Is the entanglement of photons necessary for interference?

[alexepascual]

I can only repeat what I've said before and I will try to with better examples but think about this some more before we go on.

OK. I'll give it some thought. Thanks again.

-Alex

 

[alexepascual]

I can only repeat what I've said before and I will try to with better examples but think about this some more before we go on.

I have been studying a little about the details of entangled photons but I haven't made much progress. Actually I haven't spent enough time on it. But I'll continue to look into it and eventually I'll understand it.
On the other hand there is something I thought about yesterday. If an atom emits a photon, then the photon will be entangled with some degrees of freedom of the atom. So, applying your argument to this situation, being that it is entangled to something else, a description of this photon in some basis (position for example) should be given by a reduced density matrix and it should not produce interference if it goes through a double slit.
But it happens that light is frequently produced by emision from atoms and it does produce interference after going through double slits. I know this example is a little complicated and I would not want at this point to get into the details, as this would not help me in understanding the more basic facts. But I was wondering if in this example you would consider the photon as producing interference or not (of course after a few from the same transition have gone through the slits).

 

[jambaugh]

I have been studying a little about the details of entangled photons but I haven't made much progress. Actually I haven't spent enough time on it. But I'll continue to look into it and eventually I'll understand it.
On the other hand there is something I thought about yesterday. If an atom emits a photon, then the photon will be entangled with some degrees of freedom of the atom. So, applying your argument to this situation, being that it is entangled to something else, a description of this photon in some basis (position for example) should be given by a reduced density matrix and it should not produce interference if it goes through a double slit.
But it happens that light is frequently produced by emision from atoms and it does produce interference after going through double slits. I know this example is a little complicated and I would not want at this point to get into the details, as this would not help me in understanding the more basic facts. But I was wondering if in this example you would consider the photon as producing interference or not (of course after a few from the same transition have gone through the slits).

Whether the photon entangled with the atom will or will not form an interference pattern depends on which variables in particular are being entangled. Also remember that when we do use atomic sources of photons there is a selection process going on. For example the photons coming from a laser are produced by stimulated emission and only those with very confined momenta are produced in great numbers. There is a selection process here which is distinct from say trying to see an interference pattern from photons emitted by arbitrary excited atoms (even if those atoms are all initially produced in the same excited sharp quantum mode).

Ultimately you should ask in specific cases "what do we know about the photons?" That knowledge if it exists comes from a physical selection process. Photons coming out of a laser are photons we know quite a bit about because one specific mode of emission has been amplified dramatically through the stimulated emission process.

 

[alexepascual]

Whether the photon entangled with the atom will or will not form an interference pattern depends on which variables in particular are being entangled. Also remember that when we do use atomic sources of photons there is a selection process going on. For example the photons coming from a laser are produced by stimulated emission and only those with very confined momenta are produced in great numbers. There is a selection process here which is distinct from say trying to see an interference pattern from photons emitted by arbitrary excited atoms (even if those atoms are all initially produced in the same excited sharp quantum mode).
Ultimately you should ask in specific cases "what do we know about the photons?" That knowledge if it exists comes from a physical selection process. Photons coming out of a laser are photons we know quite a bit about because one specific mode of emission has been amplified dramatically through the stimulated emission process.

I was not thinking about a laser. I was rather thinking about an atom whose conduction electron has been excited and then it spontaneously transitions to a lower level. You may consider any source of energy that is sufficient to elevate that electron to a level that will allow the desired transition. I understand sodium for example has two famous lines that are very close together (589.0nm and 589.6nm) . Of course sodium has other lines. But they are weaker and you could post-select those two using a monochromator or a filter. With respect to what we "know" about the photons, I guess we only know that they have a narrow wavelength spectrum centered around some known value (589.3nm in the case of sodium) that corresponds to that line and that's all. I had also in mind considering the emission from a single atom and one photon at a time.

 

[jambaugh]

I was not thinking about a laser. I was rather thinking about an atom whose conduction electron has been excited and then it spontaneously transitions to a lower level. You may consider any source of energy that is sufficient to elevate that electron to a level that will allow the desired transition. I understand sodium for example has two famous lines that are very close together (589.0nm and 589.6nm) . Of course sodium has other lines. But they are weaker and you could post-select those two using a monochromator or a filter. With respect to what we "know" about the photons, I guess we only know that they have a narrow wavelength spectrum centered around some known value (589.3nm in the case of sodium) that corresponds to that line and that's all. I had also in mind considering the emission from a single atom and one photon at a time.

Yes but don't confuse the spectral lines with interference patterns. Take the light from one spectral line of a sodium lamp and put it through a double slit. You won't get an interference pattern because the light is not coherent.

More precisely to the question at hand, there is no way knowing when or in what direction the emitted photon has gone nor when or in what direction the emitting atom has recoiled. They are entangled so these positions are correlated and the magnitude of the momentum is known so you know the relationship between when the emission occur and the distance the two ejecta are from the original position but until you observe one you have no information about the other.

You might get an interference pattern by putting a double slit far enough away but you are selecting out only those cases where the photon got to the double slit instead of Timbuktu. You are thereby making a measurement = "preparing" the photon which breaks the entanglement.

I say "might" as I am not sure if even then you'll get one. It depends on how one is setting up the repeated trials to get enough data to see the pattern.

 

[alexepascual]

Yes but don't confuse the spectral lines with interference patterns. Take the light from one spectral line of a sodium lamp and put it through a double slit. You won't get an interference pattern because the light is not coherent..

My understanding was that you don't need coherent light to get interference. Even though the separate photons are incoherent because they come from different atoms, each phton will interfere with itself. So you can have light from any source that produces a fairly narrow line and it will produce interference even if it is totally incoherent. I understand that you first have to make the light pass through a narrow slit so that all the waves arrive at the slits at the same angle. Do you agree with this?

More precisely to the question at hand, there is no way knowing when or in what direction the emitted photon has gone nor when or in what direction the emitting atom has recoiled. They are entangled so these positions are correlated and the magnitude of the momentum is known so you know the relationship between when the emission occur and the distance the two ejecta are from the original position but until you observe one you have no information about the other.

I think I agree on this.

You might get an interference pattern by putting a double slit far enough away but you are selecting out only those cases where the photon got to the double slit instead of Timbuktu. You are thereby making a measurement = "preparing" the photon which breaks the entanglement.

Does a measurement break the entanglement? In a EPR experiment, measuring the photon on one side does not seem to break the entanglement because you can always expect the other photon to have the correlated polarization (when you measure on the same axis).

I say "might" as I am not sure if even then you'll get one. It depends on how one is setting up the repeated trials to get enough data to see the pattern.

I agree with this.