[hamdani yusuf (OP)]

As you say, any classical theory which assumes, as many people do, that a polarising filter just filters has to give the wrong answer.

Not much of a "theory" if it gives the wrong answer.

I'm fairly sure that Maxwell's eqns (which are classical- neither QM, nor explicitly relativistic) will give the right answer.

So why did Dirac make this statement? Don't you think he understood Maxwell equations pretty well?

From section 4, Superposition and indeterminacy, pp, 12-14

The general principle of superposition of quantum mechanics... requires us to assume that between these states there exist peculiar relationships such that whenever the system is definitely in one state we can consider it as being partly in each of two or more other states. The original state must be regarded as the result of a kind of superposition of the two or more new states, in a way that cannot be conceived on classical ideas.

[Bored chemist]

Superpositions are weird non-classical things.
But the transmission (or not) of EM radiation through a polariser is a classical phenomenon.
Once you talk about individual photons (having states) then you are no longer talking about classical physics.
Quanta of light are clearly in the realm of quantum mechanics.

[Colin2B]

Superpositions are weird non-classical things.

Superposition also applies to classical waves in both acoustics and emr (and gravity waves eg water)

[Bored chemist]

Superpositions are weird non-classical things.

Superposition also applies to classical waves in both acoustics and emr (and gravity waves eg water)

Yes, And if that's the superposition you mean then there's no problem with the three polarizers.

But if you mean a superposition of states of a photon , or a well known cat, then they are quantum things.

[hamdani yusuf (OP)]

My latest experiments using twin polarizers to interact with microwave are meant to resolve this problem. So far the results seem to agree with classical explanation, including the case where those polarizers are electrically connected.

[hamdani yusuf (OP)]

My latest experiments using twin polarizers to interact with microwave are meant to resolve this problem. So far the results seem to agree with classical explanation, including the case where those polarizers are electrically connected.

When I say classical explanation, it just means that it doesn't involve quantum weirdness such as collapse of the wave function or entanglement. It isn't  necessarily in full agreement with Maxwell's theory of electromagnetism.

[yor_on]

You might find this interesting hamdani yusuf .  https://skullsinthestars.com/2020/02/05/visualizing-the-geometric-phase-of-light/

[hamdani yusuf (OP)]

Here is a problem I found in some explanation of the three polarizers experiment. When a polarizer is at 45°, it is interpreted that 50% of photons coming out from the polarizer is polarized vertically, while 50% is polarized horizontally. This interpretation can't distinguish it with the result if the polarizer is at -45°.
IMO, it is better interpreted that 100% of photons coming out from the polarizer is polarized according to the polarizer's easy axis. In the case above, it's at 45°.