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Rydberg atoms crossing one by one a high Q microwave cavity can be entangled to the field stored in this cavity, leading to fundamental tests in quantum physics. The amplitudes of the quantum states superpositions involved in the entangled states can be controlled and adjusted at will, realizing a kind of engineered entanglement at variance with the spontaneous entanglement achieved in photon cascade or down conversion experiments. The atomic coupling to the cavity field can be either resonant or dispersive. In the resonant case, entanglement results from the reversible quantum Rabi oscillation which coherently mixes atomic energy and photon number eigenstates. In the dispersive case, the atom and field undergo reciprocal frequency shifts, which produces phase dependent entanglement. By combining resonant and dispersive techniques, three or more atoms could be entangled together. The atom-cavity system is also ideal to entangle an atom to a mesoscopic coherent field made of several photons. Superpositions of coherent fields of the kind imagined by Schrödinger in his famous cat metaphor have been generated and studied. The decoherence of these states has been observed, providing new insight into the quantum-classical boundary. The generalization of these experiments to complex systems, involving more atoms, more photons and more cavities, is under way.

DOI: 10.1007/978-94-017-1454-9_12

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