A qubit (quantum + bit; pronounced /kyoobit/ [1] ) 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>.
See also: quantum computation
(A qubit is not to be confused with a cubit, which is an ancient measure of length equal to about 40 or 50cm.)
