Non-destructive quantum measurements

He analyzed the fundamental properties of measurement in quantum physics and its relationship to the concepts of complementarity, entanglement and decoherence. He then focused on the analysis ofQuantum Non-Demolition (QND) measurements. These project the measured system into one of the eigenstates of the measured observable and can be repeated a large number of times, giving the same result as long as the system is not perturbed. This property makes them potentially useful for ultra-sensitive detection of small perturbations. The history of QND measurements of light was reviewed, and recent cavity quantum electrodynamics experiments enabling the non-destructive detection of single photons were described. These experiments are performed on fields of a few photons trapped for a fraction of a second in a cavity with a very high quality factor. They provide a new way of " see " light, by tracking stochastic photon number trajectories in real time and recording the quantum jumps associated with the loss or creation of a single quantum. These QND measurements enable complete reconstruction of the quantum state of the field in the cavity, paving the way for detailed study of " Schrödinger cat " states, coherent superpositions of photonic states of different phase or amplitude.