[radioactivist]

While this is a nice demonstration of the polarization of light, this is not a demonstration of quantum mechanics, or quantum computing (though it does have pedagogical value, if qualified properly).

Polarizers essentially just project the electric field of the wave onto some axis, zeroing out the perpendicular component. Keeping in mind that light intensity is the square of the electric field strength, all of this can be explained through straightforward classical electrodynamics.

An analogous statement would be that interference of light (say through a pair of slits) is also quantum mechanical in nature. This isn't strictly wrong (since basically everything is quantum mechanical in nature when you get down to it) but is a misleading way to present something that can (and was) understood perfectly well before quantum mechanics came along.

Note: These kind of experiments for a single particle (e.g. photons, electrons, etc) are a different story and do provide a demonstration of quantum mechanics (and the combination of wave-like and particle-like properties intrinsic to it).

[ahelwer]

This is incorrect [edit: it isn't] - see the standard "three polarizing filter" experiment [0] which is impossible to explain with classical mechanics [edit: it can actually be explained, see coolgod's comment below]. Polarizing filters don't just zero out the perpendicular component of an electric wave, they measure each individual photon that passes through and either blocks it or permits it to pass with some probability proportional to the angle between the photon's polarization and the filter's polarization.

[0] https://youtu.be/zcqZHYo7ONs

[coolgod]

The three polarizing filter experiment using common sources of light can be explained perfectly without using quantum mechanics [0]. These effects only need QM to explain in the single photon regime. The polarized light from the LED display is definitely not in the single photon regime thus this experiment does not demonstrate any quantum effects. Without any quantum effects, it is much more difficult to justify the quantumness of this supposed quantum computer.

[0] http://alienryderflex.com/polarizer/

[ahelwer]

I was going to say, this wouldn't explain it because the electric field strength is projected to the polarization axis (with strength 0.707 at 45 degrees) but to match quantum measurement it should be 0.5 strength (0.707 squared). But as the grandparent comment stresses, light intensity is the square of electric field strength... so the measurement matches. Makes sense! I will amend my above comment.

[radioactivist]

The article/your video make the assumption that light is particle and that those particles are independent and each carry polarization. While we know those assumptions to be true, the classical theory of light is not a theory of particles, but of fields/waves. So when you're asking whether there is a classical explanation that's where you should look. And if you do so, you get something perfectly consistent.

[I.e. if you assumed quantum mechanics didn't exist and that Maxwell's equations were the ground truth, you could explain this behaviour without any issue (with some leeway to define a polarizer)].

I think if you try and use that line of thought strictly (only call an explanation "classical" if you can explain with discrete photon particles), you'd probably have to argue that basically all of electromagnetism is fundamentally quantum mechanical. Again, while not strictly wrong (given our present knowledge), this goes too far for me (and I'd imagine most people).

Also: The "three-polarizer" experiment from your quoted video has a perfectly simple explanation in terms of electromagnetic waves. You use a polarizer to get light polarized along say "y" == (0,1). If you put another polarizer in front of it along "x" == (1,0) then the projection is of the E-field is zero and no light passes through. Now add another polarizer at 45 deg between the two: it then projects the E-field onto its axis, mapping it from (0,E) to (E/2,E/2) (magnitude is 1/sqrt(2)). The E-field now has a non-zero component along "x". So light comes out the final polarizer.

[justinpombrio]

Here's how to do the "three polarizing filter" experiment. You can do it at home if you have three pairs of polarized sunglasses.

1. Take two of the lenses, hold them an inch apart, and shine a light so that it goes through both. The amount of light that goes through depends on their relative angle; at the right angle (90 degrees difference), no light will pass through. Hold them like this, so that no light gets through.

2. Insert a third polarizing filter between the two at a 45 degree angle. Amazingly, some light will now get through. You added an obstacle, and more light passes through.

[jes]

Thank you for an interesting experiment to try. I’m looking forward to doing it myself.

I want to ask, though. Is it correct to call the third filter an obstacle? In the quantum realm, it’s not really an obstacle, is it?

[justinpombrio]

> Is it correct to call the third filter an obstacle? In the quantum realm, it’s not really an obstacle, is it?

Evidently not!

[dhruvp]

Hi - thanks for pointing this out. I've added a note clearing up that as you correctly write the quantum interpretation makes sense at the single photon level. Obviously it's hard to generate and manipulate single photons without the right equipment (especially with a phone like in my case) but I do believe this still provides a nice intuition for what's going on. Thanks for your suggestion!

[shakow]

> Polarizers essentially just project the electric field of the wave onto some axis

Where goes the energy of the orthogonal component of the field? Absorbed by the polarizer, reflected, ... ?

[radioactivist]

Yes, those are both possibilities for building a polarizer [1].

For example, a simple polarizer could be a grid of thin metal wires whose spacing is smaller than the wave-length of the incoming light. For the component of the E-field parallel to the wires currents can be induced freely along their length, and so the grid behaves much like a solid metal plate and reflects that part of the wave. For the component of the E-field perpendicular to the wires, significant currents can't be generated (since the wires are thin) and that part of the wave passes through.

[1] https://en.wikipedia.org/wiki/Polarizer

[2] https://en.wikipedia.org/wiki/Polarizer#Wire-grid_polarizers

[jlokier]

> Where goes the energy of the orthogonal component of the field? Absorbed by the polarizer, reflected, ... ?

It depends on the type of polariser.

The type used in LCD displays and 3D cinema glasses absorbs, that's why everything looks darker through them but they don't look like mirrors.

A polarising beam splitter reflects one mode and passes the other. It looks like a half-mirror.