In quantum computing, a qubit ( /ˈkjuːbɪt/) or quantum bit 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, 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.
Read more about Qubit: Bit Versus Qubit, Representation, Qubit States, Entanglement, Variations of The Qubit, Physical Representation, Qubit Storage, Origin of The Concept and Term