Quantum entanglement and information with cold atoms

The first lesson reviewed the essential concepts of quantum information, qubits and quantum gates. The link between logic gates, entanglement and non-destructive measurement was recalled, as well as the definition of mesoscopic superpositions of states and the essential properties of decoherence. Some examples of quantum gates involving intrinsic nonlinearities were briefly reviewed (atom-photon gates in cavity quantum electrodynamics and gates between trapped ions). The case of photonic gates realized with separator blades was presented in a little more detail. In this case, there is no intrinsic nonlinearity, but a post-selection process introduces an effective nonlinearity, enabling the realization of probabilistic gates conditional on measurement results. This is the principle, briefly recalled, of the "all-optical" quantum computer. The lesson concluded with the idea that cold atom physics, with its matter waves coupled together by the intrinsic nonlinearities associated with inter-atomic collisions, had advantages over ordinary optics for the demonstration of entanglement and quantum information processes.