Mach–Zehnder interferometer question, path detection

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Discussion Overview

The discussion revolves around the Mach–Zehnder interferometer and the possibility of determining which path a photon takes by measuring the momentum change of the mirrors. Participants explore the implications of path detection on interference patterns and the role of quantum mechanics in these measurements.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • Some participants propose that measuring the change in momentum of the mirrors could reveal which path the beam took, suggesting that if path A was taken, the mirror on path A would be "pushed" and vice versa.
  • Others express uncertainty about whether the momentum of the mirrors can indeed provide which-path information, noting that interference patterns typically indicate that such information is not available.
  • A participant questions the apparent contradiction between the momentum of the mirrors encoding which-path information and the observed interference, suggesting that the momentum of the mirrors may be "hidden" in some way.
  • It is noted that for a detectable change in momentum, the initial momentum uncertainty of the mirrors must be small compared to that of the photon, which may not be achievable in practice.
  • Some participants discuss the implications of preparing the mirrors in a very precise momentum state, suggesting that this could lead to a lack of interference due to the Uncertainty principle affecting the position state of the mirrors.
  • One participant argues that if the quantum state of the mirror changes significantly upon photon interaction, it could lead to decoherence, thereby destroying interference.
  • A later reply inquires about the application of "weak measurements" in the context of the Mach–Zehnder interferometer, indicating interest in alternative measurement approaches.

Areas of Agreement / Disagreement

Participants express multiple competing views regarding the feasibility of using mirror momentum to determine which path was taken, and the implications this has for interference. The discussion remains unresolved, with no consensus on the validity of these approaches.

Contextual Notes

Limitations include the dependence on the initial momentum uncertainty of the mirrors and the unresolved nature of how momentum transfer relates to interference in quantum mechanics.

msumm21
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In a Mach–Zehnder interferometer experiment, can you measure which path the beam took by measuring the change in momentum of the (fully silvered) mirrors (i.e. if path A was taken then the mirror on path A was "pushed" a bit, if path B was taken then the mirror on path B was "pushed" a bit)?
 
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msumm21 said:
In a Mach–Zehnder interferometer experiment, can you measure which path the beam took by measuring the change in momentum of the (fully silvered) mirrors (i.e. if path A was taken then the mirror on path A was "pushed" a bit, if path B was taken then the mirror on path B was "pushed" a bit)?

i don't know if the path can be found out via push on the mirrors...

however

there are a couple of other ways to find out which path (in mach-zehnder, double-slit, delayed erasure etc) the photon/beam took.

the interference does not happen when the path is "found out"/known
 
The reason I ask is because every time the experiment is done it would seem like the which-path information would be encoded in the momentum of the mirrors, and theoretically you could always go back and look at the momentum of the mirrors to see which path was taken, but then doesn't QM say there is no interference if that is the case?

Somehow the beam always goes into the same detector (there is interference) so evidentially the momentum of the mirrors is hidden somehow? I think I am missing something basic here.
 
In order to really measure the change in momentum induced by the photon, the initial momentum uncertainty of the mirror should be small compared to the momentum of the photon. Otherwise the momentum transfer is not detectable in principle. In any typical realization of a Mach-Zehnder interferometer the mirror momentum uncertainty is too large to allow which-way measurements utiliziung the momentum transfer by the photon.
 
Theoretically (maybe this is not possible in practice) if you very precisely knew the momentum of the mirrors before the experiment, would the interference then not occur? So if you know the momentum of the mirrors well then the beam could pass to either detector?

Thanks for the info Cthugha.
 
Right, if you prepared the mirrors in a VERY precise momentum state (and therefore a very unprecise position state via the Uncertainty principle), you would not see the interference, because the position of the mirrors would be out of wack!
 
msumm21 said:
In a Mach–Zehnder interferometer experiment, can you measure which path the beam took by measuring the change in momentum of the (fully silvered) mirrors (i.e. if path A was taken then the mirror on path A was "pushed" a bit, if path B was taken then the mirror on path B was "pushed" a bit)?
If you are able to detect a change in the mirror, then the quantum state of the mirror after the income of the photon must be significantly different from that before the income of the photon. In that case, these two states of the mirror are nearly orthogonal, i.e., their scalar product is close to zero. But that means that the mirror causes decoherence of the photon state, which means that interference is destroyed.

Conversely, if you can see interference, then it means that there is no decoherence, and consequently that you are not able to detect a change in the mirror.

This is discussed more quantitatively in the book
B. Schumacher, B. D. Westmoreland: Quantum Processes, Systems, and Information (2010)
 
Demystifier, have the "weak measurements" you always talk about been done for the Mach–Zehnder interferometer experiment?
 

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