A **Qubit**, or **quantum bit** is the fundamental unit of quantum information, it is a mathematical concept satisfying particular properties. The Qubit is the quantum counterpart of the *classical bit*.

A classical bit can be in one of two different states: 0 or 1, whereas a Qubit can be in the quantum states , but also in any *superposition* of these two. That is, the state of a Qubit will be of the form

,

where and are complex number satisfying

.

The states and form the so-called *computational basis* . Then, as a mathematical concept, a Qubit is a vector normalized to length 1 in a complex vector field.

Just as it’s classical analogue, a Qubit can have many physical realizations: The polarization of a photon, the spin of an electron or a nucleus, the localization of a charge, among other two-state quantum systems.

Go to the Dictionary of Quantum Information and Quantum Computation.