Have you ever struggled with the concept of quantum measurement, feeling it’s unnecessarily abstract? You’re not alone. Enter this guide by [Mithuna] from Looking Glass Universe, where she circles back on the concept of measurement basis in quantum mechanics using a rather simple piece of calcite crystal. We wrote about similar endeavours in reflection on Shanni Prutchi’s talk at the Hackaday SuperConference in 2015. If that memory got a bit dusty in your mind, here’s a quick course to make things click again.
In essence, calcite splits a beam of light into two dots based on polarization. By aligning filters and rotating angles, you can observe how light behaves when forced into ‘choices’. The dots you see are a direct representation of the light’s polarization states. Now this isn’t just a neat trick for photons; it’s a practical window into the probability-driven nature of quantum systems.
Even with just one photon passing through per second, the calcite setup demonstrates how light ‘chooses’ a path, revealing the probabilistic essence of quantum mechanics. Using common materials (laser pointers, polarizing filters, and calcite), anyone can reproduce this experiment at home.
If this sparks curiosity, explore Hackaday’s archives for quantum mechanics. Or just find yourself a good slice of calcite online, steal the laser pointer from your cat’s toy bin, and get going!
There is no choice here. Calcite birefringence is simply that the trigonal crystal system is not isotropic in all directions. At best you can use a calcite crystal as a separator. Whilst polarization foils act as pure filters.
But in the end the photons carry the same quantum information as before entering the crystal.
I’m not saying this content is not good or anything. It merely proves that unpolarized light carries photons with all types of polarization. So this is not a measurement.
It’s not a measurement, it’s a lab demo. but it does give insight into how measurement is achieved.
I agree. The crystal isn’t randomly selecting photons and sending them different directions, it is just separating them by polarization that was already there.
A lot of QM woo seems to arise from a rather naive conception of what constitutes an “observer”
No, it’s not. It’s projecting the incoming light onto two directions of polarization – those directions are related to the calcite, not the light. That’s what a quantum measurement is: you force an arbitrary state onto a new set of output states.
Yes, it is. A birefringent material can only modify an existing polarization, it cannot create a new polarization from nothing. A half wave-plate for example, can only rotate an existing polarization, not randomly polarize a photon.
“not randomly polarize a photon.”
All photons are polarized. A non polarized source just puts out random polarization vectors each time. Photons can be in any polarization state, but they cannot be in “not a state.”
Let me clarify that a bit more:
Birefringence means that the index of refraction depends on polarization. In other words, the propagating modes inside the calcite have a different basis in polarization space. The only way something can propagate inside calcite is if it’s polarized along one of those modes.
The reason a half-wave plate acts the way it does is because of the combination of the thickness and the birefringence. You only allow the birefringence to act for a short amount of time to induce a controlled phase shift: the two propagating eigenmodes in the birefringent material don’t have enough time to separate and become incoherent relative to each other.
Technically, yes, a propagating beam through calcite doesn’t necessarily force the photon into a polarized state, because you don’t lose the phase information. That necessarily happens when it gets “detected” – meaning when it is detected “yes/no” by whatever sees the light and the phase information is lost. For instance, here, technically, when the light hits the camera’s CCD.
But from a practical point of view, once you’ve propagated well through the calcite it’s going to be functionally impossible to recombine the two divergent paths coherently and it’s done. This is the difficulty with saying what is or isn’t a quantum measurement. It’s literally all about whether or not the phase information’s lost. It technically isn’t lost until the final interaction happens but it practically can be lost beforehand.
Pay no attention to the man behind the curtain! I am the great and powerful Quanta!
“At best you can use a calcite crystal as a separator. ”
that’s… the way measurement devices work
you take the thing you want to measure, and you separate it into different results based on the states
“Quantum measurement” literally means projecting (separating) an arbitrary input state onto result eigenstates.
That guy just literally wanted to write anything, hahaha
Maybe if you spent less time writing ‘Whilst” and spent more time actually reading the few words that were the basis of your… response… you’d have read that no where in the write-up did it say that the crystal polarizes the light. Even if it did originally use those exact words, the term polarization can mean both the use of a physical filter to separate polarization vectors or physically imparting polarization on previously unpolarized waves. For someone as pedantic as you seem to be in your aimless criticism, you sure are pretty stupidly wrong about your position.
I can’t wait until we figure out which parts of QM are legit and which parts are total misconceptions, the suspense is getting to me. We need to start producing more Austrian Ashkenazi physicists again, where did all of those guys go? We haven’t figured out squat since they stopped making that type of guy
At a hunch I’d say the parts that are measurable might have some merit to them
I’m taking this as antisemitism
I’m taking this as anti-abortion
So it’s a slit experiment?
This was explained in the last episode of Dr. Stone!