In quantum computing, * qubit* or

**, is a unit of quantum information—the quantum analogue of the classical bit.**

*quantum bit*A

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, but 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.qubit