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.
Superconducting quantum computing
where formula_23 is the shape of the microwave pulse in time, formula_24 is the resonance frequency of the second qubit, formula_25 are the Pauli matrices, formula_26 is the coupling coefficient between the two qubits via the resonator, formula_27 is the qubit detuning, formula_28 is the stray (unwanted) coupling between qubits and formula_29 is Planck constant divided by formula_30. The time integral over formula_23 determines the angle of rotation. Unwanted rotations due to the first and third terms of the Hamiltonian can be compensated with single qubit operations. The remaining part is exactly the controlled-X gate.