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We have performed multiparticle entanglement experiments with circular Rydberg atoms crossing one at a time a high Q superconducting microwave cavity. Two-level atoms and a zero or one photon field stored in the cavity act as qubits carrying quantum information. Controlled qubit entanglement is produced by the quantum Rabi oscillation coupling the atom to the cavity field. Qubit state superpositions are produced and analyzed by classical microwave pulses before and after the atom cross the high Q cavity, using a Ramsey interferometer arrangement. We have demonstrated the coherent operation of a quantum phase gate and used it to perform for the first time a quantum nondestructive measurement of a single photon. Combining this gate with quantum Rabi oscillations of various durations, we have entangled step by step three subsystems — two atom and one field mode — by a controlled succession of one and two qubit operations. Once some limitations of our experiment are overcome, it will be generalized to larger number of particles, opening the way to the study of even more complex entangled states. [ABSTRACT FROM AUTHOR]