Click here for the publication.

Having performed this experiment, I have gotten clean results. Essentially, a double slit is made by putting a photon beam in the way of a wire with orthogonal polarizers on either side. This destroys the expected interference pattern since the polarized filters "measure" the path of the photons. However if one places a 45-degree polarizer that allows the orthogonal light waves to both pass through, the interference pattern restores. According to the article, this is a "quantum eraser" since the wave nature was destroyed with the perpendicular polarizers and restored afterwards with the 45-degree filter.

This being said, I also understand that the classical Fresnel-Arago laws state that orthogonal waves do not interfere. Wikipedia also mentions that when [particle detectors are at the slits][], the wave function should collapse. But it also states that this experiment has never been published. Here we have an experiment that places a "detector" at the slits, and as far as Scientific American says, it has collapsed and even restored the wave function. Now, I can only think of 2 conclusions to this:

  1. The Fresnel-Arago Laws were a precursor to quantum mechanics and there is no interference because the information has been leaked into the outside environment
  2. This is purely a classical experiment and can be explained as such

Is this experiment just a demonstration of classical optics or is there actually a quantum nature to this? I also wonder if Fresnel and Arago had an explanation to the nature of orthogonal light waves, or if the quantum mechanical wave collapse due to observation is the only reason. Does anybody have information on this?

Much gratitude for your thoughts! This is for a science fair project for my high school, so I would greatly appreciate it since I no longer know whether I should present it as a classical twist to the double-slit experiment or a true quantum mechanical phenomenon.


3 Answers 3


This experiment can be completely explained within classical physics. It must, because laser pointers produce coherent states which exactly match the predictions of classical electrodynamics. However, it is a very good analogy for the paradoxes you would face in a quantum eraser experiment with an electron beam.

The reason the analogy is good is because the light in the classical treatment is described by a wave equation that is very similar to the Schrödinger equation for a single massive particle. Thus, wavefunctions will diffract to create single blobs if given a single slit, and interfere to make fringes if given two slits. You can further encode two different waves in a single particle using a spin-½ degree of freedom, in a manner exactly analogous to the polarization degree of freedom of an EM wave.

We don't find this situation paradoxical in classical mechanics because the light shining on the screen can't be seen as a number of discrete 'packets', and its intensity comes in a continuum. It does not make sense to ask "where did the light that makes this bright fringe come from?" because it comes from both slits. If you place a detector on each of the slits, you do observe half the power going through each slit. Within classical physics, this occurs no matter how low the laser power is.

Suppose now, though, that you replace your laser for an electron gun. Since the wave mechanics remains (much) the same, the interference fringes - or lack thereof - in the wavefunction and therefore in the detection probability will not be altered. However, electrons do behave as particles fairly often. At low enough electron fluxes, you only ever measure single-electron hits on your detector, and you can ensure only a single electron is ever present in the apparatus. If you put detectors right after the slits, you don't observe half-electrons. It is here that it starts getting paradoxical: if when I observe the slits the electron is only ever in one of the two, how come the interference pattern changes if I have access to the 'which way' information? Note, though, that it's this extra layer of particleness that makes the quantum eraser weird.

Finally, what about light? Can one do a 'quantum' version of this experiment using light? After all, light also comes in photons, and you can power down your laser low enough that only single flashes will show in the screen, right? Well, for one you need to iron out a few wrinkles. For example, you need to make sure that those single flashes are indeed a property of the light and not of the detector; there exist fairly reasonable models which explain the photoelectric effect by quantizing only the atoms and not the field. This means, in particular, that you need to change your laser for a single photon source, which is a different beast altogether.

Even then, though, the experiment is not quite enough to be a paradox. The reason for this is that photons don't really have positions or trajectories or even, really, wavefunctions. They're single excitations of the corresponding classical modes, and the modes themselves exhibit interference and wave behaviour. (Indeed, the experiments where you get photons to behave like particle waves are quite different.) Thus, while you can put together a quantum eraser measurement with single photons, the situation is more complicated and calls for a more delicate analysis.

  • $\begingroup$ @DukeWilliam That sort of empty style edit is not appropriate on this site. $\endgroup$ May 21, 2023 at 22:33
  • $\begingroup$ Mmm.... I will try to avoid such edits in future. Just thought of them as obvious grammar mistakes that I can correct without damaging the original meaning of the passage. I respect that you probably intended to use that words that way to convey some meaning or nuance or probably it's just your style. But just that readers can take those words differently than you mean it. (by the way, as those words don't mean any serious technicality it is probably unnecessary edit, at least in retrospect). sorry for the inconvenience. cheers. $\endgroup$ May 22, 2023 at 14:26

The analysis of the availability or not of the which-way information, and the consequences on the interference pattern, is purely a quantum analysis. Now, there are different experimental devices to make the which-way information appear or disappear. You don't necessarily need to use polarizers for this (you could have used mirrors, beam splitters and detectors). Even, if, in this particular experiment, polarizers have been used, the interesting analysis is not a possible classical analysis (orthogonal waves do not interfere), but it is the quantum analysis. More precisely, in your experiment, the two analysis (classical and quantum) are correct, because of the very particular experimental devices, but the fundamental analysis is quantum, and the quantum analysis will still be true, even if you don't use polarizers in your experiment, while, in this case, the classical analysis will fail.

  • $\begingroup$ The issue is that I still fail to see how classical is not enough to explain this particular experiment. If Fresnel-Arago Laws are enough to demonstrate the nature of this, then why do I need to add a quantum perspective to this? Were Fresnel and Arago really just dealing with quantum mechanics when they found that orthogonal waves don't interfere? Do they not interfere because the which way information is leaked? $\endgroup$
    – user36902
    Jan 8, 2014 at 5:27
  • $\begingroup$ As said in my answser, in this particular experiment, a classical explanation is correct. And the which-way quantum explanation is correct too. So, if you want, in this experiment, the classical explanation is enough (you don't need quantum mechanics principles). But for most which-way experimental devices, no classical explanation is available. So the only correct global point of view (for all experiments) is the quantum point of view. $\endgroup$
    – Trimok
    Jan 8, 2014 at 8:58
  • 2
    $\begingroup$ Of course you can do a quantum analysis of this experiment. But since a classical analysis is totally sufficient to explain it, presenting this particular SCIAM experiment as a demonstration of quantum effects would be silly. It is just an illustration of the distinguishability/visibility duality, and an introduction to the experiments that actually probe quantum effects (entanglement). $\endgroup$
    – Gilles
    Jul 26, 2018 at 4:55

I tried to do an experiment to answer this question using a real double slit setup. As expected I got no double-slit interference when I put the orthogonal polarizers in place. However, I did get single slit interference. This would be entirely compatible with the classical Fresnel-Arago laws because no interference between the orthogonal beams occurred but the beams interfered with themselves. On the other hand, the article you referred to claims that by introducing the polarizers the wave nature of the light was destroyed. However, the presence of the single slit diffraction patterns, which results from the wave nature of light, clearly contradicts this claim. Now, you probably did not observe this phenomenon because you did not use slits but you used a wire to separate the paths. Using simply a wire you will not be able to observe single slit patterns. So, I think it is safe to say that the setup described in the Scientific American article does not show quantum phenomenon.

  • $\begingroup$ You might want to provide details of your experiment (e.g., images of the setup & results). $\endgroup$
    – Kyle Kanos
    Nov 13, 2015 at 18:45
  • 1
    $\begingroup$ What do they mean by saying that this experiment has never been published? There are many published examples of the quantum eraser $\endgroup$ Dec 25, 2015 at 1:12

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