In quantum physics, the smallest unit of quantum information is the qubit.
The word qubit combines “quantum” and “bit.” A classical bit must be either 0 or 1, whereas a qubit can exist in a quantum superposition of both basis states until it is measured. The measurement probabilities depend on the qubit’s quantum state.
A qubit is not simply a classical bit holding extra values. Its state is represented by a two-dimensional complex vector, and the relative phase between its components can affect later interference. Qubits can also become entangled, producing correlations that have no classical equivalent.
Physical qubits can be built from different systems, including trapped ions, superconducting circuits, photons, and electron spins. Quantum algorithms manipulate qubits with carefully controlled operations, but practical machines must deal with noise, decoherence, and error correction. A qutrit instead has three basis states, while a classical bit remains a two-state unit.