, or more germane, games like Delta Green or Cthulutech, where doing research into 'extra dimensional science' or whatever other terms they use to describe the eldritch magics and alien geometries, drives the researchers insane?

    That's what quantum physics is like.

  • http://www.youtube.com/watch?v=DfPeprQ7oGc [youtube.com]

    A quote from the comments (saves time):

    jon manock 1 week ago
    "this video is extremely misleading. the electron does not know it is being observed and decide to behave like a particle.
    it is the electron interacting with the test equipment that collapses the wave function. consciousness has nothing to do with it."

  • You can not tell the difference between particles that are in a superposition of states and those that have "collapsed". If such a difference could be discerned then entangled pairs could be used for faster-than-light communication by modulating their "collapsedness". These guys are not dealing with particles but a somewhat larger system. Is it an example of macroscopic system exhibiting quantum behavior? If so, does it offer a non-magical explanation of the phenomena?
  • I couldn't figure out if this was awesome or total BS until I RTFA.

If a camel is a horse designed by a committee, then a consensus forecast is a camel's behind. -- Edgar R. Fiedler

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