Qubit
In quantum computing, a qubit () or quantum bit (sometimes qbit) is the basic unit of quantum information—the quantum version of the classical 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 states can be taken to be the vertical polarization and the 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 coherent superposition of both states/levels simultaneously, a property which is fundamental to quantum mechanics and quantum computing.
Qubit field theory
In many ordinary quantum field theories, constraining one observable to a fixed value results in the uncertainty of the other observable being infinite (c.f. uncertainty principle), and as a consequence there is potentially an infinite amount of information involved. In the situation of the standard position-momentum commutation (where the uncertainty principle is most commonly cited), this implies that a fixed, finite, volume of space has an infinite capacity to store information. However, Bekenstein's bound hints that the information storage capacity ought to be finite. Qubit field theory seeks to resolve this issue by removing the commutation restriction, allowing the capacity to store information to be finite; hence the name qubit, which derives from quantum-bit or quantised-bit.