# Real-time control of Cavity Quantum Electrodynamics Experiments

**Bruno Peaudecerf***University of Strathclyde, Glasgow*

Two key features at the boundary between the quantum world and the macroscopic one are the quantum measurement, which in its ideal form projects a quantum system in a measurement eigenstate, and the phenomenon of decoherence, which progressively destroys quantum superpositions. Cavity Quantum Electrodynamics (CQED) experiments are particularly suited to the study of both; in these experiments, long-lived microwave photons in a cavity can be probed by non-resonant atoms realising a quantum non-destructive measurement of the field. In these measurements a competition takes place between the time it takes to determine the quantum state (one needs to measure several atoms, as each atom only brings partial information), and the time it takes for decoherence to modify the quantum state through exchanges with its environment. The real-time control of such a system allows to achieve both a faster measurement, and counteract the effect of the decoherence

In this talk I will present a series of experiments performed in the CQED group of Serge Haroche in the Laboratoire Kastler Brossel during my PhD, that took advantage of the real-time control of the experiment. As the quantum non-destructive measurement of the field in a cavity requires the measurement of several atoms, real-time control can first help to optimise the use of these ressources, by tuning the parameters of an atom interferometer, realising an adaptive measurement that maximises the information acquired from each atom [Phys. Rev. Lett. 112, 080401 (2014)]. The measurement of a photon-number state can thus be sped-up by up to 45%.

Real-time control also allows to modify the quantum state of the field according to the results of measurements already performed. One can thus realise a quantum feedback loop which is able to stabilise a fragile quantum state, like a Fock state with a fixed number of photons, against the effects of decoherence. The measurement back-action makes this more difficult than classical feedback, but the long timescales associated with decoherence in our CQED setup allow to perform rather complex calculations during a single feedback loop. Two schemes have been implemented, making use of coherent photon injections [Nature (London) 477, 73 (2011)] or of resonant atomes injecting or absorbing one photon each [Phys. Rev. Lett. 108, 243602 (2012)], to stabilise Fock states with up to 7 photons against decoherence. These results pave the way to information-efficient quantum measurement and state reconstruction, and more generally to the management of the constraints of decoherence in metrology and quantum information.