Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

B Decoherence by Emission?

  1. Mar 2, 2016 #1
    In the never ending quest to understand a complicated idea for which I completely lack the academic requirements to do so, I'd like to ask a few basic questions that I hope will allow simple yes/no responses (to ease the frustration of the physicist/mathematician contributors). Sadly, the answer of "Your question makes no sense" is also a very real possibility, so we might abbreviate that response to "Ugh" for simplicity sake.

    First question: In terms of decoherence, does the emission of a photon from a particle constitute a physical interaction with the environment/system in a similar manner to the absorption of a photon?
     
    Last edited: Mar 2, 2016
  2. jcsd
  3. Mar 2, 2016 #2

    naima

    User Avatar
    Gold Member

    When excited atoms passing thru the Young slits, emit a photon it changes the density matrix . It gives some which-path information the visibility of the franges decreases.
     
  4. Mar 2, 2016 #3
    Does this provide information with regard to the location of the photon emitting particle?
     
  5. Mar 2, 2016 #4

    DrClaude

    User Avatar

    Staff: Mentor

    Last edited by a moderator: May 7, 2017
  6. Mar 2, 2016 #5

    naima

    User Avatar
    Gold Member

    Information is about the path. And so about the position.
     
  7. Mar 2, 2016 #6
    Thank you both. Let me work through the sources you've provided. I might offer a follow up question later, if that's OK.
     
    Last edited by a moderator: May 7, 2017
  8. Mar 3, 2016 #7

    Demystifier

    User Avatar
    Science Advisor

    To have a decoherence-like interaction, you need interaction with a large number of degrees of freedom. If the particle (assumed to be an elementary particle or a particle with a very simple structure) and the emitted photon do not interact with the environment, nothing like decoherence will happen. The process of emission will be continuous and unitary, nothing like a quantum jump will occur. The quantum jump requires environment with many degrees of freedom. The environment may be external (e.g. surrounding gas or measuring apparatus) or internal (a complex emitting particle).

    The same is true when emission is replaced by absorption.
     
  9. Mar 3, 2016 #8

    naima

    User Avatar
    Gold Member

    This point is very interesting.
    It is often said that the visibility interference pattern decreases because of a possible detector of photons. Do you say that the origin is the actual presence of another existing environment (the gas or something else)?
     
  10. Mar 3, 2016 #9

    Demystifier

    User Avatar
    Science Advisor

    Yes. To decrease visibility of interference pattern you need some environment, which can be a detector or something else.
     
  11. Mar 3, 2016 #10

    A. Neumaier

    User Avatar
    Science Advisor
    2016 Award

    Knowing which path gives information about the momentum in the direction of the path and about the position in two orthogonal directions orthogonal to it. Thus it gives only partial position information.
     
  12. Mar 3, 2016 #11
    Could you please expand on this?
     
  13. Mar 3, 2016 #12

    Demystifier

    User Avatar
    Science Advisor

    For internal decoherence (in a somewhat different context) see
    http://lanl.arxiv.org/abs/1406.3221 [Eur. J. Phys. 36 (2015) 045003]
    and references therein.
     
  14. Mar 3, 2016 #13
    Thank you. Most of the math is over my head, but the discussion and conclusion summary is excellent.
     
  15. Mar 3, 2016 #14

    naima

    User Avatar
    Gold Member

    In the case of a Stern Gerlach device, I see several steps in the decoherence;
    1) the degrees of freedom of the electron (spin, position) and those of the magnetic field interact and decoherence occurs. What does this "occurs" mean?
    We can have at any time t many copies of the electron prepared in the same state (mixed here). We can measure say their Wigner function. It needs that they interact with macroscopic apparatus to get an image. Varying t, we see how the Wigner function and decoherence evolve.
    This leads to the second point.
    But before measuring the state with a macroscopic device we cannot say that decoherence completely occured. It has nothing to do to the remaining presence of little off diagonal terms. It is still possible to erase what was done. the two output beams of the SG can be the inputs of a beam merger whose output is the initial electron's state.
    Things are reversible.
    2) registration of the decoherence.
    It is what happened when we measured the Wigner function. It is different from a screen in front of the SG. We get here a macroscopic spot which gives the value of spin measurement. In one case we measure the density matrix and in the other the spin. In both cases there is irreversibility.
    This scheme can easily understood in the case of an emitted witness photon. Is it the same when the atom absorbs the photon? What is then the environment?
     
  16. Mar 3, 2016 #15

    Demystifier

    User Avatar
    Science Advisor

    How do you know that absorption of the photon by the atom has occured? Before detecting the absorption by a macroscopic device, you cannot know that. So this is where environment is in the case of absorption.
     
  17. Mar 3, 2016 #16

    naima

    User Avatar
    Gold Member

    As i read again these lines, i am no more sure that they are different. When one gets a tomogram of the density matrix he has to measure quadratures and for each of them he gets random outputs just like with a screen.
    Bhobba wrote that measurement (with its random output) occurs when decoherence is done. I see it from the other side: Decoherence is done when measurements occured.
     
  18. Mar 3, 2016 #17

    vanhees71

    User Avatar
    Science Advisor
    2016 Award

    Well, decoherence is very efficient (to the dismay of quantum-computer afficionados). It was demonstrated by Zeilinger at all doing double-slit/grating experiments with Bucky balls that the emission of a few black-body photons is sufficient to make the interference go away

    http://arxiv.org/abs/quant-ph/0402146
     
  19. Mar 3, 2016 #18

    A. Neumaier

    User Avatar
    Science Advisor
    2016 Award

    This is nearly impossible, unless the experiment is done in outer space and the photon escapes into the vacuum, to be absorbed only by a very distant star (which can be ignored due to the finite speed of light).
     
  20. Mar 3, 2016 #19

    naima

    User Avatar
    Gold Member

    You see that a few photons caused the (first step of) decoherence when you look at the interferences.
    You need then a macroscopic device.
    Demystifier answered to my question about absorbed photons, that to know if the photons were absorbed you had to measure the atom with a macroscopic device. without that decoherence is reversible.
     
    Last edited: Mar 3, 2016
  21. Mar 4, 2016 #20

    vanhees71

    User Avatar
    Science Advisor
    2016 Award

    Where do you need a macroscopic device? Of course, to measure the buckyballs you need a detector. The decoherence through the random emission of thermal photons is not a macroscopic process. The interesting thing about the buckyballs is that they are "mesoscopic", i.e., with 60 or 70 atoms in a molecule they have on the one hand a lot of degrees of freedom and a quasi-continuous energy spectrum, being in this sense on the edge of becoming "macroscopic" objects. On the other hand they are not that large that you can't control them. So you can cool them down so much that they show quantum-mechanical coherence effects in their passage through gratings. Heating them up in a controlled way these interference effects cease more and more the more thermal photons are emitted. That's the demonstration of decoherence and gradual loss of quantum behavior in a pretty well managable system!
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted



Similar Discussions: Decoherence by Emission?
  1. Decoherence question (Replies: 82)

  2. Decoherence Clarification (Replies: 280)

Loading...