## Van der Waals-Zeeman Colloquium

Tuesday, 1 December 2009, 16:00

**Manipulating and probing microwave fields in a cavity by quantum nondemolition photon counting**

**Prof. dr. Michel Brune**

Laboratoire Kastler Brossel, Département de physique de l'ENS

We perform quantum nondemolition photon (QND) counting in a high Q cavity. Microwave photons are stored in a superconducting cavity for time as long as 0.1s. By using Rydberg atoms as clocks whose ticking rate is affected by light shifts induced by the cavity field, we realize an ideal projective measurement of the photon number. The experiment demonstrates all features of quantum measurement theory: random results, state projection and repeatabily.

Measuring the atoms is an efficient way for preparing by projection non-classical states such as number states or Schrödinger cat states. We use the QND measurement method for reconstructing the Wigner function of these states and to monitor their decoherence. These field manipulation methods can be applied to state preparation by quantum feedback and to demonstrate non-locality with two fields located in separated cavities.