[bamgstrom (OP)]

Demonstrations of the DIY double-slit quantum-eraser can be found on YouTube for those not familiar with the experiment.The experiment demonstrates that two beams of orthogonally polarized light do not produce an interference pattern although they do produce a diffraction pattern with the double slit experiment.

The loss of interference was explained two centuries ago by Fresnel and Arago. Their explanation was that the two orthogonal beams of polarized light interfered so rapidly and randomly that the light became incoherent and interference became impossible to detect.

The loss of interference was later explained by Skully and Druhl in 1982 as a matter of which-path information. If it is possible to determine which path the light took when going through the double slits, the interference is lost, but if the which-path information is unavailable or destroyed, the interference returns. By marking the two beams of light with polarizing filters, which-path information is available to observers therefore the interference pattern is lost.

The Skully and Druhl hypothesis has since become the choice of explanations for how the double slit experiment works. But their explanation seems dubious to me since it involves the availability of information to the observer rather than any physical interaction.

I decided to test the Skully and Druhl hypothesis by substituting circular polarizing films for the linear polarizers in the double slit experiment. Circularly polarized light does not interfere destructively the way linearly polarized light does.

I found that the two beams of light when marked with circularly polarized light provide which-path information without destroying interference contrary to the results predicted by Skully and Druhl.

I searched the literature to see if anyone has tried the double slit experiment with circular polarizers and found nothing. All I could find is two anecdotal reports that circular polarizers destroy interference the same as linear polarizers. That is contrary to my results.

I was wondering if anyone was interested in repeating the experiment to see if it confirms my results or if anyone can explain what I am doing wrong?

[bamgstrom (OP)]

My modification of the usual double-slit quantum eraser experiment is to replace the linear polarizers with circular polarizers to see if the interference pattern remains or not. My results so far suggest that the interference remains which favors the old Fresnel-Arago explanation over that of Skully-Druhl which has become the preferred explanation in quantum physics.
For my experiments, I use ordinary clear cellophane tape “Scotch tape” as a circular polarizing filter. The wide packing tape works best because of its width. I find that most sources of clear tape work as polarizers. Some are better than others and some hardly work at all so it helps to test them first.

The easiest way to test for polarization is to apply the tape to a convenient size piece of window glass (glass from picture frames works nicely) and then observe the tape through a polarizing filter or polarized glasses against the lighted background of a flat-screen LCD monitor.  Most computers and TVs have these now. The tape can also be applied directly to a computer screen but this could damage the screen. By rotating the glass you should see the tape go from clear to dark blue and almost black.

I have a “circular” polarizer for photographic work but it is also a linear polarizer. A polarizer labeled “circular” may not be circular only so that is another thing to watch for. Circular polarizers should be perfectly clear since they don’t block light the way linear polarizers do. They simply give the light a “twist” as it passes through.

Circular polarizers are quarter-wave so they first become orthogonal at 45 degrees rather than 90 degrees as with linear polarizers. They are orthogonal at 90 degrees but 45 degrees appears to work best. I like to use a microscope slide as a support for the tape. First, I cover more than half of the slide with tape at a 45-degree angle. Then cut to remove the triangle in the middle and then place another piece of tape perpendicular to the slide so the two edges of tape meet in the middle. Placing the tape over an empty gap in some kind of a frame also works.

A fun experiment is to cover a piece of glass with many layers of overlapping clear tape going in all directions and look at the glass with polarized glasses against the light of a computer screen. You should see a kaleidoscope of changing colors.

[hamdani yusuf]

I have a “circular” polarizer for photographic work but it is also a linear polarizer. A polarizer labeled “circular” may not be circular only so that is another thing to watch for. Circular polarizers should be perfectly clear since they don’t block light the way linear polarizers do. They simply give the light a “twist” as it passes through.

You can use 3D glasses for circular polarization filter. One side would be clock-wise, and the other would be counter-clock-wise.

https://en.wikipedia.org/wiki/Polarizing_filter_(photography)#Types
Circular polarizing photographic filters consist of a linear polarizer on the front, with a quarter-wave plate on the back. The quarter-wave plate converts the selected polarization to circularly polarized light inside the camera. This works with all types of cameras, because mirrors and beam-splitters split circularly polarized light the same way they split unpolarized light.[7]

Linear polarizing filters can be easily distinguished from circular polarizers. In linear polarizing filters, the polarizing effect works (rotate to see differences) regardless of which side of the filter the scene is viewed from. In "circular" polarizing filters, the polarizing effect works when the scene is viewed from the male threaded (back) side of the filter, but does not work when looking through it backwards.

So, if the incoming light is already linearly polarized, then its orientation will determine how much of it can pass the circular polarizer.

https://www.apioptics.com/about-api/api-blog/api-news/how-circular-polarization-works/
A circular polarizer is made up of two components: a linear polarized filter and a quarter-wave plate. The input light going into the linear polarizer filter is known as being randomly polarized (I prefer that term over “unpolarized light” because all light is polarized). The light exiting the linear polarizer filter is now considered linearly polarized light because the plane of polarization of the output light is in one direction instead of being random (or unpolarized).

The linearly polarized light then passes through the quarter-wave plate. Here is the critical and tricky part: the polarization axis is a vector between the electrical fields (Ex and Ey respectively).

The quarter-wave plate has what is called a Fast Axis and a Slow Axis. Note that the “Quarter Wave” designation denotes how much the Slow Axis will retard one of the electrical fields as it passes through the wave plate. To create true circularly polarized light (as opposed to elliptically polarized light), the polarizing axis must be at 45º to the fast and slow axis. Thus the relative 45º polarizer axis allows the electromagnetic fields to be parallel to the fast and slow axis of the wave plate. With all that lined up, the polarized light then exits the quarter-wave plate, with either the Ex or Ey fields shifted by a quarter of a wave.

In this link you can see the effect of circular polarizer interactively.
https://hoyafilterusa.com/pages/how-a-circular-polarizer-works

[bamgstrom (OP)]

You can use 3D glasses for circular polarization filter. One side would be clock-wise, and the other would be counter-clock-wise.
Quote

3D glasses wouldn't work for my intentions because they are linearly polarized as well as circularly polarized. Fresnel and Arago explained that linearly polarized light interferes so rapidly and randomly that the regular pattern of diffraction is lost but their explanation does not extend to circularly polarized light.

A much more recent explanation is that the act of "marking" the light beams with polarized light gives us "which-path" information and the availability of which-path information alone is responsible for destroying the diffraction pattern.

My intention is to test the theory by marking the light paths with circularly polarized light to see if the diffraction pattern remains. I find that circularly polarized light does not destroy interference which is consistent with Fresnel and Arago's explanation but it counters the explanation that the availability of which-path information destroys diffraction.

3D glasses are like ordinary linearly polarized sunglasses with clear cellophane tape at orthogonal angles placed over the lenses to make them both linearly and circularly polarized. That is why I use cellophane tape alone as a polarizer.

[Bored chemist]

. Circular polarizers should be perfectly clear since they don’t block light the way linear polarizers do. They simply give the light a “twist” as it passes through.

That's not how it works.

[hamdani yusuf]

Fresnel and Arago explained that linearly polarized light interferes so rapidly and randomly that the regular pattern of diffraction is lost but their explanation does not extend to circularly polarized light.

Where did you find the source of that information? It looks like you've been misled, or misunderstood things they tried to explain.

[hamdani yusuf]

3D glasses are like ordinary linearly polarized sunglasses with clear cellophane tape at orthogonal angles placed over the lenses to make them both linearly and circularly polarized. That is why I use cellophane tape alone as a polarizer.

Quotes in my reply#2 above has explained how circular polarizers work. Which part of it do you think unclear or inaccurate?

[hamdani yusuf]

. Circular polarizers should be perfectly clear since they don’t block light the way linear polarizers do. They simply give the light a “twist” as it passes through.

That's not how it works.

I think he's referring to optically active substance.
https://en.m.wikipedia.org/wiki/Optical_rotation

Optical rotation, also known as polarization rotation or circular birefringence, is the rotation of the orientation of the plane of polarization about the optical axis of linearly polarized light as it travels through certain materials. Circular birefringence and circular dichroism are the manifestations of optical activity.

[Bored chemist]

Does this help?
https://aapt.scitation.org/doi/10.1119/1.16432

[hamdani yusuf]

Does this help?
https://aapt.scitation.org/doi/10.1119/1.16432

I guess it can do help. Unfortunately I have no access to open the full article. I can only read the abstract.

ABSTRACT
Coherent light in the two arms of a Michelson interferometer are made circularly polarized but with opposite rotations. When the two beams recombine, the light is linearly polarized but the direction of polarization changes depending on the phase between the two beams. When a linear polarizer is used on the output and rotated, the observed interference fringe pattern shifts. If the field of view contains circular fringes, the continuous rotation of the polarizer in one direction makes the circles continuously expand or contract.

The conclusion is not stated clearly in the abstract, whether the extraction of which path information destroys the interference pattern. Perhaps it's there in the full document. Can somebody tell us?

[bamgstrom (OP)]

Fresnel and Arago explained that linearly polarized light interferes so rapidly and randomly that the regular pattern of diffraction is lost but their explanation does not extend to circularly polarized light.

Where did you find the source of that information? It looks like you've been misled, or misunderstood things they tried to explain.

My information comes from the original Fresnel-Arago article describing their three laws of polarization. 
Wikipedia also has an easy-to-find reference under “Fresnel-Arago Laws. ” 
Let me know if your interpretation is different from mine.

[bamgstrom (OP)]

Quotes in my reply#2 above has explained how circular polarizers work. Which part of it do you think unclear or inaccurate?

There is nothing wrong with your description and it follows the usual production of a “circular” polarizer. I also have a circular polarizer used for photographic purposes but a quick test of the polarization can show that the polarization is both circular and linear. This is a third type of polarization known as elliptical polarization because a digram of the electro and magnetic waves describes the shape of an ellipse.

I would like to clear up the difference between circular polarization and elliptical polarization which is, confusingly, also called circular polarization.

A circularly polarized light wave appears to spiral because the electro plane of the wave is shifted out of phase with the magnetic plane and a diagram of the wave where the two planes intersect resembles a spiral like a corkscrew which can spiral to either the left or right. The electro and magnetic axes of the wave are random when natural light is passed through a circular polarizer.

With elliptic polarization, light is first passed through a linear polarizer and then through a circular polarizer and a diagram of where the planes intersect describes an ellipse like a slightly flattened corkscrew. This is not true circular polarization because it is both circular and linear. I would not expect the two beams of elliptically polarized light from a double-slit to interfere because of their linear aspect.

On the other hand, I find that circularly polarized light (not elliptically polarized) does interfere.

[hamdani yusuf]

Fresnel and Arago explained that linearly polarized light interferes so rapidly and randomly that the regular pattern of diffraction is lost but their explanation does not extend to circularly polarized light.

Where did you find the source of that information? It looks like you've been misled, or misunderstood things they tried to explain.

My information comes from the original Fresnel-Arago article describing their three laws of polarization. 
Wikipedia also has an easy-to-find reference under “Fresnel-Arago Laws. ” 
Let me know if your interpretation is different from mine.

Here's the Wikipedia article:

The laws are as follows:[1]

Two orthogonal, coherent linearly polarized waves cannot interfere.
Two parallel coherent linearly polarized waves will interfere in the same way as natural light.
The two constituent orthogonal linearly polarized states of natural light cannot interfere to form a readily observable interference pattern, even if rotated into alignment (because they are incoherent).
https://en.wikipedia.org/wiki/Fresnel%E2%80%93Arago_laws

My interpretation for the first law is, coherent linearly polarized waves cannot produce destructive interference. They can superpose into elliptically polarized light. Depending the phase difference, the result can be circularly polarized when its +/- 90°, or linearly polarized when it's 0 or 180°.
The third law doesn't apply for laser, because it produces coherent light.

[hamdani yusuf]

On the other hand, I find that circularly polarized light (not elliptically polarized) does interfere.

Elliptically polarized light can produce destructive interference if the rotation direction and orientation of semimajor axis are the same, but the phase differs by 180°.

[hamdani yusuf]

Just in case you haven't seen the real life experiment yet.

[bamgstrom (OP)]

Just in case you haven't seen the real life experiment yet.

Thank you for the video of the experiment. It was an experiment I have never seen before. Do you have any information about what he uses as a “particle detector” and how it works?

One difficulty with both given explanations is that light acts as a wave and interferes even when sent through the apparatus as a single particle as long as both beam splitters are present leaving both paths open. But there is no interference if one of the paths is blocked even if, in theory, the blocked path is unused because there is only one photon to choose a single path.
 
The experiment is similar to an experiment done by Flores as a modification of the original Afshar experiment (2005).  Both Afshar/Flores experiments used a wire grid as a sort of “particle detector”. The grid was placed in a part of the beam where both light paths interfered in the apparatus and at a point where the wires were located in the dark areas of the predicted interference pattern.

If the wire grid normally blocks 10 percent of incoming light, the presence of a wire grid should diminish the amount of light reaching the detector if any of the light is traveling in straight lines as a particle, but light as a wave in an interference pattern could travel around the wires so there should be no loss of light. Flores found no loss of light indicating that the light from the apparatus was a wave.

In the video experiment, the particle detector in a similar position before the lens was said to cause light to behave as a particle as indicated by a loss of interference. The experiments do not agree likely because of a difference in particle detectors.

[hamdani yusuf]

Do you have any information about what he uses as a “particle detector” and how it works?

The detectors are supposed to detect extremely weak light signal. They're called Single Photon Detectors, such as:
https://www.thorlabs.com/newgrouppage9.cfm?objectgroup_id=5255
https://www.laser2000.de/en/detectors/1448-single-photon-camera.html
https://www.optica-opn.org/home/newsroom/2020/april/single-photon_detectors_build_a_super-fast_camera/
https://www.hamamatsu.com/us/en/resources/webinars/detectors/single-photon-detectors-and-detection.html

https://en.wikipedia.org/wiki/Photon_counting
Photon counting is a technique in which individual photons are counted using a single-photon detector (SPD). A single-photon detector emits a pulse of signal for each detected photon, in contrast to a normal photodetector, which generates an analog signal proportional to the photon flux. The number of pulses (but not their amplitude) is counted, giving an integer number of photons detected per measurement interval. The counting efficiency is determined by the quantum efficiency and the system's electronic losses.

Many photodetectors can be configured to detect individual photons, each with relative advantages and disadvantages.[1][2] Common types include photomultipliers, geiger counters, single-photon avalanche diodes, superconducting nanowire single-photon detectors, transition edge sensors, and scintillation counters. Charge-coupled devices can be used.

Advantages
Photon counting eliminates gain noise, where the proportionality constant between analog signal out and number of photons varies randomly. Thus, the excess noise factor of a photon-counting detector is unity, and the achievable signal-to-noise ratio for a fixed number of photons is generally higher than the same detector without photon counting.[3]

Photon counting can improve temporal resolution. In a conventional detector, multiple arriving photons generate overlapping impulse responses, limiting temporal resolution to approximately the fall time of the detector. However, if it is known that a single photon was detected, the center of the impulse response can be evaluated to precisely determine its arrival time. Using time-correlated single-photon counting (TCSPC), temporal resolution of less than 25 ps has been demonstrated using detectors with a fall time more than 20 times greater.[4]

Disadvantages
Single-photon detectors are typically limited to detecting one photon at a time and may require time between detection events to reset. Photons that arrive during this interval may not be detected. Therefore, the maximum light intensity that can be accurately measured is typically low. Images/measurements composed of small numbers of photons intrinsically have a low signal-to-noise ratio due to shot noise caused by the randomly varying numbers of emitted photons. This effect is less pronounced in conventional detectors that can concurrently detect large numbers of photons, mitigating shot noise. Therefore the signal-to-noise ratio with photon counting is typically much lower than conventional detection, and obtaining usable images may require long acquisition times.

[Zer0]

Can Anyone provide a Simplistic four lines explanation of what the
" Quantum Eraser Experiment " is?

ps - i am tired of using Utube!

[hamdani yusuf]

Can Anyone provide a Simplistic four lines explanation of what the
" Quantum Eraser Experiment " is?

ps - i am tired of using Utube!

I'll let Google Bard answer your question.

The Quantum Eraser Experiment is a fascinating experiment in quantum mechanics that demonstrates several fundamental principles, including:
Quantum superposition: Particles like photons can exist in multiple states simultaneously, like being both "up" and "down" in a spin measurement.
Quantum entanglement: When two particles are entangled, their fates are linked, even if they are separated by vast distances. Measuring one particle's state instantly determines the state of the other, no matter how far apart they are.
Complementarity: Observing certain aspects of a quantum system, like which slit a photon passes through, destroys information about other aspects, like whether it interfered with itself.
Here's how the experiment typically works:

A beam of photons is directed towards a double slit: This allows the photons to pass through either one of the two slits.
An interference pattern is observed: If we don't know which slit the photons passed through, they "interfere" with themselves, creating a characteristic bright and dark pattern on a screen behind the slits. This is because each photon acts like a wave and can diffract through both slits simultaneously.
A detector is placed near one of the slits: Now, when a photon triggers the detector, we know which slit it went through. Interestingly, the interference pattern disappears!
The "eraser" comes in: In some versions of the experiment, another measurement is made on the photons after they pass through the slits. This can involve measuring their polarization or phase. Depending on the type of measurement, the interference pattern can reappear, even though we "know" which slit the photons went through!
This seemingly paradoxical behavior highlights the strange nature of quantum mechanics. Measuring one aspect of a quantum system affects other aspects, even if they appear unrelated. The "eraser" doesn't actually erase the past path of the photons, but it somehow negates the information gained by the first measurement, allowing the interference pattern to re-emerge.

There are several interpretations of the Quantum Eraser Experiment, each with its own implications. Some physicists believe it suggests that information about the past path of the photons is never truly lost, even if we can't measure it directly. Others argue that the experiment demonstrates the non-local nature of quantum entanglement, where measurements on one particle instantly affect the state of another, no matter how far apart they are.

Overall, the Quantum Eraser Experiment is a powerful tool for exploring the mysteries of quantum mechanics and its implications for our understanding of the universe.

Instead of four lines, it answers in four paragraphs. I hope you don't mind.

[alancalverd]

Except that it doesn't make sense.