The Qubit reference article from the English Wikipedia on 24-Apr-2004
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Qubit

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A qubit is not to be confused with a cubit, which is an ancient measure of length.

A qubit (quantum + bit; pronounced /kyoobit/ [1] ) is a quantum state in a two-dimensional complex vector space. A qubit 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 qubit's most important distinction from a classical bit, however, is not the continuous nature of the state (which can be replicated by any analog quantity), but the fact that multiple qubits can experience quantum entanglement. Entanglement is a nonlocal interaction that allows a set of qubits to express superpositions of different binary strings (01010 and 11111, for example) simultaneously. Such "quantum parallelism" is the key to the potential power of quantum computation.

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>.

See also