Here is the link to the Dirac's three polarizer experiment. https://www.informationphilosopher.com/solutions/experiments/dirac_3-polarizers/

When a 90$^o$ and 0$^o$ polarizers are placed in front of each other, no light comes out of the other end. When a 45$^o$ polarizing filter is placed between 90$^o$ and 0$^o$ filters, we surprisingly see light coming out from the setup.

A quantum explanation of polarization of light was given by Dirac.

However, I found a few sources where this is explained classically. The crux of their argument is that there are long molecules in the polarizer film. They not only attenuate the light but also rotate it. This combined effect can be used to explain the phenomenon classically.

http://alienryderflex.com/polarizer/ https://qr.ae/pGqOMb

Is this a good experiment to teach about Quantum Superposition. Is there is any flaw in the second argument?


1 Answer 1


I'm not sure I accept all of the attempted classical explanation to the three-polarizer problem using long molecules that rotate the light as being sufficient for the purposes of explaining properties of quantum mechanics as needed in quantum computing/quantum information. The proposal fails to enlighten from whence the security of the BB84 protocol comes. I would also argue that such an explanation is wholly insufficient for quantum computation, as the explanation is silent on entanglement and violations of Bell's inequality.

For example, the proposed classical solution appears to require a continuous stream of light and long molecules that can attenuate the light and rotate the stream appropriately. However, there is no indication in the protocol about the probability of rotating a quanta of light - something implicit in the way we describe the BB84 protocol. Alice doesn't send a stream of light to Bob to slowly rotate the stream in the different bases; she sends an individual photon of light to Bob in different bases.

Additionally, and more to the point about quantum computing, the solution explicitly ignores entangled particles, and cannot therefore make any predictions or have any explanatory power about Bell's Theorem. Entanglement appears to be necessary for quantum computation, as emphasized in many questions on this site.


I also don't believe the attempted classical solution can be described as explaining anything about superposition. A stream of light at 45$^\circ$ is not described as being in a "superposition" of a stream of light at 0$^\circ$ and 90$^\circ$, as far as I can tell, as the explanation appears to envision some large molecules that can affect the rotation of the light.

The explanation is not without merit, though, because it might be simpler than the quantum perspective, but it makes some testable predictions that are falsifiable. For example, what are these large molecules made of?

  • $\begingroup$ The 3 polarizer experiment is there to explain about superposition and not entanglement. Understanding entanglement needs using bbo crystals and not polarizers, so there is no chance this experiment would reveal anything about entanglement. $\endgroup$ Dec 16, 2021 at 5:15

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