# Feedback loops applied to the measurement and control of light in a cavity

### Thursday, 23 Jan. 2014, 13:00 - 14:00

**Presenter:** Dr. Bruno Peaudecerf, Laboratoire Kastler Brossel, Paris

**Host:** Prof. Markus Arndt

**Where:** Zimmer 55, 1st Floor, Strudlhofgasse 4, 1090 Vienna

I will present the real-time control experiments that were performed in the cavity QED group of Serge Haroche at LKB (Paris) during the time of my PhD. In our experiments, a stream of Rydberg atoms interacting dispersively with the microwave field in a cavity perform a quantum non-demolition measurement of the photon number by a succession of weak measurements. Taking into account all measurement outcomes, the measurement back-action and all experimental imperfections, a control computer can estimate the state of the field in real time. This allows for the implementation of feedback loops in the experiment, aiming at improving the measurement efficiency, or controlling the quantum state of light.

We first used this real-time information to prepare and stabilize a target Fock state of light. Upon each atomic detection one can for example inject in the cavity a small classical field, the amplitude of which is optimized to bring the state closer to the target. In another strategy we also used resonant atoms, injecting or removing one photon at a time from the field, to stabilize efficiently the photon number in the cavity. This way Fock states from 1 to 7 photons could be prepared on demand and stabilized against decoherence.

The information from past measurements can also be used to adapt the measuring apparatus for future measurements. We have realized the adaptive non-demolition measurement of the photon number, where by adapting the parameters of the atomic measurements in real-time, we improve the extraction of information from the field, and thus the time it takes to project it in a Fock state. We demonstrated a significant improvement of convergence times. This improvement, combined with control strategies, could allow to reach more fragile, higher energy states.