Physical and logical qubits
In quantum computing, a qubit is a unit of information analogous to a bit (binary digit) in classical computing, but it is affected by quantum mechanical properties such as superposition and entanglement which allow qubits to be in some ways more powerful than classical bits for some tasks. Qubits are used in quantum circuits and quantum algorithms composed of quantum logic gates to solve computational problems, where they are used for input/output and intermediate computations.
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.