Repeated quantum non-demolition measurements

Michel Bauer, Tristan Benoist, Denis Bernard

J.-Ph. Poizat, Jean-François Roch, Philippe Grangier

J. Mlynek, Gerhard Rempe, Stephan Schiller +1 more

Daniel F. Walls, Gerard J Milburn

Paul Busch, PEKKA J. LAHTI

Abstract The theory of unsharp quantum measurements is reviewed as a generalization of the von Neumann‐Lüders measurement theory and applied to measurements of continuous quantities. A generalized notion of repeatability is proposed which is applicable even to intrinsically unsharp continuous observables. As an illustration a precise formulation of a quantum non‐demolition (QND) measurement performed on a harmonic oscillator is given.

J.-Ph. Poizat, Jean-François Roch, Philippe Grangier

We present a review of the criteria developped to characterize the efficiency of quantum non-demolition measurement devices for travelling electromagnetic waves. These criteria describe respectively the quality of the measurement, the non-destruction of the signal, and the quantum correlations between the meter read-out and the signal output. Quantitative limits can be defined with respect to these criteria, delimiting classicad and quantum domains of operation. Moreover, some questions related to the gain on the signal channel, and to repeated QND measurements, will be also discussed.