Two guys — Stern and Gerlach — did an experiment in 1922. They wanted to measure magnetism caused by electron orbits. At the time, they didn’t know about particles having angular momentum due to spin. So — as explained by [The Science Asylum] in the video below — they clearly showed quantum spin, they just didn’t know it and Physics didn’t catch on for many years.

The experiment was fairly simple. They heated a piece of silver foil to cause atoms to stream out through a tiny pinhole. The choice of silver was because it was a simple material that had a single electron in its outer shell. An external magnet then pulls silver atoms into a different position before it hits some film and that position depends on its magnetic field.

If electrons randomly flew around the nucleus like a cloud, you’d expect a cloudy line on the film. If the electrons had a fixed number of possible electron orbits, the film would show a series of points. In the end, the result was a big surprise — it was neither of the expected patterns. Instead, they got something shaped like the outline of some lips.

They realized that the horizontal deflection occurred even without the magnet, so what looked like two lines were really two points, and that implies that the electrons must be in one of two positions. However, the truth is more complicated.

In fact, Schrödinger’s equations appeared later and shed more light on how the electrons could orbit. It also seemed to imply that the earlier experiment should have been a single spot on the film. The answer turned out to be quantum spin.

According to the video, this was a lucky mistake. The experiment was perfect for measuring quantum spin, but it was unlikely that anyone would have thought to perform it for that purpose. By trying to prove one thing, they had actually proved another thing that no one understood yet. Science is strange and wonderful.

Spin is a big deal in many quantum computers. If you need a refresher on electron orbitals, it is a topic we cover periodically.