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Quantum Non-Demolition counting of photons in Cavity QED

Serge Haroche, Ecole Normale Supérieure

(Session 1 : Friday from 08:45-09:30)

Abstract. Rydberg atoms crossing one by one a high-Q cavity extract information from the field stored in it, without absorbing the photons. The procedure realizes an ideal quantum-non demolition (QND) measurement of light. Initially prepared in a coherent state, the field quickly collapses into a Fock state of well-defined photon number, then undergoes successive jumps towards vacuum due to cavity relaxation. We have checked Planck's law and the predictions of quantum field theory by performing a statistical analysis of thousands of individual quantum trajectories recorded in this way. As the photon number is pinned down to a single value by the QND procedure, the field's phase is blurred. The first stage of this blurring process, induced by a single atom, prepares a photonic Schrödinger cat in the cavity, i.e. a coherent superposition of two field states with different phases. By displacing this cat state in phase space and performing a QND measurement on the translated field, we have reconstructed its Wigner function. It exhibits two classical components and, between them, an interference feature presenting negative parts. which is a signature of the cat state quantum coherence. This interference component vanishes much faster than the decay of the field intensity. This tomographic procedure opens the way to a direct investigation of the decoherence process on cat states containing up to a few tens of photons.