In
quantum computing, a
qubit or
quantum bit (sometimes
qbit) is a unit of
quantum information—the quantum analogue of the classical
bit. A qubit is a
two-state quantum-mechanical system, such as the
polarization of a single
photon: here the two states are vertical polarization and horizontal polarization. In a classical system, a bit would have to be in one state or the other. However quantum mechanics allows the qubit to be in a
superposition of both states at the same time, a property which is fundamental to quantum computing.