Why No Decoherence in Double Slit Experiment?

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In summary, the conversation discusses the concept of decoherence driven quantum collapse in relation to the double slit experiment. It is explained that decoherence destroying the interference pattern occurs when the state of the interacting object is affected to the point that you can determine which path the photon has taken. It is also mentioned that the type of interaction determines what is "measured" in the environment. The conversation ends with a discussion about information being the fundamental substance of existence rather than matter.
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I'm hoping that someone has the patience to help a poor ignorant layman understand... and be warned that I am mathematically impotent. My struggle is in wrapping my mind around the concept of decoherence driven quantum collapse. As I understand it, the interaction between particles causes a quasi-measurement that "collapses" the quantum state of those particles (and all other entangled entities). If that is accurate, why does the photon strike in the double slit experiment not produce decoherence in the quantum state? Why the interference pattern?
 
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Suppose you have some experimental setup in which the photon interacts strongly with an object in the interference experiment, e.g. the photon bounces off a mirror if it mves through one of the two slits.


Decoherence destroying the interference pattern will then occur if the state of the mirror would be affected by the photon to such an extent that you could, in principle, examine the mirror and tell that the photon had bounced off the mirror and hence tell which path the photon has taken.


If the photon bounces off the mirror elastically then the momentum of the photon changes so, the mirror must absorb some momentum. So, it seems that the "which path information" does exist. But this is not true. The mirror (and rest of the world if themirror is fixed to the ground) doesn't have a precisely defined momentum. The mirror has a very precisely defined position and that causes the momentum of the mirror to be undetermined to some extent due to the Uncertainty relation.


Clearly, if position of the mirror were not determined to witin a wavelength of the light, you wouldn't be able to see an interference pattern. If the position of the nirror is determined to within a fraction of the wavelength of the light, then you can show that the uncertainty of the mometum is necessarily larger than the momentum of the photon.


So, the collision of the photon with the mirror causes the wavefunction of the mirror as a function of momentum to shift a bit, but this shift is much smaller than the width of the momentum distribution. This means that the mometum of the mirror gives you almost no information about the path the photon took and the inteference pattern will be almost unaffected.


Now, the reason why macroscopic objects have a well defined position is explained by decoherence. So, in a way, decoherence in the macroworld explains why the photon does not decohere and why we can observe interference phenomena.
 
  • #3
Count Iblis said:
Now, the reason why macroscopic objects have a well defined position is explained by decoherence.
If you or anyone else have more information about this, I'd be interested. Why does the environment "measure" the position of everything, rather than some other observable, like momentum?
 
  • #4
Fredrik said:
If you or anyone else have more information about this, I'd be interested. Why does the environment "measure" the position of everything, rather than some other observable, like momentum?

I think the observable that is "seemingly" measured depends on the type of interaction.

Environment -- Coherent state encompasses a large number of phenomena.

If you have a up spin-impurity ( a large quasi-classical particle) that interacts with a down-spin electron, environment measures the momentum of the spin because by checking the spin of your impurity after the interaction, you could conclude that the initial state (momentum) of the electron.

I can think of other examples, like the interaction of a large contact with a one-level molecule, etc...
 
  • #7
Fredrik said:
If you or anyone else have more information about this, I'd be interested. Why does the environment "measure" the position of everything, rather than some other observable, like momentum?

I think that the simple answer to that is that most interactions are mainly dependent on distance (think, say, Coulomb interaction) ; that is, the interaction hamiltonian can be diagonalized in the position basis. That is not universally so, and in fact, most of the time it is not the position basis in which one decoheres, but in a kind of coherent state basis of "position and momentum" coherent states.
 
  • #8
Count Iblis said:
Suppose you have some experimental setup in which the photon interacts strongly with an object in the interference experiment, e.g. the photon bounces off a mirror if it mves through one of the two slits.


Decoherence destroying the interference pattern will then occur if the state of the mirror would be affected by the photon to such an extent that you could, in principle, examine the mirror and tell that the photon had bounced off the mirror and hence tell which path the photon has taken.



Thank you for you response Count (and others). Your feed back helps some, but I'm still somewhat foggy (afraid that's my typical state of mind). If in our double slit experiment, rather than a mirror we substitute photographic emulsion, the photon strike will have a "strong interaction", and record its point of impact by eliciting molecular changes in the emulsion. Yet, from what I have been told, the uncertainty of which slit it passed through might remain, thus maintaining its quantum superposition. As subsequent strikes accumulate over time. an interference pattern will be demonstrated on the emulsion. This would appear to be a macroscopic manifestation of an unresolved quantum system.
Given your example, and the follow-up explanations regarding the types of interactions/measurements that result in decoherence, I am left with the impression that the critical issue is in the information content of the process... not in any material change that occurs in the involved particles. Am I reading this right? If so, should I conclude that information is the fundamental substance of existence rather than matter and energy?
 

1. What is the double slit experiment?

The double slit experiment is a famous experiment in quantum physics that demonstrates the wave-particle duality of light and matter. It involves shining a beam of particles (such as electrons or photons) through two parallel slits and observing the resulting pattern on a screen. This experiment has been used to show that particles can behave like waves and exhibit interference patterns, suggesting that they have both particle-like and wave-like properties.

2. Why is decoherence important in the double slit experiment?

Decoherence refers to the loss of quantum coherence, or the ability of a particle to exist in multiple states simultaneously, due to interactions with its environment. In the double slit experiment, decoherence is important because it determines whether the interference pattern is observed or not. If decoherence occurs, the particle will behave like a classical object and the interference pattern will disappear.

3. Why is there no decoherence in the double slit experiment?

There is no decoherence in the double slit experiment because the particles used in the experiment (such as electrons or photons) are so small that they do not interact significantly with their environment. This means that they can maintain their quantum coherence and exhibit interference patterns on the screen.

4. Can decoherence be prevented in the double slit experiment?

Decoherence can be prevented in the double slit experiment by using particles that are less likely to interact with their environment, such as photons or electrons. Additionally, keeping the experiment in a controlled and isolated environment can help prevent interactions that could lead to decoherence.

5. What are the implications of no decoherence in the double slit experiment?

The lack of decoherence in the double slit experiment has important implications for our understanding of quantum mechanics and the nature of reality. It suggests that particles can exist in multiple states simultaneously and that our observation of them can affect their behavior. This has sparked ongoing debates and research in the field of quantum physics.

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