I'm reading Preskill's notes on quantum information theory, and in chapter 2 (full list here) he in particular explains how qubits are different from a probabilistic classical bit. Among examples there is this paragraph

Suppose that a photon beam is directed at an $x$ analyzer, with a $y$ analyzer placed further downstream. Then about half of the photons will pass through the first analyzer, but every one of these will be stopped by the second analyzer. But now suppose that we place a 45${}^\circ$-rotated analyzer between the $x$ and $y$ analyzers. Then about half of the photons pass through each analyzer, and about one in eight will manage to pass all three without being absorbed. Because of this interference effect, there is no consistent interpretation in which each photon carries one classical bit of polarization information. Qubits are different than probabilistic classical bits.

Is this example really sufficient to illustrate the difference between the classical and quantum properties of light polarization? As far as I can tell, an interpretation where a polarizer is a classical probabilistic machine that either stops a photon or polarizes it (along its own axis) works just fine.

  • $\begingroup$ The discrepancy comes from the number of photons that pass through, which is not (in my opinion) given enough emphasis in your quote. this video helped me understand the phenomenon better. $\endgroup$ Jun 9, 2021 at 9:51
  • $\begingroup$ The point is that he assumes that there is one bit of polarization information associated with each photon. If you use a direction as polarization information then it works. $\endgroup$ Jun 9, 2021 at 16:53
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    $\begingroup$ As to the title: everything is ‘inherently quantum’. $\endgroup$
    – my2cts
    Dec 22, 2023 at 11:02

1 Answer 1


I would agree with you that the experiment does not show quantum behaviour.

But polarization is a quantum phenomenon, we have 2 possible states which is quantum .... and all the interactions with the filters are based on probability.

As Marius commented, the video shows that 85% pass thru when the filter is 22.5 degrees, the value is based on a cos squared law which is probability behaviour.

Another thought for the 3 filter experiment is that the same results would be obtained even if all the photons were of the same polarity to begin with .... thus the experiment does not reveal the 2 state quantum nature of photons.

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    $\begingroup$ Two posdible states do not automatically mean that it is a quantum phenomenon - classical EM waves also have polarization. $\endgroup$
    – Roger V.
    Jun 9, 2021 at 16:57

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