Fire a beam of atoms through a magnetic field and classical physics predicts a smear. Instead the beam splits into exactly two spots — nothing in between. That's the Stern-Gerlach experiment (1922), and it's the cleanest proof that an electron's spin has exactly two values: up or down. Spin is the atom's single bit — the weld point of the whole atom-as-computing story.
A spinning charge is a tiny magnet, so a non-uniform field pushes it up or down by an amount set by how much its spin points along the field. Classically that projection could be anything from full-up to full-down — a continuous smear on the screen. But spin is quantised: an electron's spin along any axis can only ever read +½ or −½. Two answers, so two spots. The gap in the middle is the whole point — there is no "sideways."
Two states, nothing between — that is the definition of a bit. It's the same ×2 that turns the orbital ladder (1·3·5·7·9) into the electron capacities (2·6·10·14·18): every orbital holds two electrons because spin offers exactly two slots, ↑ and ↓. It's the fourth number in an electron's address (the one that lets two share an orbital). And it's where the atom and the computer actually touch — not a metaphor, a literal two-state quantum variable. A real qubit is built on exactly this.
The honest line: almost everything else in this cluster is "real physics wearing a computing costume." Spin is the exception — it is binary, measured as two dots on a plate in 1922. This is the one place the analogy is the physics.