Quantum Institute: Visitor Schedule

The Quantum Lunch is regularly held on Thursdays in the Theoretical Division Conference Room, TA-3, Building 123, Room 121.
For more information, contact Diego Dalvit.

February 14, 2008
Thursday, 3 PM to 4 PM

Speaker: Serge Haroche,
Ecole Normale Supérieure and Collège de France, Paris

TOPIC: Quantum Non Demolition Counting of Photons and Schrödinger Cat Tomography in Cavity QED Experiments

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 amounts to 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 get pinned down to a single value by the QND procedure, the field phase is blurred. This is a manifestation of complementarity. 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 QND measurements 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.