What is a qubit? Just as a classical bit has a state – either 0 or 1 – a qubit also has a state. Two possible states for a qubit are the states Ket 0 and Ket 1, which are similar to the states 0 and 1 of a classical bit.

The difference between bits and qubits is that, unlike bits, qubits can be in a state other than Ket 0 or Ket 1. It is perfectly legal to have a linear combination of these two states.

This phenomenon is called quantum superposition.

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