Single Microwave Photons Can Be Measured Nondestructively

A deeply held tenet of quantum mechanics, dating back to its infancy, is that one can't measure a system without disturbing it. That doesn't mean, however, that one can't influence the form such a disturbance takes. Over the past 25 years, researchers have been developing schemes for controlling the effects of measurements so that the properties of interest emerge unscathed. Now a group led by Serge Haroche at the Ecole Normale Supérieure (ENS) in Paris has demonstrated such techniques at the most fundamental level—detecting the presence of a single photon in a nondestructive way.