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.
Quantum circuit
For quantum circuits a similar composition of qubit gates can be defined. That is, associated to any "classical assemblage" as above, we can produce a reversible quantum circuit when in place of "f" we have an "n"-qubit gate "U" and in place of "g" we have an "m"-qubit gate "W". See illustration below: