# Coherency and entanglement

#### San K

Trying to understand coherence and entanglement and have some questions:

1. Do incoherent photons interfere?
a) Single particle interference through double slit
b) Multiple photons through double slit
Is it that the interference pattern in the above cases is not a clear fringe pattern however there is still interference?
2. Why does coherence increase when light is passed through a pin hole?
3. Why does brighter light have higher coherence?
4. What does coherence within a single photon mean?
5. Can coherence be changed (is it reversible) within the same photon?
i.e. Is entanglement changeable between two photons?
i.e. can the experimenter play with coherence and entanglement with the same set of entangled photons?
i.e. decrease coherence and watch the effect of increased entanglement, Increase coherence and watch the effect of decreased entanglement?
6. In a single particle, double -slit experiment are the wave-functions passing through each slit assumed to be entangled with each other? in the case of an "in-coherent" photon?

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#### mfb

Mentor
1. Do incoherent photons interfere?
What do you mean with "incoherent photons"?
There are no photons you can point to and say "this cannot have interference".
You can get interference effects with all light, but if the different photons in that light are not coherent (to each other!), you might be unable to notice this.

2. Why does coherence increase when light is passed through a pin hole?
Depends on the setup. You select some specific part of your light, which might have some special properties (with the obvious one: "went through that hole").
3. Why does brighter light have higher coherence?
This is not a general rule. It might depend on the setup.
5. Can coherence be changed (is it reversible) within the same photon?
What do you mean with that?
i.e. Is entanglement changeable between two photons?
You can transfer entanglement between different photon pairs, and modify the entangled properties. If that does not answer your question, I don't understand it.
i.e. can the experimenter play with coherence and entanglement with the same set of entangled photons?
i.e. decrease coherence and watch the effect of increased entanglement, Increase coherence and watch the effect of decreased entanglement?
What do you mean with that?
6. In a single particle, double -slit experiment are the wave-functions passing through each slit assumed to be entangled with each other?
It is a single wave function.

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