Abstract
In the fourth lesson, we showed how the superconducting phase qubit could be used in quantum information processing operations. After briefly describing the relaxation of superconducting qubits, analyzing some essential causes of dissipation and giving orders of magnitude of their coherence times, we described experiments carried out in John Martinis' group at the University of California, Santa Barbara. We began by describing a few concrete devices coupling together two or more qubits, giving an idea of the complexity of experiments currently possible. We then described in greater detail an experiment involving the manipulation of an isolated qubit (observation of Rabi oscillations), followed by an experiment involving the entanglement of two qubits, an operation which, combined with rotations of the qubit states, amounts to the realization of a " control-not " logic gate. We have also shown how the independent measurement of qubits using SQUIDS enables their density operator to be reconstructed by quantum tomography. Finally, we describe an experiment that tested Bell's inequalities for the first time in a mesoscopic system.
