Qubit
In quantum computing, a qubit () or quantum bit (sometimes qbit) is the basic unit of quantum information—the quantum version of the classical binary bit physically realized with a two-state device. A qubit is a two-state (or two-level) quantum-mechanical system, one of the simplest quantum systems displaying the peculiarity of quantum mechanics. Examples include: the spin of the electron in which the two levels can be taken as spin up and spin down; or the polarization of a single photon in which the two states can be taken to be the vertical polarization and the horizontal polarization. In a classical system, a bit would have to be in one state or the other. However, quantum mechanics allows the qubit to be in a coherent superposition of both states/levels simultaneously, a property which is fundamental to quantum mechanics and quantum computing.
Bell state
In this example, Alice is trying to communicate two bits of classical information, one of four two bit strings: formula_13or formula_14. If Alice chooses to send the two bit message formula_15, she would perform the phase flip formula_16to her qubit. Similarly, if Alice wants to send formula_17, she would apply a CNOT gate; if she wanted to send formula_14, she would apply the formula_19gate to her qubit; and finally, if Alice wanted to send the to bit message formula_20, she would do nothing to her qubit. Alice performs these quantum gate transformations locally, transforming the initial entangled state formula_21 into one of the four Bell states.