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Decoherence and Entanglement

  1. Feb 5, 2010 #1
    What is the relationship between decoherence and entanglement? Does the decoherence destroy entanglement of system and environment?
  2. jcsd
  3. Feb 5, 2010 #2


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    Yes and no, decoherence destroys coherence (the systems become "classical") which in turns destroys the entanglement; so it is an indirect effect.
    But it also turns out to be impossible (or at least extremely difficult) to come up with entanglement schemes that do not also create problems with decoherence. This is for eample why it is relatively easy to create a single qubit, but extremely difficult to build a practical quantum computer.
  4. Feb 5, 2010 #3
    One aspect of this discussion can be seen in the following situation:

    Imagine a photon travelling horizontally to reach a beam splitter oriented at 45 degrees, so that, after reaching it the photon will be in a coherent superposition of states (travelling horizontally after the BS and travelling vertically after the BS)

    The first state corresponds to transmission in the BS and the second, to reflection.

    If your system is the photon, the passing through the BS made the photon to enter in a coherent superposition of states, creating therefore, coherence in your system.

    Now if your systems are the horizontal and vertical channels (occupied or nor by quanta of energy) the description changes a little bit:

    Now you have, before, one quantum of occupation in channel 1 (horizontal) and zero quantum in channel 2 (vertical) and, after the BS, you turn to have channel 1 entangled with channel 2 (vertical) as the new state may be written as |1 0 > + | 0 1 >.

    Thus, at least in this particular case: coherence and entanglement may stand for different alternative ways to describe phenomena.

    I have already been informed that this is not the whole truth about these two concepts.

    Best Regards,

  5. Feb 5, 2010 #4
    ...And yet it seems that coherence and superposition of states is critical in everyday photosynthesis! At room temperature no less. Some conditions clearly destroy this relationship, but in nature these effects seem more ubiquitous than not. I have no idea how or why.
  6. Feb 5, 2010 #5


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    If you consider a coherent superposition of states interacting with a bath of weakly-coupled perturbing states, then the coherence will decay with time, however it may persist long enough for faster phenomena (like charge-transfer in the photosynthesis example), to occur. I think this is the "hand-waving" explanation that can be applied in your example of photosynthesis at room-temperature.
  7. Feb 5, 2010 #6
    Ahhh... ok I can see how that works. Thanks SCat!
  8. Feb 10, 2010 #7
    in fact, this explanation with reference to weak perturbative bath is not true, specially in the case of photosynthetic proteins.

    according to earlier theoretical predictions coherences between electronic states, even in relatively simple molecules, were expected to decay very rapidly (approx. on a time scale of 10fs). this is precisely why the experimental results from last week are causing such a stir. photosynthetic pigements are relatively strongly coupled to the protein environment. it is therefore remarkable to find coherences between excitonic states, delocalized over pigments separated by 20 angstroms, survive for over several hundreds of femtoseconds at room temperatures.

    one explanation that is being forwarded is that the protein environment rather than serving to decohere the system helps to maintain the coherence. it is thought that the bath induced fluctuations experienced by the different pigment molecules are correlated because they share the protein environment. It is uncorrelated environmental fluctuations that lead to decoherence.

    finally i would like to say that such long lived coherences are hardly shown to be ubiquitous in nature as somebody else suggest. it may turn out to be the case that we find this to be the case; however, at present the publication last week was the first experimental observation of long lived coherences in any natural system.
  9. Feb 10, 2010 #8
    The first, yes, but if it is present in the photosynthetic process then 'widespread' is a pretty good definition. The notion that predictions of quantum behaviour in a biological room-temp system could be so far off should perhaps ring alarm bells. As for ubiquity beyond plants... wait, photosynthesis is pretty damned ubiquitous. Part of skepticism is to require evidence to support claims, but that doesn't rule out some very basic assumptions when it is discovered that freakishly long-lived coherent states seems to play a role in the metabolism of PLANTS.

    I would expect more of the same in the future. Of course, there will always be theoreticians to explain just why they were wrong before, but NOW they have the right of it. No sooner do baffling results arrive than an equally unlikely explanation for the behaviour is upon its heels. :rofl:

    EDIT: Last WEEK? We're thinking of different experiments. If you know of one last week, it is not the first to find this. I would very much like to see this new study if you have a link? You've made the point well that this is a region of the science lacking in hard data, and we all know data = mental candy.
  10. Feb 10, 2010 #9


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    First of all, thanks for the heads up on the paper (Nature 463, [2010] pp. 644-647). I hadn't seen it yet .. extremely cool result! However, I don't think you meant what you said above exactly the way it sounded, my original statement:

    is certainly true in the general sense. There is always a time-dependent decoherence term due to external (i.e. not part of the coherent state) interactions that acts to destroy the coherent state. Thus there is always a competition between decoherence and other processes, as I indicated. Furthermore, this is true in this photosynthesis example as well, however there are some other features of the system that have evolved to allow the coherence to persist for an anomalously long time.

    Now, it looks like you are correct that the approximation of a weakly coupled bath is wrong in this case. In most cases, photosynthetic dyes are weakly coupled to the antenna proteins ... this case is anomalous in that the dyes are covalently bound to the protein. However the magnitude of the coupling doesn't significantly change the qualitative description I gave.

    And anyway, I *said* it was hand waving. :wink:

    I guess it depends on what you mean by "natural system" ... it was the first experimental observation of long-lived coherences *at room temperature* ... Greg Engel's group demonstrated it a couple years ago in for a cryogenic protein sample.(Nature 446, [2007] pp. 782-786).
    Last edited: Feb 10, 2010
  11. Feb 10, 2010 #10
    Let me get this straight... coherence in photosynthesis strikes you as 'anamolous'. Hm. There are a lot of plants, and other things (lichen, some bacteria, etc...) that use photosynthesis. In fact, that would be the VAST majority of life, first to emerge as well (edit for clarity: well, not first, but early).

    That strikes me as downright common. It does lead one to wonder, given the vast array of proteinaceous life on Earth, just how many other such 'anomalies' are simply part of the norm.
  12. Feb 10, 2010 #11


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    C'mon man ... I clearly meant that it is anomalous in the context of everything we were given to expect about coherent states from previous experiments .. that is why I said "persists for an anomalously long time". As khany said, that is why this result is so spectacular.

    Furthermore, this has been demonstrated to be true in *exactly one case* at RT ... the only other time it was experimentally observed for a cryogenic sample. So you can hardly say that it is ubiquitous at this point. Even the authors of those papers only state that this *may* indicate that long-lived coherent states at biological temperatures are more common than we have been led to expect.

    Careful, or we make you change your handle to "Word Twister", or "Conclusion Jumper" :tongue:
  13. Feb 10, 2010 #12
    Sir, I say it is ubiquitious, ubiquitous as the ether which pervadeth the heavens betwixt the fixed stars!!


    Sorry, I don't know what happened to me for a moment. What I meant to say was, "whoops". I did misunderstand you, and your riposte is well taken. I was raised by wolves and jesuits hence the rhetoric. Forgive me?! :smile:
  14. Feb 10, 2010 #13


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    You are making me laff. :smile:
  15. Feb 10, 2010 #14
    The tenacity and loyalty of wolves and the meticulous education of the jesuits... hmmm... maybe I could write a terrible book... :rofl:

    Glad you're laughing though, we all need it sometimes. This field is wonderful and exciting, but it can make one's head feel a bit cramped.
  16. Feb 10, 2010 #15
    hmm. i think i came off sounding too confrontational. my apologies.

    long lived coherence in proteins *at room temperature* is anomalous because nobody had predicted nor observed it before. it could, and i suspect it will, turn out to ubiquitous in photosynthetic pigment protein complexes. here i agree with SpectraCat, however, we can hardly make the claim that the phenomenon is ubiquitous based on a single observation of this phenomenon by a single research group in a very special variety of marine algae.

    i would like to point out that the pigment molecules are 'generally' not weakly coupled to the protein bath (not withstanding the lack of covalent bonding of the pigments to the protein matrix). in fact, the strength of the coupling to the bath (that causes decoherence) is of the same order as the coupling between different pigment molecules (coupling that results in delocalized excitonic states which allow for coherent evolution).


    i am also a skeptic on this issue like Frame Dragger. the two dimensional photon echo (2DPE) techniques used to reveal long lived coherences are very new. as the use of these spectroscopies becomes more widespread i wouldn't be surprised if we begin to find such long lived coherences in other non-exotic structures. the scholes group has already shown the existence of such long lived coherences for polymer chains.
    Last edited: Feb 10, 2010
  17. Feb 10, 2010 #16


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    No need to apologize ... I only responded to clarify my earlier statements. Regarding the coupling .. well, I will have to go back and read up on it ... looks like I am also guilty of over-generalization here as well.

    I also will not be surprised if it turns out that evolution has figured out a way to tune the inter- and intramolecular interactions to stabilize the coherent state so everything is "just right" to support a long-lived coherent state (and also to destabilize it when the organism needs to turn off the channel). Furthermore, I would again not be surprised if this turned out to be a fairly general phenomenon.
  18. Feb 10, 2010 #17
    No need to apologize, as I appreciate a vigorous debate. Thank you for the olive branch nonetheless. I understand that you and SpectraCat clearly meant the same thing, and I misunderstood. No big deal.

    Now, on your point about 2DPE, I would LOVE to see if the same might hold in the mechanisms of cyanobacteria, microtubules within neurons, etc. That said, from one result it's just interest and curiosity... maybe photosynthesis is really a very unique process resulting from rich CO2 levels in earth's history.

    Then again... maybe the pigmant cells bound to actin and other proteins in such animals as cuttlefish and octopi might have similar properties. Here's a question too... is this quantum behaviour critical or incidental to the process? It seems fun to think that this is increasing the yield for a given photosynthetic cell, but maybe it's just a side-effect. Regardless, we have so many new and exciting tools to observe nature through, even if the LHC is the one that gets most of the press.

    Peace to you as well.

    EDIT: SpectraCat, here's a question physicist rarely have to ask themselves; if quantum behaviour such as 'long' lived coherence in a biological system IS beneficial, then it probably played a role in the evolution of photosynthetic life. Quantum effects CLEARLY altering or defining the course of how plants METABOLIZE?! That's an amazingly long reach for a science such as QM which is often seen as theory or engineering. What other quantum interactions have had such a direct effect on our evoltion in the realm of advantage and not just adaptation to cope. The basis of our food chain seems to harness energy through means we can't reproduce if paid to do so. A bit galling to be outdone by a fern or a pine tree. :rofl:
    Last edited: Feb 10, 2010
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