Quantum bits, or “qubits,” are the quantum-mechanical analogue of the minimum information unit in a classical computer. A conventional electronic binary digit, abbreviated “bit,” can have only one of two values: on or off, 0 or 1, as represented by electrical charges, voltages or magnetization. A quantum bit, however, can be 0, 1, or a “superposition” of both at once, owing to the inherently indeterminate nature of unmeasured quantum states.

Examples include photon qubits, whose orientation can be a superposition of horizontal and vertical polarization; electron spin qubit, where the two states are spin-up and spin-down orientations of the electron’s spin; flux qubit, where the two states correspond to two magnetic flux values in a Josephson junction.