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Classical states and decoherence

  1. Feb 26, 2015 #1
    It is stated that classical states are robust against decoherence.. what would happen if classical states can decohere too? Or how do you imagine it for example occurring to a table.. How would the table look like if it suffers decoherence too? Would you fall down if you sit on one?
     
  2. jcsd
  3. Feb 26, 2015 #2
    I think you have misunderstood what decoherence is. Decoherence is where a sufficiently large quantum object interacts with its environment. Because of the nature of the interaction the resulting state describing the system looks like classical probabilities -- the interference vanishes (although in principle the quantum system + environment remain in superposition).
     
  4. Feb 26, 2015 #3

    bhobba

    Staff: Mentor

  5. Feb 27, 2015 #4
    StevenTNZ and Bhobba.. I was referring to Einselection (Environment-Induced Superselection).

    http://cnls.lanl.gov/~dalvit/Publications_files/PRA-72-062101-2005.pdf

    "Persistent monitoring of an open quantum system by its
    environment can single out a preferred set of states, known
    as pointer states. Pointer states are the most robust in spite of
    the interaction with the environment. That is, they entangle
    least with the environment, and, hence, are least perturbed by
    it. This is the essence of environment-induced superselection.

    Hence the pointer states, or classical states are robust against decoherence. Contrary to Bill statement that "Classical states are decohered by the environment".

    Can someone describe what really is Einselection (Environment-induced superselection). I'm confused by it and can't connect it with standard QM. What is its counterpart in the standard QM. Again. What would happen if classical states decohere too. Then perhaps we won't have an observable of position in our daily life but momentum? Is this what it's saying? It's like particles can choose either position or momentum.. and if decoherence occur even to classical or pointer states, then we should have observable of momentum and not position. so we won't have two feet to walk in position basis but momentum (whatever or how-ever it is).
     
  6. Feb 27, 2015 #5

    bhobba

    Staff: Mentor

    Its not contrary to it - its an example of it.

    Its simple. Decoherence is an interaction between systems, usually the environment, that transforms a pure state into a mixed state in a particular basis. Most of the time its in the position basis. And that basis is stable meanting it does not change as the interaction evolves. The technical detains of why you can find in page 83 of:
    https://www.amazon.com/Decoherence-Classical-Transition-Frontiers-Collection/dp/3540357734

    Its got to with most interactions are of the radial type.

    The whole issue is examined in chapter two of the above reference which, if it interests you, I strongly suggest you get a copy of.

    First though do you understand the difference between a pure state and a mixed state?

    Thanks
    Bill
     
    Last edited: Feb 27, 2015
  7. Feb 27, 2015 #6
    I own the book. Have just reread page 83 and got the points but it speaks of particles. I also know the difference between a pure state and mixed state. I'm talking of macroscopic object and specifically the following passage:

    http://www.nature.com/news/2004/041223/full/news041220-12.html

    "The difficulty arises because directly finding out something about a quantum system by making a measurement inevitably disturbs it. "After a measurement," say Wojciech Zurek at Los Alamos National Laboratory in New Mexico and his colleagues, "the state will be what the observer finds out it is, but not, in general, what it was before."

    Because, as Zurek says, "the Universe is quantum to the core," this property seems to undermine the notion of an objective reality. In this type of situation, every tourist who gazed at Buckingham Palace would change the arrangement of the building's windows, say, merely by the act of looking, so that subsequent tourists would see something slightly different."

    Bhobba. What the above is implying is that if Einselection is temporary switched off, the Buckingham Palace window would keep changing depending on the gaze of each tourist. Here the position observable is still used, but I'm thinking the billions of atoms in the windows is decohered by the environment, so position is already fixed.. so if Einselection is suppressed, how would it make the windows rearrange every time a different tourist look at it? This is what I couldn't understand for years.
     
  8. Feb 27, 2015 #7

    bhobba

    Staff: Mentor

    Exactly how do you do that?

    Its inherent in the interaction Hamiltonian.

    QM does undermine the notion of objective reality. Its related to the problem of outcomes which is also discussed in that book. However decoerence is observer independant.

    Thanks
    Bill
     
  9. Feb 27, 2015 #8
    the interaction Hamiltonian is in the levels of particles... here positions or energy or momentum can be chosen. But in the Buckingham Palace, it is in the positions of each particles.. but how does the window rearrange when you are not changing position to momentum but just positions. So even without Einselection, the positions of each millimeter of the window is still decohered. Is the window a valid example or a wrong example of the application of Einselection?
     
  10. Feb 27, 2015 #9

    bhobba

    Staff: Mentor

    Come again. Windows and typical macro objects are decohered to be in eigenstates of position - thats the exact import of the page number I gave. Most Hamilitonians are radial.

    Thanks
    Bill
     
  11. Feb 27, 2015 #10
    I was wondering if the Nature author gave a vague example. In the book particles can be eigenstates of position or momentum or energy. But in the article. The palace window is rearranging itself.. you are saying this is still a good example of eigenstates of positions being controlled by Einselection?
     
  12. Feb 27, 2015 #11

    bhobba

    Staff: Mentor

    I am saying that because of the the radial nature of virtually all interactions encountered in practice (ie interacting with the air, stray photons etc) a window will be in eigenstates of position. The palace window rearranging itself? I have zero idea why you would think such would happen.

    Thanks
    Bill
     
  13. Feb 27, 2015 #12
    Didnt you read the article

    http://www.nature.com/news/2004/041223/full/news041220-12.html

    it says without quantum darwism, the palace window would rearrange after each tourist stares at it. my main question is... without quantum darwism or how we get information from reflected photon and not directly interacting with it.. it is indeed possible the window rearrange itself after each tourist gaze?
     
  14. Feb 27, 2015 #13

    bhobba

    Staff: Mentor

    That's wrong.

    From decoherence the position basis is singled out and we have an improper mixed state in it. The general interpretation is that the mixed state is a proper one which means it is actually in that state - but we don't know which one.

    I am not an expert on Quantum Darwinism but my limited knowledge of it is its trying to give a purely quantum explanation of that interpretative assumption.

    Added Later:
    That's a lay article - to disentangle it you need to get the professional paper its based on. I reread the bit on Quantum Darwinism in Schlosshauer and it seems to be saying simply what I said above. We want a fully quantum explanation of how the improper mixed state becomes a proper one.

    Thanks
    Bill
     
    Last edited: Feb 27, 2015
  15. Feb 27, 2015 #14
    The Nature layman article is based Zurek paper http://arxiv.org/abs/quant-ph/0105127
    Basically Quantum Darwism is as the paper quotes it "Intercepting fragments of the environment allows observers to find out (pointer) state of the system without perturbing it". So the nature layman article is saying that because of quantum darwism, the Buckingham Palace window can't be perturbed by the stare of each tourist. But let's say there was no quantum darwism, can the palace window be perturbed if we send a photon to look at it (say a flashlight) such that the window can change shape? But the window is not in pure state, the positions of each particle is in improper mixed state which looks like proper mixed state (apparent collapse). Can a proper mixed state still be perturbed by the environment and re-prepare to pure state and back to another improper mixed state? I just want to know how accurate is that specific article about the palace window being able to change shape if tourist stares at it (if there was no quantum darwism). Let's just focus on the window thing accuracy.
     
  16. Feb 28, 2015 #15

    atyy

    User Avatar
    Science Advisor

    Yes, the window analogy is more or less accurate. Let's go with the simplistic version of collapse just for the idea. A measurement P collapses the wave function randomly into an eigenstate of P. Then if a different measurement Q is made the wave function will randomly collapse into an eigenstate of Q. So for example if I measure position, the wave function will collapse into a narrow peak. Now if I measure momentum, the wave function will collapse into a spread out wave. If I alternate between position and momentum measurements, the wave function will keep jumping between being peaked and spread out. So in Zurek's analogy, each tourist is making a different measurement and so collapsing into an eigenstate of the respective measurements, so reality will be all jumpy.

    In addition to Zurek's approach, other lines to explaining the conditions under which repeated or continuous measurements give classical results are:
    http://arxiv.org/abs/1305.2517
    http://arxiv.org/abs/1407.8090

    I think in those papers, one does get a stochastic differential equation describing a jumpy reality. But as long as the jumps are "small", one will have recovered classical trajectories.
     
    Last edited: Feb 28, 2015
  17. Feb 28, 2015 #16
    First of all, it was not Zurek's analogy, the Nature article was written by Philip Ball. Secondly, let me clarify, are you saying macroscopic object can change shape by perturbing it if quantum darwism is suppressed? But thermal vibrations can make each atom classical, so how do you make the whole object change shape? Or does it only work for system in pure state? not macroscopic object? or since a macroscopic object are improper mixed state, then you can make improper mixed state jumpy such that the window can really change shape?
     
  18. Feb 28, 2015 #17

    atyy

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    No, it is not that quantum darwinism can be suppressed. Rather, the emergence of the pointer states via decoherence is just one aspect of how classicality emerges from the quantum formalism. One still has to understand what happens when a sequence of measurements occurs after decoherence. Quantum darwinism is a different analysis of the same quantum formalism, trying to understand the role of repeated measurements for the emergence of classicality.
     
  19. Feb 28, 2015 #18
    So why doesn't the window rearrange itself everytime a tourists passes by it? Note in the article, it is arguing that because of quantum darwism, quantum barrier to perturbation seems to exist so we can't change the window. This is why I concluded that if there is no quantum darwism, the window can rearrange itself. Please confirm if the window is just analogy for microscopic system or it is also valid for macroscopic system. If so, how come we don't have experiment where window can change shape or how do we do the experiment to show it?
     
  20. Feb 28, 2015 #19

    atyy

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    Your question doesn't make any sense. There is no distinction between microscopic and macroscopic systems. They are both governed by quantum mechanics. Under some circumstances, quantum mechanics can be well approximated by classical mechanics. Deocherence and quantum darwinism are ways to explain under what circumstances the classical approximation is good.
     
  21. Feb 28, 2015 #20
    So how do you do the experiment. Let's take the case of a painting. If we put it inside a totally sealed box and no photons can get inside and we installed a camera and one active emitter. How do we make the painting change shape such that the picture in the painting would move? The article says quantum darwism acts as shield to avoid perturbing the object, so with the sealed box, no photons are being reflected off that object (hence suppressing quantum darwism). What must you transmit in the emitter to make the painting rearrange itself (just like the window in the article)?
     
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