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A curious observer’s guide to quantum mechanics, Pt. 6: Two quantum spooks

Throughout our quantum adventures to date, we’ve seen a bunch of interesting quantum effects. So for our last major excursion, let’s venture into a particularly creepy corner of the quantum wood: today, we’re going to see entanglement and measurement order.

Together, these two concepts create some of the most counterintuitive effects in quantum mechanics. They are so counterintuitive that this is probably a good time to re-emphasize that nothing in this series is speculative—everything we’ve seen is backed by hundreds of observations. Sometimes the world is much stranger than we expect it to be.

I’ve always considered the world of spies and espionage to be strange and spooky, so maybe it is fitting that one of the applications of what we are talking about today is cryptography. But there’s a lot to go over before we get there.

After a long walk through an increasingly dark and gloomy forest, gnarled trees dripping with vines, we unexpectedly emerge into a meadow sparkling in bright sunshine. Blinking in the light, we pull out our polarized sunglasses.

If you’ve never played with polarized sunglasses, it is great fun to look at different objects while tilting your head in various directions (or holding the glasses in front of you and rotating them if you want to look slightly less like a doofus—whatever you do, please don’t look at the Sun.) Some things will look the same regardless of how you rotate the glasses: concrete, trees, and houses all reflect unpolarized light. Others will dramatically change brightness as you rotate the glasses: reflections off of water and car windows, the blue sky (tangential to the direction to the Sun), and most LCD computer monitors. That’s because these objects reflect or emit polarized light, which has its own set of rules for how it interacts with the glasses.

The lenses in polarized sunglasses are polarized in the vertical direction, meaning that when worn normally, they will allow light polarized vertically to pass through but will completely block horizontally polarized light. This is advantageous, because light glinting off of water is mostly horizontally polarized; a lens that only allows vertically polarized light through will greatly reduce the reflected glare. Read from source….