A classical bit is 0 or 1. A qubit is described by two complex numbers called amplitudes; measuring it gives 0 or 1 with probabilities equal to the squared size of those amplitudes. With n qubits there are 2ⁿ amplitudes, which is why exact classical simulation becomes expensive so quickly.
The three ideas that matter
- Superposition — a qubit can carry amplitude for 0 and 1 at once.
- Entanglement — qubits can be correlated in ways that cannot be described one qubit at a time (try the Bell state in the Lab: 01 and 10 never appear).
- Interference — amplitudes add like waves; algorithms arrange for wrong answers to cancel and right answers to reinforce.
Why most useful work is hybrid
Today’s devices are small and noisy. Practical workflows use classical computers to prepare problems, optimise parameters and check results, and call a quantum processor only for selected steps. QAISS is designed around that reality, and requires a classical baseline for every comparison.
Every term used here is defined in the Learning Hub (/learn).