Origin of Photons Irrelevant in Double Slit Experiment?

[Orionabc]

If I "create" a stream of photons in the lab or if I spectographically select a stream of photons from a star, I get the same experimental result in a double slit experiment? In other words there is no difference between photons whose point of origin can be deduced to be eons ago vs those originating in the lab for this type of experiment; is there any quantum experiment where there would be a difference? My knowledge of quantum mechanics is not great, but, I don't think history prior to the measurement plays a role. What happens to the photon after the measurement also is not of consequence. The only thing that matters is the measurement itself. Is that right?

 

[Joseph14]

The temporal and spatial coherence of the stream of photons matters for interference experiments. This will vary for different sources. If the spatial coherence is less then your slit separation or the temporal coherence is less than 1/f the interference will not be produced. So I would say considering the source of photons is important.

 

[Orionabc]

Thanks for answer. I agree source matters and coherence is why, but...does the distance to the source matter?

 

[Joseph14]

I believe that spatial coherence increases with distance, so in that way the source being farther away would be better.

 

[arkajad]

Thanks for answer. I agree source matters and coherence is why, but...does the distance to the source matter?

Given the source geometry and detectors geometry, the distance should not be too small, and it should not be too large. Within reasonable limits it does not matter - like with everything.

 

[Orionabc]

Interesting, thanks Joseph14 and Arkajad.

So for large distances...to maintain spatial coherence requires having the same wavelength. In a classical universe, where we have continuously variable wavelengths, it gets harder the further out you go. You would have to wonder, given a continuously variable wavelength, how could you ever produce two photons with the same wavelength. Then you would have to wonder how far out could you see stars given the now finite probability that coherence would decay as a function of distance due to very small perturbations in wavelength. In a quantum universe, discrete wavelength, it is easier. Infact, without having something interact with the stream of photons it becomes a sort of a modern equivalent of Newtons first law...coherent photons with wavelength X moving in a straight line will remain coherent unless acted upon by another photon.

For short distances, in a classical universe, all photons would be spatially coherent, i.e. it takes a delta time and distance to deconvolute and that delta is a function of the wavelength of the photons. For a quantum universe I am not sure what coherence means as the delta for time and space gets smaller. Assume for a moment my source is a single atom being pumped into an excited state and then returning to a lower state. I am limited in how closely together in time and space I can cause two photons to come to be...whatever that means.

Am I all wet here or am I understanding your answers correctly?

 

[arkajad]

Once we are discussing such scales, perhaps you should also take into account that not all physicists are 100% sure that photons are strictly massless. There are experimental upper bounds for their mass, but nevertheless if non-zero, it will set its own length-scale.

 

[Cthugha]

So for large distances...to maintain spatial coherence requires having the same wavelength. [...]
For short distances, in a classical universe, all photons would be spatially coherent,[...]

Am I all wet here or am I understanding your answers correctly?

It is exactly the other way around. The larger the distance between source and your double slit is, the higher the spatial coherence will be. An easy picture for this behavior is given if you consider the differences in travel distances and times from the source to your slit.
Imagine a rectangular flat light source and a double slit.
If you place your source directly in front of your slits, the shortest possible distance from a point at the source to a point at the slit is a straight line, where the photon is emitted at an angle normal to the plane of the double slit. The longest possible distance belongs to a photon emitted from one corner of the rectangular light source which goes to the far corner of the slit at the other side. This photon will not be emitted at an angle normal to the plane of the double slit, but at a large angle phi. So the longest travel distance is R and the shortest is R cos (phi). The different travel lengths correspond to different emission times. Of course the longer this distance/duration is, the lower the coherence will be.

If you now place the light source far away from the slit, the angle under which the photons emitted from one corner of the rectangle to the far side of the slit at the other side are emitted, is much closer to the normal angle and the travel path differences will be much smaller. Accordingly your coherence is increased.

This is why starlight has very high spatial coherence. In fact, one of the first experiments which can be considered a quantum optics experiment, the HBT experiment, was performed using light emitted from Sirius B.

 

[Naty1]

In other words there is no difference between photons whose point of origin can be deduced to be eons ago vs those originating in the lab for this type of experiment; is there any quantum experiment where there would be a difference?

correct...no experimental difference

One photon is like another except for energy...

I suppose photons from eons ago might be polarized...but they don't "age" or decay

 

[Orionabc]

Putting it in the vernacular...thats totally cool. It means that the age of a photon can only be determined by plotting its point of origin in the universe. To do that I need to know its current momentum vector don't I? Or at least what it was at the time I detected it?

 

[Naty1]

...momentum vector don't I? Or at least what it was at the time I detected it?

I guess that's a start... but it seems to be a question loaded with practical (experimental) problems...

As one example: (happens to be regarding CMBR)

The anisotropy of the cosmic microwave background is divided into two sorts: primary anisotropy, due to effects which occur at the last scattering surface and before; and secondary anisotropy, due to effects such as interactions with hot gas or gravitational potentials, between the last scattering surface and the observer.

http://en.wikipedia.org/wiki/CMBR

Lots of other considerations in that article...