A qubit is a quantum system in which the Boolean states 0 and 1 are rep- resented by a prescribed pair of normalised and mutually orthogonal quantum states labeled as ${|0⟩, |1⟩}$
According to [1]. Then a quantum register $\mid x_1x_2...x_n\rangle, x_i\in\{0,1\}$ is defined to be collection of n qubits.
Now I often see expressions like $\mid x_1, ... x_n \rangle$ where the $x_i$ belong to some $S \subset \mathbf{Z}$.
- Can the individual constituents $\mid x_i \rangle$ be called qubits even though they are non-binary?
- Would it be appropriate to call $\mid x_1, ... x_n \rangle$ a qubit register in this case?
- What is the physical interpretation of such a register?