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Any two-level quantum system can be used as a qubit.

Multilevel systems can be used as well, if they possess two states that can be effectively decoupled from the rest ( e. g., ground state and first excited state of a nonlinear oscillator ).

There are various proposals.

Several physical implementations which approximate two-level systems to various degrees were successfully realized.

Similarly to a classical bit where the state of a transistor in a processor, the magnetization of a surface in a hard disk and the presence of current in a cable can all be used to represent bits in the same computer, an eventual quantum computer is likely to use various combinations of qubits in its design.