QUROPE Quantum Information Processing and Communication in Europe

S. Haroche (P2a CNRS), Exploring the nature of light in a photon box

M. Brune (P2a CNRS), invited talk, Quantum theory of measurament at work by photon counting in a box

I.Dotsenko (P2a CNRS), invited talk, Quantum feedback for preparation and protection of quantum states light

Igor Dotsenko (P2a CNRS), invited talk, Quantum measurament and Quantum feedback on light trapped in a cavity

http://www.sciencesetavenir.fr/actualite/fondamental/20110831.OBS9571/un-nouveau-piege-a-photons-ultra-performant.html

Le Monde, sept. 3rd, 2011, p.16: Un piègfe à lumière éternel tendu par des physiciens français

http://www2.cnrs.fr/en/1898.htm

Feedback loops are central to most classical control procedures. A controller compares the signal measured by a sensor (system output) with the target value or set-point. It then adjusts an actuator (system input) to stabilize the signal around the target value. Generalizing this scheme to stabilize a micro-system’s quantum state relies on quantum feedback, which must overcome a fundamental difficulty: the sensor measurements cause a random back-action on the system. An optimal compromise uses weak measurements, providing partial information with minimal perturbation.

We propose an engineered reservoir inducing the relaxation of a cavity field towards nonclassical states. It is made up of two-level atoms crossing the cavity one at a time. Each atom-cavity interaction is first dispersive, then resonant, then dispersive again. The reservoir pointer states are those produced by an effective Kerr Hamiltonian acting on a coherent field. We thereby stabilize squeezed states and quantum superpositions of multiple coherent components in a cavity having a finite damping time.

We explore experimentally the fundamental projective properties of a quantum measurement and their application in the control of a system’s evolution. We perform quantum non-demolition (QND) photon counting on a microwave field trapped in a very-high-Q superconducting cavity, employing circular Rydberg atoms as non-absorbing probes of light. By repeated measurement of the cavity field we demonstrated the freeze of its initially coherent evolution, illustrating the back action of the photon number measurement on the field’s phase.