It is often a source of irony. We teach, or learn, something by dealing with the simplest case first, devoid of complexities. Only after that do we start learning more complex situations.
Yet, in many cases, it is extremely difficult to duplicate, in practice, this simplest case. Quantum mechanics is one such example. While we learn about QM at the intro level by looking at the case of an infinite potential well, a finite potential well, 1-electron central force, etc... trying to actually get a clear experiment on this is actually quite difficult. This is because we have to isolate the system that we want to measure from the rest of the world, so that only the simplest, most fundamental parameters are involved in the experiment.
This is one such case. The experimenters claims to finally being able to directly measure the elements of a density matrix. Yet, if you have done any amount of QM, you would have seen this density matrix in your QM classes. I remember encountering it when I was using Merzbacher's text. So this is a classic, text-book item that we are all taught in school. Yet, it is not an easy thing to measure directly, till now. This is possible due to advancements in the so-called "weak measurement" that have previously produced Bohn's Pilot-Wave-like results.
Still, it is nice to know that what you learn in those textbooks are actually correct! :)
Zz.
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