Quantum non-demolition measurements in optics

Quantum non-demolition measurements are designed to circumvent the limitati ons imposed by Heisenberg's uncertainty principle when performing repeated measurements of quantum states. Recent progress in quantum optics has enabl ed the experimental realization of quantum non-demolition measurements of t he photon flux of a light beam. This achievement bears on fundamental issue s about the ultimate sensitivity of measurements, and may open the way for applications such as noise-free information tapping in optical telecommunic ations.