In quantum computing, a qubit (/ˈkjuːbɪt/) or quantum bit is a basic unit of quantum information; a binary qudit – the quantum version of the classic binary bit physically realized with a two-state device. A qubit is a two-state (or two-level) quantum-mechanical system, one of the simplest quantum systems displaying the peculiarity of quantum mechanics. Examples include the spin of the electron in which the two levels can be taken as spin up and spin down; or the polarization of a single photon in which the two spin states (left-handed and the right-handed circular polarization) can also be measured as horizontal and vertical linear 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 coherent superposition of multiple states simultaneously, a property that is fundamental to quantum mechanics and quantum computing.

The coining of the term qubit is attributed to Benjamin Schumacher. In the acknowledgments of his 1995 paper, Schumacher states that the term qubit was created in jest during a conversation with William Wootters.

A binary digit, characterized as 0 or 1, is used to represent information in classical computers. When averaged over both of its states (0,1), a binary digit can represent up to one bit of information content, where a bit is the basic unit of information. However, in this article, the word bit is synonymous with a binary digit.

In classical computer technologies, a processed bit is implemented by one of two levels of low direct current voltage, and whilst switching from one of these two levels to the other, a so-called "forbidden zone" between two logic levels must be passed as fast as possible, inasmuch as electrical voltage cannot change amplitude instantaneously.

There are two possible outcomes for the measurement of a qubit, usually taken to have the values "0" and "1", like a bit. However, whereas the state of a bit can only be binary (either 0 or 1), the general state of a qubit according to quantum mechanics can be an arbitrary coherent superposition of all computable states simultaneously. Moreover, whereas a measurement of a classical bit would not disturb its state, a measurement of a qubit would destroy its coherence and irrevocably disturb the superposition state. It is possible to fully encode one bit in one qubit. However, a qubit can hold more information, e.g., up to two bits using superdense coding.

A bit is always completely in either one of its two states, and a set of n bits (e.g. a processor register or some bit array) can only hold a single of its 2ⁿ possible states at any time. A quantum state can be in a superposition state, which means that the qubit can have non-zero probability amplitude in both its states simultaneously (popularly expressed as "it can be in both states simultaneously"). A qubit requires two complex numbers to describe its two probability amplitudes, and these two complex numbers can together be viewed as a 2-dimensional complex vector, which is called a quantum state vector, or superposition state vector. Alternatively and equivalently, the value stored in a qubit can be described as a single point in a 2-dimensional complex coordinate space.

Furthermore, a set of n bits can be represented by n binary digits, simply by concatenating the representations of each of the bits, whereas a set of n qubits, which is also called a register, requires 2ⁿ complex numbers to describe its superposition state vector.

In quantum mechanics, the general quantum state of a qubit can be represented by a linear superposition of its two orthonormal basis states (or basis vectors). These vectors are usually denoted as ${\displaystyle |0\rangle ={\bigl [}{\begin{smallmatrix}1\\0\end{smallmatrix}}{\bigr ]}}$ and ${\displaystyle |1\rangle ={\bigl [}{\begin{smallmatrix}0\\1\end{smallmatrix}}{\bigr ]}}$. They are written in the conventional Dirac—or "bra–ket"—notation; the ${\displaystyle |0\rangle }$ and ${\displaystyle |1\rangle }$ are pronounced "ket 0" and "ket 1", respectively. These two orthonormal basis states, ${\displaystyle \{|0\rangle ,|1\rangle \}}$, together called the computational basis, are said to span the two-dimensional linear vector (Hilbert) space of the qubit.