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Quantum feedback

The stability of complex systems, as well as many of our everyday actions rely on feedback loops. A probe measures the system state, a controller determines the action leading it towards the chosen operating point and an actuator realizes this action. Juggling, for instance, relies on numerous feedback loops from the eye to the hands through the brain.
Transferring the feedback concept to the quantum world is an important goal. Quantum physics allows one to realize tasks out of the reach of classical systems, such as precision measurements or complex information processing. The quantum states used for theses tasks are fragile, the most useful being also the most fragile. They must be protected. Feedback is a possible protection mechanism.
Quantum feedback must face a difficulty : quantum measurement changes the state of the system. The “Cavity Quantum Electrodynamics” group at LKB, in cooperation with Mines-ParisTech and INRIA, realized the first continuous quantum feedback experiment. They maintain a constant number of photons in a ’photon box’, a high-quality cavity. Atoms perform quantum measurements of the photon number, weak enough to avoid spoiling the state. The controller and the actuator correct the effect of this measurement and of the losses. Each loss of a photon in the environment is detected and corrected in real time. This experiment is an important step towards the control of complex and potentially useful quantum states.

Bibliography :

Real-time quantum feedback prepares and stabilizes photon number states, C. Sayrin, I. Dotsenko, X. Zhou, B. Peaudecerf, T. Rybarczyk, S. Gleyzes, P. Rouchon, M. Mirrahimi, H. Amini, M. Brune, J.M. Raimond et S. Haroche, 1er septembre 2011.

Contacts :

Jean-Michel Raimond l T 33 (0)1 44 32 34 88 l jmr@lkb.ens.fr
Serge Haroche l T 33 (0)1 44 32 34 20 l haroche@lkb.ens.fr