Station 2
Bloch Sphere
A bit knows two values. A qubit knows the entire surface of a sphere.
On site
Pick up the model and turn it. At the top is |0⟩, at the bottom |1⟩ — and everything in between is a real, allowed state.
Where to next
The sphere in 30 seconds
North pole
|0⟩ — the measurement is guaranteed to give 0.
South pole
|1⟩ — the measurement is guaranteed to give 1.
Equator
Perfect superposition: 50 % to 50 %.
Longitude
The phase — invisible in a single measurement, but crucial for interference.
Why a sphere and not a line?
Because it takes two numbers to describe a qubit: how strongly it leans towards 0 or 1 (the latitude) and which phase it has (the longitude). Two degrees of freedom → a surface → a sphere.
And with several qubits?
That's where the nice picture sadly ends. Two qubits already need a space that can no longer be drawn — and that is exactly where entanglement lives, the real superpower. The sphere only works for a single qubit.