What is the basic unit of quantum information?

A qubit is the basic unit of quantum information. Like a classical bit, it is associated with two reference states commonly labeled 0 and 1, but quantum mechanics also permits a qubit to exist in a coherent superposition of those states.

A qubit is modeled mathematically as a two-level quantum system. Examples include an electron’s spin, a photon’s polarization, or two selected energy levels in an atom or ion. When measured, a qubit produces a definite result, but the probabilities of those results depend on its state before measurement.

Superposition does not mean that a qubit simply stores both ordinary values in a readable form. Measurement gives one outcome, and extracting information changes the quantum state. Quantum gates manipulate amplitudes and phases, while entanglement links multiple qubits. A quantum register is a collection of qubits used together in a quantum computer.