9 ms·

A new experiment hints at surprising hidden mechanics of quantum superpositions

89 points by kylnew 8y ago

amelius 8y ago
Question about the basics. They say: > And it gets weirder: Measuring which slit such a particle goes through will invariably indicate it only goes through one—but then the wavelike interference (the “quantumness,” if you will) vanishes. The very act of measurement seems to “collapse” the superposition. “We know something fishy is going on in a superposition,” says physicist Avshalom Elitzur of the Israeli Institute for Advanced Research. “But you’re not allowed to measure it. This is what makes quantum mechanics so diabolical.” But (afaik) measuring means disturbing, because you have to exchange some energy with the system in order to perform the measurement. So in the above quote, if you replace "measuring" by "disturbing", then all of a sudden, the whole paragraph doesn't make any sense ... Can anyone clarify this?
- lisper 8y ago
  > you have to exchange some energy with the system in order to perform the measurement That's true, but what matters is not the energy exchange so much as the resulting entanglement. It is entanglement, not measurement per se, that "destroys" the interference. See: http://www.flownet.com/ron/QM.pdf http://www.flownet.com/ron/QM.pdf or the movie version: https://www.youtube.com/watch?v=dEaecUuEqfc https://www.youtube.com/watch?v=dEaecUuEqfc for more details.
- russdill 8y ago
  You might want to look up the quantum eraser experiments. You can have a particle interact with the system in such a way that the result of the measurement cannot be known.
  - drbw 8y ago
    There's a good episode of PBS SpaceTime about this: https://www.youtube.com/watch?v=8ORLN_KwAgs https://www.youtube.com/watch?v=8ORLN_KwAgs
- gdavisson 8y ago
  >But (afaik) measuring means disturbing, because you have to exchange some energy with the system in order to perform the measurement. This isn't necessarily true; QM allows what's called interaction-free measurement. Say you have a particle that's in a superposition of two states (or places, or whatever), which I'll call A and B. You do something that'll interact (and detect) it if it's in A. If you didn't detect it, that means you've effectively measured it as being in B. You can also turn it around, and use a particle in a superposition to see if an object (that it would interact with if it's in state A) exists; if the object exists, the possibility of interacting with it will destroy interference effects between the parts of the particle's superposition, allowing you to infer the existence of the object. The Elitzur–Vaidman bomb tester (https://en.wikipedia.org/wiki/Elitzur–Vaidman_bomb_tester https://en.wikipedia.org/wiki/Elitzur–Vaidman_bomb_tester) is an extreme example of this: it lets you verify that a bomb is "live" (will explode if interacted with), with only a 50% chance of setting it off. Another example is the interaction-free version of the quantum Zeno effect. The quantum Zeno effect is that (under the right circumstances) a particle can be trapped in a particular state by continuously measuring whether it's still in the state. In the interaction-free version, you use something that'll interact if the particle ever leaves the state... and it never does. This has actually been done; see https://www.nature.com/articles/ncomms7811?WT.ec_id=NCOMMS-20150415 https://www.nature.com/articles/ncomms7811?WT.ec_id=NCOMMS-2...
  - Moodles 8y ago
    Very interesting, thanks. This is a nice video to expose the weirdness of the double slit experiment: https://www.youtube.com/watch?v=DfPeprQ7oGc https://www.youtube.com/watch?v=DfPeprQ7oGc
    - skipperr 8y ago
      Does anyone know of actual videos of the experiment? All the videos I find online only show the wave interference pattern, but not the particle-like pattern shown when measuring/observing the particles.
      - spindle 8y ago
- deleted 8y ago
  [deleted]
- Certhas 8y ago
  For context, in classical mechanics we also have to perturb the system to measure it, but we can make the perturbations arbitrarily small. That's why we call it a measurement rather than an interaction, and can get away with not explicitly modeling it. A seminal paper on the question in classical mechanics was the Bohr Rosenfeld 1933 Paper on measuring Electrodynamic fields, that showed that classical EM fields can indeed be measured accurately (I can't find it online right now, but the follow up on QED is [1]). The question appears in the context of General Relativity as the question when extended bodies follow geodesics. These are all different to the situation in QM. There is an irreducible alteration of the state in QM. It resembles in many ways the updating of a probability distribution if you gain new information. Imagine you have two envelopes with a blue and a red card in them. You mix them and send one of them far away. Now you know that the far away envelope has a 50/50 chance of being blue. It is in a superposition of red and blue. If you open your envelope and look in, the superposition collapses, and you suddenly know exactly what state the other envelope is in, even though you didn't touch it, didn't disturb it at all. QM measurements behave just like this "updating of probabilities upon learning something" _except_ that we know from other, more subtle properties (Bell's inequalities) that they are _not_ probabilities referring to a hidden state. [1] https://journals.aps.org/pr/pdf/10.1103/PhysRev.78.794 https://journals.aps.org/pr/pdf/10.1103/PhysRev.78.794
- ttctciyf 8y ago
  The issue is that no-one has a cast-iron definition of what exactly amounts to a measurement, outside of something entangling with the system being measured. You can "disturb" a two-slit system, for example by creating a third slit which will change the "interference pattern" or pattern of probability of detection on the screen, without this being a measurement, though, so the concepts of "disturb" and "measure" are distinct.
cfadvan 8y ago
This is a horrendous article, clearly written by someone who doesn’t grasp the basics. I gave up when they conflated how the double-slit experiment demonstrates wave-particle duality with a demonstration of superposition. I can’t believe how much of a joke SciAm has become, it makes me sad.
- pessimist 8y ago
  Your criticism is totally incorrect. In fact the 2-slit experiment does demonstrate superposition. Wave-particle duality and superposition of states are really just complementary ways of describing the same situation. To understand this, please try to read any undergrad level treatment of quantum mechanics. For example - See Feynman's lectures volume 3.
  - cfadvan 8y ago
    I have that right here, can you give me a page number?
    - pessimist 8y ago
      Please read chapter 3 "Probability Amplitudes". http://www.feynmanlectures.caltech.edu/III_03.html http://www.feynmanlectures.caltech.edu/III_03.html "When a particle can reach a given state by two possible routes, the total amplitude for the process is the sum of the amplitudes for the two routes considered separately. In our new notation we write that ⟨x|s⟩both holes open=⟨x|s⟩through 1+⟨x|s⟩through 2. (3.4)"
- kylnew 8y ago
  I’m admittedly pretty basic on the subject matter myself and wouldn’t have been able to notice that. Any suggested light reading that would clarify this distinction?
  - cfadvan 8y ago
    I’m not sure about light reading, but http://theoreticalminimum.com http://theoreticalminimum.com is a great starting point.
oldandtired 8y ago
What intrigues me about the entire situation is that the scientists are not standing back and looking at all the assumptions they use. Simple things like that the belief that there are "virtual" particles "popping" in and out of existence and they don't affect the path of "real" particles like photons or electrons. Simple things like what processes is the matter that exists each side of the slits undergoing in relation to the particles that pass through the slits and how each affects the other. Simple things like what is a photon or other such particle. Is the particle/wave duality idea hindering or helping the further understanding of these processes? Is quantum mechanics the best approximation that we have or are there other ideas that would simply the models in use? I liken it to a programming project that has gone down one path based on a series of assumptions and when these assumptions are found to be incorrect or different from what is actually there, it becomes very difficult to change direction without doing a complete rebuild. If one looks at the history of investigations over the last 100 years or so, one finds that there are a variety of ideas that never gained any traction at the time of proposal. Yet, today appear to give a handle on some of the puzzles that are being found. These ideas are ignored because they didn't gain traction at the time of first proposal. Mayhaps, it would be worth spending some time to investigate to see if they have any merit. They may not, but it can't hurt to see if they have.
- millstone 8y ago
  Notions like wave-particle duality, virtual particles "popping in and out", etc. aren't input assumptions. They're metaphors that come out of the equations, which are precise and well-founded. These have a useful role as levers to spur intuition, but ought not to be confused with the theories themselves. It turns out questions like "what is a photon" are not that simple. For example, the very phrase: "a photon." Well, the particle number is an operator which is subject to the uncertainty principle and doesn't commute with other measurements, so already you're at risk... Our intuition misleads us in the quantum realm.
  - oldandtired 8y ago
    Yes, they are metaphors. However, they come out of prior assumptions and they are used as input assumptions for later work. The equations are simplified mathematical characterisations of the experimental evidence. Certain simplifying assumptions are made to create these equations. Theories are an attempt to clarify what is believed to be happening and are always based on some kind of simplifying assumptions. Often, it is difficult to take a step back and take another long hard look at what the evidence is. The point of my mentioning "what is a photon?" is that we make certain assumptions (as per your operators, uncertainty principle, etc) which we don't step back from and see if there are alternatives. Even trying to get a handle on what the uncertainty principle means gets us into a philosophical quagmire. Our view of the "quantum realm" is coloured by a huge amount of background assumptions, including the "intuitions" of those investigating the subject matter. There is an oft stated saying: "The science is settled" over some matter or another. Yet, the whole point of scientific endeavour is that it is not settled, we are still investigating (or at least should be).
bob_theslob646 8y ago
The website is down.
- scottyelich 8y ago
  http://www.thewebsiteisdown.com/ http://www.thewebsiteisdown.com/
jerry40 8y ago
Does it mean we can create some machine (let's call it timegraph): - before some sport event (NBA finals 7th match) we write a question 'who will win, A (1st team name) or B (2nd team name)?' - do 2 experiments with photon and slits: * (A) * (B) - do not measure their results until tomorrow - tomorrow (after the match is over) you measure result of the experiment of the team that won and destroy results of the another experiment - (returning to the previous day): look which experiment shows inteference and which doesn't
- yorwba 8y ago
  > look which experiment shows inteference and which doesn't This is also a kind of measurement, and one you can't meaningfully perform before the other. By the time you know the interference pattern, it's already too late to decide whether you want to measure the intermediate position. You might try to measure without looking at the results, but that doesn't change anything about the fact that the interaction happened. (Superpositions do not collapse by being observed by a human, they collapse by interacting with the rest of the world.)
  - jerry40 8y ago
    > Superpositions do not collapse by being observed by a human, they collapse by interacting with the rest of the world. But the article says: "Measuring which slit such a particle goes through will invariably indicate it only goes through one—but then the wavelike interference (the “quantumness,” if you will) vanishes. The very act of measurement seems to “collapse” the superposition." "Aharonov’s approach is called the two-state-vector formalism (TSVF) of quantum mechanics, and postulates quantum events are in some sense determined by quantum states not just in the past—but also in the future. " Doesn't it mean that future measurement 'pushes' superposition into a definite state in the past?
vorg 8y ago
> The apparent vanishing of particles in one place at one time—and their reappearance in other times and places—suggests [...] a particle’s presence in one place is somehow “canceled” by its own “counterparticle” in the same location. [...] These putative counterparticles should possess negative energy and negative mass, allowing them to cancel their counterparts. I thought negative energies in Quantum Mechanics give rise to senseless infinities that can't be eliminated. What gives?

89 points by kylnew 8y ago

Question about the basics. They say: > And it gets weirder: Measuring which slit such a particle goes through will invariably indicate it only goes through one—but then the wavelike interference (the “quantumness,” if you will) vanishes. The very act of measurement seems to “collapse” the superposition. “We know something fishy is going on in a superposition,” says physicist Avshalom Elitzur of the Israeli Institute for Advanced Research. “But you’re not allowed to measure it. This is what makes quantum mechanics so diabolical.” But (afaik) measuring means disturbing, because you have to exchange some energy with the system in order to perform the measurement. So in the above quote, if you replace "measuring" by "disturbing", then all of a sudden, the whole paragraph doesn't make any sense ... Can anyone clarify this?

lisper 8y ago
> you have to exchange some energy with the system in order to perform the measurement That's true, but what matters is not the energy exchange so much as the resulting entanglement. It is entanglement, not measurement per se, that "destroys" the interference. See: http://www.flownet.com/ron/QM.pdf http://www.flownet.com/ron/QM.pdf or the movie version: https://www.youtube.com/watch?v=dEaecUuEqfc https://www.youtube.com/watch?v=dEaecUuEqfc for more details.
russdill 8y ago
You might want to look up the quantum eraser experiments. You can have a particle interact with the system in such a way that the result of the measurement cannot be known.
- drbw 8y ago
  There's a good episode of PBS SpaceTime about this: https://www.youtube.com/watch?v=8ORLN_KwAgs https://www.youtube.com/watch?v=8ORLN_KwAgs
gdavisson 8y ago
>But (afaik) measuring means disturbing, because you have to exchange some energy with the system in order to perform the measurement. This isn't necessarily true; QM allows what's called interaction-free measurement. Say you have a particle that's in a superposition of two states (or places, or whatever), which I'll call A and B. You do something that'll interact (and detect) it if it's in A. If you didn't detect it, that means you've effectively measured it as being in B. You can also turn it around, and use a particle in a superposition to see if an object (that it would interact with if it's in state A) exists; if the object exists, the possibility of interacting with it will destroy interference effects between the parts of the particle's superposition, allowing you to infer the existence of the object. The Elitzur–Vaidman bomb tester (https://en.wikipedia.org/wiki/Elitzur–Vaidman_bomb_tester https://en.wikipedia.org/wiki/Elitzur–Vaidman_bomb_tester) is an extreme example of this: it lets you verify that a bomb is "live" (will explode if interacted with), with only a 50% chance of setting it off. Another example is the interaction-free version of the quantum Zeno effect. The quantum Zeno effect is that (under the right circumstances) a particle can be trapped in a particular state by continuously measuring whether it's still in the state. In the interaction-free version, you use something that'll interact if the particle ever leaves the state... and it never does. This has actually been done; see https://www.nature.com/articles/ncomms7811?WT.ec_id=NCOMMS-20150415 https://www.nature.com/articles/ncomms7811?WT.ec_id=NCOMMS-2...
- Moodles 8y ago
  Very interesting, thanks. This is a nice video to expose the weirdness of the double slit experiment: https://www.youtube.com/watch?v=DfPeprQ7oGc https://www.youtube.com/watch?v=DfPeprQ7oGc
  - skipperr 8y ago
    Does anyone know of actual videos of the experiment? All the videos I find online only show the wave interference pattern, but not the particle-like pattern shown when measuring/observing the particles.
    - spindle 8y ago
deleted 8y ago
[deleted]
Certhas 8y ago
For context, in classical mechanics we also have to perturb the system to measure it, but we can make the perturbations arbitrarily small. That's why we call it a measurement rather than an interaction, and can get away with not explicitly modeling it. A seminal paper on the question in classical mechanics was the Bohr Rosenfeld 1933 Paper on measuring Electrodynamic fields, that showed that classical EM fields can indeed be measured accurately (I can't find it online right now, but the follow up on QED is [1]). The question appears in the context of General Relativity as the question when extended bodies follow geodesics. These are all different to the situation in QM. There is an irreducible alteration of the state in QM. It resembles in many ways the updating of a probability distribution if you gain new information. Imagine you have two envelopes with a blue and a red card in them. You mix them and send one of them far away. Now you know that the far away envelope has a 50/50 chance of being blue. It is in a superposition of red and blue. If you open your envelope and look in, the superposition collapses, and you suddenly know exactly what state the other envelope is in, even though you didn't touch it, didn't disturb it at all. QM measurements behave just like this "updating of probabilities upon learning something" _except_ that we know from other, more subtle properties (Bell's inequalities) that they are _not_ probabilities referring to a hidden state. [1] https://journals.aps.org/pr/pdf/10.1103/PhysRev.78.794 https://journals.aps.org/pr/pdf/10.1103/PhysRev.78.794
ttctciyf 8y ago
The issue is that no-one has a cast-iron definition of what exactly amounts to a measurement, outside of something entangling with the system being measured. You can "disturb" a two-slit system, for example by creating a third slit which will change the "interference pattern" or pattern of probability of detection on the screen, without this being a measurement, though, so the concepts of "disturb" and "measure" are distinct.

cfadvan 8y ago

This is a horrendous article, clearly written by someone who doesn’t grasp the basics. I gave up when they conflated how the double-slit experiment demonstrates wave-particle duality with a demonstration of superposition. I can’t believe how much of a joke SciAm has become, it makes me sad.

pessimist 8y ago
Your criticism is totally incorrect. In fact the 2-slit experiment does demonstrate superposition. Wave-particle duality and superposition of states are really just complementary ways of describing the same situation. To understand this, please try to read any undergrad level treatment of quantum mechanics. For example - See Feynman's lectures volume 3.
- cfadvan 8y ago
  I have that right here, can you give me a page number?
  - pessimist 8y ago
    Please read chapter 3 "Probability Amplitudes". http://www.feynmanlectures.caltech.edu/III_03.html http://www.feynmanlectures.caltech.edu/III_03.html "When a particle can reach a given state by two possible routes, the total amplitude for the process is the sum of the amplitudes for the two routes considered separately. In our new notation we write that ⟨x|s⟩both holes open=⟨x|s⟩through 1+⟨x|s⟩through 2. (3.4)"
kylnew 8y ago
I’m admittedly pretty basic on the subject matter myself and wouldn’t have been able to notice that. Any suggested light reading that would clarify this distinction?
- cfadvan 8y ago
  I’m not sure about light reading, but http://theoreticalminimum.com http://theoreticalminimum.com is a great starting point.

oldandtired 8y ago

What intrigues me about the entire situation is that the scientists are not standing back and looking at all the assumptions they use. Simple things like that the belief that there are "virtual" particles "popping" in and out of existence and they don't affect the path of "real" particles like photons or electrons. Simple things like what processes is the matter that exists each side of the slits undergoing in relation to the particles that pass through the slits and how each affects the other. Simple things like what is a photon or other such particle. Is the particle/wave duality idea hindering or helping the further understanding of these processes? Is quantum mechanics the best approximation that we have or are there other ideas that would simply the models in use? I liken it to a programming project that has gone down one path based on a series of assumptions and when these assumptions are found to be incorrect or different from what is actually there, it becomes very difficult to change direction without doing a complete rebuild. If one looks at the history of investigations over the last 100 years or so, one finds that there are a variety of ideas that never gained any traction at the time of proposal. Yet, today appear to give a handle on some of the puzzles that are being found. These ideas are ignored because they didn't gain traction at the time of first proposal. Mayhaps, it would be worth spending some time to investigate to see if they have any merit. They may not, but it can't hurt to see if they have.

millstone 8y ago
Notions like wave-particle duality, virtual particles "popping in and out", etc. aren't input assumptions. They're metaphors that come out of the equations, which are precise and well-founded. These have a useful role as levers to spur intuition, but ought not to be confused with the theories themselves. It turns out questions like "what is a photon" are not that simple. For example, the very phrase: "a photon." Well, the particle number is an operator which is subject to the uncertainty principle and doesn't commute with other measurements, so already you're at risk... Our intuition misleads us in the quantum realm.
- oldandtired 8y ago
  Yes, they are metaphors. However, they come out of prior assumptions and they are used as input assumptions for later work. The equations are simplified mathematical characterisations of the experimental evidence. Certain simplifying assumptions are made to create these equations. Theories are an attempt to clarify what is believed to be happening and are always based on some kind of simplifying assumptions. Often, it is difficult to take a step back and take another long hard look at what the evidence is. The point of my mentioning "what is a photon?" is that we make certain assumptions (as per your operators, uncertainty principle, etc) which we don't step back from and see if there are alternatives. Even trying to get a handle on what the uncertainty principle means gets us into a philosophical quagmire. Our view of the "quantum realm" is coloured by a huge amount of background assumptions, including the "intuitions" of those investigating the subject matter. There is an oft stated saying: "The science is settled" over some matter or another. Yet, the whole point of scientific endeavour is that it is not settled, we are still investigating (or at least should be).

bob_theslob646 8y ago

The website is down.

scottyelich 8y ago
http://www.thewebsiteisdown.com/ http://www.thewebsiteisdown.com/

jerry40 8y ago

Does it mean we can create some machine (let's call it timegraph): - before some sport event (NBA finals 7th match) we write a question 'who will win, A (1st team name) or B (2nd team name)?' - do 2 experiments with photon and slits: * (A) * (B) - do not measure their results until tomorrow - tomorrow (after the match is over) you measure result of the experiment of the team that won and destroy results of the another experiment - (returning to the previous day): look which experiment shows inteference and which doesn't

yorwba 8y ago
> look which experiment shows inteference and which doesn't This is also a kind of measurement, and one you can't meaningfully perform before the other. By the time you know the interference pattern, it's already too late to decide whether you want to measure the intermediate position. You might try to measure without looking at the results, but that doesn't change anything about the fact that the interaction happened. (Superpositions do not collapse by being observed by a human, they collapse by interacting with the rest of the world.)
- jerry40 8y ago
  > Superpositions do not collapse by being observed by a human, they collapse by interacting with the rest of the world. But the article says: "Measuring which slit such a particle goes through will invariably indicate it only goes through one—but then the wavelike interference (the “quantumness,” if you will) vanishes. The very act of measurement seems to “collapse” the superposition." "Aharonov’s approach is called the two-state-vector formalism (TSVF) of quantum mechanics, and postulates quantum events are in some sense determined by quantum states not just in the past—but also in the future. " Doesn't it mean that future measurement 'pushes' superposition into a definite state in the past?

vorg 8y ago

> The apparent vanishing of particles in one place at one time—and their reappearance in other times and places—suggests [...] a particle’s presence in one place is somehow “canceled” by its own “counterparticle” in the same location. [...] These putative counterparticles should possess negative energy and negative mass, allowing them to cancel their counterparts. I thought negative energies in Quantum Mechanics give rise to senseless infinities that can't be eliminated. What gives?