Ch 25.1 The quantum analogue of a classical bit: a two-level quantum system $|\psi\rangle = \alpha|0\rangle + \beta|1\rangle$ with $|\alpha|^2 + |\beta|^2 = 1$. Unlike a classical bit, a qubit can be in a superposition of 0 and 1. The fundamental unit of quantum information. *See also:* Bloch sphere