Concept 1 of 7 · The medium

Qubits and superposition

A classical bit is a switch (on / off). A qubit is an arrow in 3D space that can point anywhere on a sphere — but when you measure it, you only ever get a single 0 or 1.

What it actually means

A classical bit has exactly two values, 0 or 1, like a light switch. A qubit is a vector pointing from the centre of a sphere to some point on its surface. The north pole is |0⟩, the south pole is |1⟩, and every other point is a superposition — a specific combination of the two, weighted by amounts called amplitudes.

The catch: when you measure a qubit you don’t see the arrow. You see either 0 or 1, with a probability set by how the arrow was oriented. After measurement the arrow snaps to the pole you observed — the superposition is gone.

So why bother? Because before you measure, the qubit can take part in operations that depend on it being in both states at once. With N qubits you hold a joint state of 2^N amplitudes and manipulate all of them with a single gate. The power isn’t “trying many things in parallel” — it’s setting up interference so the right answer reinforces and wrong answers cancel.

Why it matters for our problem

A molecule’s wavefunction lives in an exponentially large space: for N electrons over M orbitals there are C(M, N) arrangements, and the ground state is a superposition of all of them with specific amplitudes. Classical computers track every amplitude separately; for ~50 electrons that is astronomical.

A quantum computer stores the same superposition natively — one amplitude per basis state, encoded across qubits. In 03_wh_binding.ipynb, the WH⁻ wavefunction lives in 6 qubits = 2⁶ = 64 amplitudes. The full fusion problem would need 200+ qubits = 2²⁰⁰ ≈ 10⁶⁰ amplitudes. No classical computer will ever store that vector; a 200-qubit chip encodes it natively.