# Double Slit Experiment

DrChinese

You wrote

And to be fair, you are splitting hairs! Photons absolutely obey the HUP. They have position and momentum operators, which do not commute. They also have spin operators that do not commute. Yes, it is true that a photon does not have exactly the same operators as electrons but that is because they are different particles.

For a photon, its energy (which implies mass) is proportional to its frequency (and inversely proportional to its wavelength). The speed is c (in a vacuum), and it moves in a direction so it has a velocity. Combine these elements and you have momentum. Problem solved!

Thanks for this clarification of the range of HUP. And thanks to ZapperZ too. You have both convinced me of the dangers of paying too much heed to the content of wikipedia!

I am simply talking about a normal double slit setup, where you get interference one way (no which path) and two bars the other (knowing the slit). I am not referring to the "near field" format where interference is not an element.

An example of such a setup would be: laser light source oriented at a 45 degree angle and aimed at 2 slits; place polarizers in front of each slit, one oriented at 0 degrees and the other at 90 degrees. You will see 2 bars, because any photon that can pass through one slit cannot pass through the other; and there can be no interference.

Remove the polarizers (no other changes), and the normal interference pattern appears.
You have been around long enough to know this one, just repeating you error does not help. You are simply wrong.
Refer us to one example that documents an interference pattern on a test screen that when some means of marking which slit is used produces "Two Bars".

Or draw an example covering one slit up - then in red outline "The One Bar" on the screen highlight or shade it edge to edge to indicate intensity.
Then cover the other slit and outline your "second bar" in green on the same screen.

You claim separate red and green bars!
I claim just one wide dispersion pattern highlighted in red and green.

Just where do you draw in the interference pattern with both slits open?
Outside the boundaries of your two bars?
Or inside the boundaries of my single wide dispersion pattern?

-do you now concur, this phenomena has nothing to do with “entanglement”.
You have a lot of credibility on forum so you have a responsibility to get it right and correct your errors.

DrChinese
Gold Member
...Or draw an example covering one slit up - then in red outline "The One Bar" on the screen highlight or shade it edge to edge to indicate intensity.
Then cover the other slit and outline your "second bar" in green on the same screen.

You claim separate red and green bars!
I claim just one wide dispersion pattern highlighted in red and green.

Probably easiest to see at this site:

Only one slit open at a time (about 1/4 way down page) "In the first experiment we send particles or waves from the source S through a diaphragm which has alternately the left and the right slit open, but never both at the same time, as illustrated by the following animation."

(And I agree that entanglement has nothing to do with this example, hope nothing I said implied otherwise. The oddity is that entangled light through a double slit does produce the single dispersion pattern you describe. That is a specific difference between entangled light and unentangled light. I thought you might have been referring to that scenario.)

Probably easiest to see at this site:

Only one slit open at a time (about 1/4 way down page) "In the first experiment we send particles or waves from the source S through a diaphragm which has alternately the left and the right slit open, but never both at the same time, as illustrated by the following animation."

(And I agree that entanglement has nothing to do with this example, hope nothing I said implied otherwise. The oddity is that entangled light through a double slit does produce the single dispersion pattern you describe. That is a specific difference between entangled light and unentangled light. I thought you might have been referring to that scenario.)

Good example,
it shows the overlapping dispersion patterns I described,
and how the inerferance pattern only appears in the area of overlap.
And NOT the two bar result you described
which would have required the interference pattern to display outside the two separate "bars" of light.

Hope that clears it up for you.

“The oddity is that entangled light through a double slit does produce the single dispersion pattern you describe.”

You already gave an experimental reference in another thread that showed that is not true.

IN post #10 of “How do you determine that a particle is/was entangled?” you gave:
http://arxiv.org/abs/quant-ph/0106078

Read the first two lines of sect V.
Also, review the comments between formulas 6 & 7 to confirm that this actually preformed experiment was conducted in a “Far Field” set up. A requirement to produce the interference pattern with or without the beam on the double slit originating from a entanglement source.
And the results that deny your “oddity” are reported in figure 2.

Since no filters are in place anywhere in this test 100% of the S photons are used, even if the correlation counter is turned off the same 100% of the S photons would be detected in the same place as shown in Fig 2 - an interferance pattern from a single beam from a "entalgement source"

JesseM
Good example,
it shows the overlapping dispersion patterns I described,
and how the inerferance pattern only appears in the area of overlap.
And NOT the two bar result you described
How about the standard http://grad.physics.sunysb.edu/~amarch/ [Broken]? (not the 'delayed choice' version). In this experiment, putting different polarizers in front of the slits allows you to determine which slit each photon went through, and this destroys the interference pattern:
To make the "which-way" detector, a quarter wave plate (QWP) is put in front of each slit. This device is a special crystal that can change linearly polarized light into circularly polarized light. The two wave plates are set so that given a photon with a particular linear polarization, one wave plate would change it to right circular polarization while the other would change it to left circular polarization.

With this configuration, it is possible to figure out which slit the s photon went through, without disturbing the s photon in any way. ... The presence of the two quarter wave plates creates the possibility for an observer to gain which-way information about photon s. When which-way information is available, the interference behavior disappears.
RandallB said:
IN post #10 of “How do you determine that a particle is/was entangled?” you gave:
http://arxiv.org/abs/quant-ph/0106078

Read the first two lines of sect V.
The first two lines say to refer to figure 2 to see that there was an interference pattern when the entangled photons were sent through the slits with the quarter-wave plates absent, but if you look at the text that goes with fig. 2 it shows they are talking about an interference pattern in the coincidence count, they aren't saying the total pattern of entangled photons would show interference:
Fig. 2. Coincidence counts vs. detector Ds position with QWP1 and QWP2 removed. An interference pattern due to the double-slit is observed.
As I said in post #15 of the How do you determine that a particle is/was entangled? thread, if you doubt that the total pattern of entangled photons doesn't show interference in a double-slit setup, others have given you plenty of references to show you're wrong:
bruce2g gave you an example in this thread awhile back, showing a graph in post #20 where interference was seen in the coincidence count but where they also showed the total pattern of signal photons without coincidence counting, and no interference was seen. In that post he also mentioned that the paper derived the predicted probability distribution for the total pattern of signal photons, and that it was simply a constant function, obviously implying no interference. So both on a theoretical and experimental level, this paper shows that photons entangled in a certain way will not show interference when you look at the total pattern going through a double slit.

I also mentioned some other papers which also seem to show experimental demonstrations of this in post #24 of that thread. You never responded to either bruce2g's post or mine.
Please actually look at these papers before repeating your claims that there is no evidence that entangled photons show a non-interference pattern.
RandallB said:
Since no filters are in place anywhere in this test 100% of the S photons are used, even if the correlation counter is turned off the same 100% of the S photons would be detected in the same place as shown in Fig 2 - an interferance pattern from a single beam from a "entalgement source"

Nonsense, the paper never says anything about 100% of the photons showing up in the coincidence count, the total pattern for entangled photons shows non-interference as the references I gave above show. The experiment in the paper you're talking about is actually the same one discussed on the http://grad.physics.sunysb.edu/~amarch/ [Broken], read that to get a better understanding of what's going on in this experiment. Note that even with the quarter-wave plates absent (the first diagram in the 'Experimental Investigation' section of the page), they only graph cases where an s-photon was registered at Ds and the entangled p-photon was registered at Dp, there might be plenty of cases where an s-photon was registered at Ds but they didn't graph it because the entangled p-photon missed the narrow range covered by Dp. If you put an array of photon detectors at different positions alongside Dp and called them Dp-1, Dp-2, Dp-3, etc., so that close to 100% of the p-photons would be detected by one of the Dp-detectors, then some s-photons at Ds would have their entangled p-photons register at Dp-1, some would have their entangled p-photons register at Dp-2, and so forth. Then the Ds/Dp-1 coincidence count would show interference, as would the Ds/Dp-2 coincidence count and so forth, but if you added all these separate coincidence counts together I believe you'd get a non-interference pattern.

Last edited by a moderator:
DrChinese
Gold Member
Good example,
it shows the overlapping dispersion patterns I described,
and how the inerferance pattern only appears in the area of overlap.
And NOT the two bar result you described

I guess my definition of two bars is different, but that is what it is often called. I did not mean to imply it was a literal bar, just that there are no interference fringes. If you use 3 slits, you get 3 bars, 4 slits leads to 4 bars, etc.; as long as you know which slit it passes through, the extra "bars/fringes" don't appear.

I think JesseM covered the entangled version pretty well. It is clear from that reference that a single dispersion pattern (I call it a wide bar) results when all of the photons are considered. The "interference" pattern is only present when coincidence counting, i.e. when a select subset is examined.

When you think about it, it is odd that photons act differently going through a double slit depending on whether they are/were entangled or not. But they must, in order to match the requirements of the HUP (and no FTL signaling).

I guess my definition of two bars is different, but that is what it is often called. I did not mean to imply it was a literal bar, just that there are no interference fringes. If you use 3 slits, you get 3 bars, 4 slits leads to 4 bars, etc.; as long as you know which slit it passes through, the extra "bars/fringes" don't appear.

I think JesseM covered the entangled version pretty well. It is clear from that reference that a single dispersion pattern (I call it a wide bar) results when all of the photons are considered. The "interference" pattern is only present when coincidence counting, i.e. when a select subset is examined.

When you think about it, it is odd that photons act differently going through a double slit depending on whether they are/were entangled or not. But they must, in order to match the requirements of the HUP (and no FTL signaling).
Remember you have novices on these forums.
Understanding the double slit requires thin slits that produce dispersion NOT BARS is important and bars is misleading – the term Bars hurts rather than helps a new reader on the topic.

Even the good example you linked to is “extreme” as the two “peaks” you want to represent as “Bars” are never actually seen in real DS experiments.
The real overlap is much more complete in real experiments.

As far as the detecting entanglement as existing by only using a Double Slit without a working counter that debate should return to the other thread.

I’ve yet to see Jesse or anyone provide or site results from such a simple to run experiment that shows me wrong, Just near field hand waving arguments and I apply the DNFTT rule to that nonsense.

And as to HUP; how does it not not use equation (7) in the experiment you sited there.
Did you even re-read what I suggested you review there?

JesseM