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A qubit (quantum + bit; pronounced "Q-bit") is a two-state quantum system which is the smallest unit of quantum information. Its two basic states are conventionally labeled |0⟩ and |1⟩ (pronounced: ket 0 and ket 1). A pure qubit state is a linear quantum superposition of those two states. This is significantly different from the state of a classical bit, which can only take value 0 or 1.

A number of qubits taken together is a qubit register[?]. Quantum computers perform calculations by manipulating qubits.

It is also possible to have a three-state system, called a qutrit[?], whose states are conventionally labeled |0⟩, |1⟩ and |2⟩.

(A qubit is not to be confused with a cubit, which is an ancient measure of length equal to about 40 or 50cm.)

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