A quantum bit or qubit is a unit of quantum information. It is the quantum analogue of the classical bit. It is described by a state vector in a two-level quantum-mechanical system, which is formally equivalent to a two-dimensional vector space over the complex numbers. Bit versus qubit A bit is the basic unit of computer information. Regardless of its physical realization, a bit is always understood to be either a 0 or a 1. An analogy to this is a light switch— with the off position repr… (More on Qubit)