A coin on the table, a coin in the air

Every ordinary computer, from a wristwatch to the biggest machine on earth, works on bits. A bit is one switch, on or off, and there is nothing in between. Eight billion of them on a chip, each one on or off, and that is the whole story.

A qubit is different in one way. Until you look at it, it is not on or off. It is in a mix of the two, called a superposition, and the mix has a lean: maybe mostly on with a little off, maybe half and half. When you look, which is called measuring, it lands on one side, on or off, with odds set by the lean. Then it stays there.

So the coin picture is this. A bit is a coin on the table. A qubit is a coin spinning in the air. You can shape how it spins. But the moment you slap it flat to read it, it is heads or tails and the spin is gone.

The thing that trips most people up: the qubit is not secretly heads or tails the whole time with you not knowing. Einstein thought it must be. The Bell tests, further down, showed it is not. Before you look, there is no answer yet. That is the physics, not a gap in our knowledge.

One more thing the coin picture gets wrong, and it matters for everything after. A coin in the air has a chance of heads, say 70%. A qubit has something underneath the chance, called an amplitude, which can be positive or negative or point in any direction on a clock face. The chance is the amplitude squared. Two positive amplitudes add up. A positive and a negative cancel. Chances never cancel; amplitudes do. Hold onto that, because it is the whole trick.