Double slit with time measurement

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The discussion revolves around a proposed measurement involving a double-slit experiment with time measurements to determine photon paths. It highlights the impact of coherence time on interference patterns, emphasizing that if the time difference between paths is significant, interference will not occur. The conversation references Feynman's path integral formulation, noting that paths leading to detection events at different times cannot interfere. Participants argue about the implications of coherence length and the conditions necessary for interference, particularly in setups involving single photons and parametric down-conversion. Ultimately, the consensus is that a clear understanding of coherence and timing is crucial for analyzing interference in quantum mechanics.
  • #61
DParlevliet said:
The experiment above is about the same as a double slit.

No, it is clearly not. A double slit measures spatial coherence. A Mach-Zehnder interferometer measures temporal coherence. These are two different quantities which are not "about the same".

DParlevliet said:
A spatial detector which can show waves.

And what is that supposed to be? I do not think this thread is going anywhere and you do not seem to be willing to understand the things in detail or read up on them, so I am afraid this is getting pointless.
 
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  • #62
Cthugha said:
No, it is clearly not. A double slit measures spatial coherence. A Mach-Zehnder interferometer measures temporal coherence. These are two different quantities which are not "about the same".
If a single photon from the sun goes throught a double slit, then the coherence time of each wave, going throught each slit, is in the femtosecond range?
 
  • #63
It does not matter what you do to it. Unless you filter the light spectrally, you always get a coherence time of maybe 100 femtoseconds or something like that for light from the sun.

Whether you will see an interference pattern or not still depends on spatial coherence, though. So for example it matters strongly, whether you place a pinhole to filter the light.
 
  • #64
Cthugha said:
It does not matter what you do to it. Unless you filter the light spectrally, you always get a coherence time of maybe 100 femtoseconds or something like that for light from the sun.

Whether you will see an interference pattern or not still depends on spatial coherence, though. So for example it matters strongly, whether you place a pinhole to filter the light.

I would give up, the fact you haven't though strongly indicates your patience, so kudos. :smile:
 
  • #65
DParlevliet said:
If a single photon from the sun goes throught a double slit, then the coherence time of each wave, going throught each slit, is in the femtosecond range?

It's in the Planck range actually but meh...
 
  • #66
Cthugha said:
It does not matter what you do to it. Unless you filter the light spectrally, you always get a coherence time of maybe 100 femtoseconds or something like that for light from the sun.
Of course a spectral filter is needed because one needs monochromatic light. It must be as much as possible be comparable with single photons, so not broad bandwith. Does that matter much?
 
  • #67
DParlevliet said:
Of course a spectral filter is needed because one needs monochromatic light. It must be as much as possible be comparable with single photons, so not broad bandwith. Does that matter much?

Eh? Single photons are Fock states with a fixed photon number of 1. That is all there is. They do not have to be monochromatic. They often are far from that in reality. Sorry, but Bone234 seems to be right. This discussion seems entirely pointless.
 
  • #68
closed for moderation
 

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