# B Poor man's double slit etc. setup

#### vanhees71

Gold Member
A direct observation that make me think "this can't be classical" is rather very trivial. Its large divergence of laser beam when it is partially blocked by a barrier versus small divergence of freely propagating laser beam. I don't see that this can be explained by Huygens principle (or bullet type particles).
Again: A "laser beam" is to a pretty good approximation a coherent state and not a single-photon state. It's well described as a classical electromagnetic wave, and I don't understand what you mean by that this example shows something non-classical. It's just diffraction at an edge, which has been fully understood (i.e., beyond Kirchhoff's approximate solution) by A. Sommerfeld, who was an expert in the theory of optical diffraction. His optics textbook is simply marvelous:

A. Sommerfeld, Lectures on Theoretical Physics, Vol. IV (Optics), Academic Press (1954)

Huygens's principle has to be used with a grain of salt. The mathematically fully correct version is found through using the retarede Green's function of the wave equation (in 3D space!),
$$G_{\text{ret}}(t,\vec{x})=\frac{1}{4 \pi |\vec{x}|} \delta \left (t-\frac{r}{c} \right), \quad r=|\vec{x}|.$$
For details, see Morse&Feshbach, Methods of theoretical physics, Vol. 1.

#### zonde

Gold Member
Again: A "laser beam" is to a pretty good approximation a coherent state and not a single-photon state. It's well described as a classical electromagnetic wave, and I don't understand what you mean by that this example shows something non-classical. It's just diffraction at an edge, which has been fully understood (i.e., beyond Kirchhoff's approximate solution) by A. Sommerfeld, who was an expert in the theory of optical diffraction.
I suppose I rather don't understand non-diffraction of freely propagating laser beam. Can you volunteer some explanation? I would like to understand what can be explained classically. I can start a new thread if it's more convenient.

Staff Emeritus
while in any DIY experiment you should expect a solid dose of wishful thinking
But I just want to test it and see the result for myself..
It seems to me that Blue Scallop has a healthy scientific attitude, one that will not be satisfied by experimental noise plus "wishful thinking". Unfortunately, it's still not clear exactly what the OP is looking for, as well as a general confusion among participants in this thread on "entanglement" vs "superposition".

#### Blue Scallop

If you don't see the 10 inside the globe above.. you are color blind.. similarly I want a quantum test akin to the following:

If the Stern-Gerlach would have spin up, the person would see "KEY"
If the Stern-Gerlach would have spin down, the person would see "BOARD".

I want to test if I or my friends can see both the superposition of up and down.. in other words if we can see both branches at same time or the "KEY BOARD" displayed instead of just KEY or BOARD. I want to confirm if the unitarity in qm is infallible and an unbroken law of nature. Also before projection (collapse or single outcome in MWI), it is supposed to be in superposition not just of up and down.. but in all angles located in configuration space.. I just to also test if I and my friends can see the superposition of all angles in configuration space. Remember wave functions are located in configuration space and not in physical space.

The point to do experiment is just to teach and verify for yourself. Why do your school has dynamo to demonstrate Maxwell's physics or others. It is to demonstrate to students.

So where to buy the Stern-Gerlach setup or can you think of a simpler setup that can do that above test. Nurgatory polarizer test is just a test where the result is the same even if one can see all branches at same time (this is what I want to demonstrate that it is not possible or unitarity in QM is a law set in stone). Thank you.

#### Nugatory

Mentor
So where to buy the Stern-Gerlach setup
These aren't things that you can buy off the shelf. Each experimenter will have their own requirements so there's no possibility of building a standardized product and putting it in a catalog or up on Amazon. The building blocks (micrometers, polarizes, lenses, power supplies, oscilloscopes, electrical wire, timers, photodetectors, coincidence counters and other electronic bits, ....) are easily purchased, but it's up to the experimenter to figure out how to assemble these into an apparatus that they can use for whatever experiment they're conducting.

#### Nugatory

Mentor
I want to test if I or my friends can see both the superposition of up and down..
Superpositions are pretty much by definition not something that can be seen. The best you can do is to come up with experiments like the polarizer one that I suggested, in which the results can only be easily explained by assuming the existence of a superimposed state.

Staff Emeritus
Also, a single Stern-Gerlach experiment doesn't tell you anything about superposition.

I think you need to think more carefully - and this probably involves books - exactly what it is you want to show.

#### Blue Scallop

Also, a single Stern-Gerlach experiment doesn't tell you anything about superposition.

I think you need to think more carefully - and this probably involves books - exactly what it is you want to show.
In the double slit. There seems to be two kinds of superposition to wathc out for.. the superposition of going through left or right slit and the branches (the different spots in the detector) or the eigenvalues. Just want a device to prove there is no way to see the electron going via left and right at the same time. And there is no way to see all eigenvalues (screen positions) at all branches at the same time. Unitarity forbids them I know. But won't it be possible Unitarity can be distributed among Quantum sytem, Spacetime and Observer? Got this insight from Ken G when he suggested entanglement may be the norm and ordinary unentangled particles just a special case. Whatever even if he was wrong, just want to see it for myself Unitarity is 100% true all the time without any exception. If there is no a single double slit device anywhere in Hong Kong or Singapore.. I guess there is one in Japan, where?

Is there no Entanglement Museum anywhere in the world where you can see all these devices plus the Clauser, Aspect Bell devices? I'm just curious just like how kids want to see prism gets rainbows. Aren't you the public curious to see such actual devices?

#### jtbell

Mentor
Its large divergence of laser beam when it is partially blocked by a barrier versus small divergence of freely propagating laser beam. I don't see that this can be explained by Huygens principle
"Far enough" away from a circular aperture (the technical term here is "Fraunhofer approximation"), the angular width θ of the central maximum of the diffraction pattern is proportional to λ/d where λ is the wavelength and d is the diameter of the aperture. This is well-established classical wave optics, see e.g. here:

http://hyperphysics.phy-astr.gsu.edu/hbase/phyopt/cirapp2.html#c2

The aperture that produces a laser beam is much wider than a pinhole, therefore light emerging from a laser aperture spreads much less than light emerging from a pinhole.

You get the same λ/d dependence in single-slit diffraction, which is easier to derive because one deals only with the transverse dimension across the slit, not along it. (With a circular aperture one has to deal with two dimensions, usually in polar coordinates, and introduce Bessel functions. ). A Google search for "single slit diffraction intensity derivation" brings up numerous mathematical derivations, of which this one is probably typical (I did this sort of thing in an intermediate optics course that I taught for many years):

https://courses.physics.ucsd.edu/2011/Winter/physics4d/files/Intensity in 1-slit pattern.pdf

Here's a graphical version involving phasors ("phase vectors") which one sometimes sees in first-year physics courses:

http://hyperphysics.phy-astr.gsu.edu/hbase/phyopt/sinint.html

Last edited:

#### rrogers

Concerning the original question: Have you tried youtube? For instance

#### Blue Scallop

A lab tried to build its own Mach-Zehnder Interferometer experiment.. but the length of upper path and lower path has to be kept at the nano-meter level or both detector A and detector B would be triggered. A right setup should only have one detector trigger.. and they can't do it..

According to http://www.users.csbsju.edu/~frioux/two-slit/MZ-WhichWay.pdf "The detection of the photon exclusively at Dx is the equivalent of the appearance of the interference fringes in the double-slit experiment."

It is said 9 out of 10 use interfometer for which way path experiment. I'm looking for description of actual experiments (the 1 out of 10) where they use double slits.. single photon source.. and single photon detector.. why is single photon detector so difficult? and what is the complete setup for this test? So even if you hide a tiny camera in one of the slits.. the photon or electron knows? how about any that won't cause decoherence or is the explanation of the following correct?

https://www.st-andrews.ac.uk/~kd1/PH5015/Rempe.pdf

"The principle of complementarity refers to the ability of quantum-mechanical entities to behave as particles or waves under different experimental conditions. For example, in the famous double-slit experiment, a single electron can apparently pass through both apertures simultaneously, forming an interference pattern. But if a`which-way' detector is employed to determine the particle's path,the interference pattern is destroyed.This is usually explained in terms of Heisenberg's uncertainty principle, in which the acquisition of spatial information increases the uncertainty in the particle's momentum, thus destroying the interference."

Is this explanation correct? If not, what is the correct explanation? Thank you.