Repeated Quantum Non-Demolition measurement of light: Fock states and quantum trajectories

Measurement postulates, which define a Quantum Non-Demolition (QND) measurement, predict that the minimal perturbation on a measured object is a projection of its state. The measurement apparatus usually used work far beyond this minimal limit. In particular photodetectors absorb the photons they detect, thus destroying them. In our cavity quantum electrodynamics experiment, circular Rydberg atoms and microwave photons confined in a superconducting cavity interact in the strong coupling regime. After the interaction, the two systems are entangled: each of them carries information about the other. In the detuned case, the effect of the interaction is a simple energy shift of the atomic levels, inducing a phase shift of the dipole proportional to the photon number, that can be measured by Ramsey interferometry. Atoms thus deliver information about the field photon number without having modified it. Following this principle, we could implement a repeated QND measurement of the photon number, thanks to our very long cavity lifetime. The photon number evolution in the presence of relaxation reveals sudden jumps called quantum jumps. Our experiment allowed the first observation of this behaviour for light. By describing the information delivered by each atomic detection according to Bayes law, we could follow the progressive collapse of a coherent state towards Fock states containing up to seven photons. The statistical analysis of our results gives a very clear illustration of quantum measurement postulates. (author)