Photons and free quantum particles

[hokhani]

TL;DR

Are photons localised?

Consider a light source, like lamp, that radiates photons in vacuum. Can we detect a particular photon, moving in a specific direction, always at one point or the photon is localised and moves from one point to another? In other words, I would like to know that like free quantum particles, photons also have the probability of presence everywhere?

 

[PeterDonis]

Can we detect a particular photon, moving in a specific direction, always at one point or the photon is localised and moves from one point to another?

These are two different questions.

"Moving in a specific direction" is a question about momentum. Photons do have a well-defined momentum operator, so yes.

"Localised" is a question about position. Photons do not have a well-defined position operator, so no.

 

[pines-demon]

Consider a light source, like lamb,

I know this is a typo but lambs do emit photons (even in vacuum).

 

[hokhani]

These are two different questions.

"Moving in a specific direction" is a question about momentum. Photons do have a well-defined momentum operator, so yes.

"Localised" is a question about position. Photons do not have a well-defined position operator, so no.

Thanks for your answer. If photon is like wave, I expect after turning off the lamp to be able to detect again in it's first-detected place. Isn't it?

 

[bhobba]

Thanks for your answer. If photon is like wave, I expect after turning off the lamp to be able to detect again in it's first-detected place. Isn't it?

I am unsure of your exact question.

Can you elaborate?

Thanks
Bill

 

[PeterDonis]

If photon is like wave

It isn't. It's a quantum field.

I expect after turning off the lamp to be able to detect again in it's first-detected place. Isn't it?

Light from an ordinary lamp isn't going to exhibit any useful quantum effects; it's well described by classical electrodynamics, in which light is a wave, but there are no photons.

That said, like @bhobba, I'm also confused as to what you're asking. If you turn off the lamp, you don't detect light coming from it any more.