HAVING YOUR PHOTON AND SEEING IT TOO. Measuring a photon repeatedly without destroying it has been achieved for the first time, enabling researchers to study an individual quantum object with a new level of non-invasiveness. Physicists have long realized that it is possible to perform non-destructive observations of a photon with a difficult-to-execute technique known as a "quantum non-demolition" (QND) measurement. After many years of experimental effort, researchers in France (Serge Haroche, Ecole Normale Superieure, 011-33-1-4432-3420, haroche@physique.ens.fr) have demonstrated the first QND measurement of a single quantum object, namely a photon bouncing back and forth between a pair of mirrors (a "cavity"). A conventional photodetector measures photons in a destructive manner, by absorbing the photons and converting them into electrical signals. "Eating up" or absorbing photons to study them is not required by fundamental quantum mechanics laws and can be avoided with the QND technique demonstrated by the French researchers. In their technique, a photon in a cavity is probed without absorbing any net energy from it. (Of course, Heisenberg's Uncertainty Principle ensures that counting a photon still disturbs the "phase" associated with its electric and magnetic fields.) In the experiment, a rubidium atom passes through a cavity. If a photon is present, the atom acquires a phase shift which can easily be detected. Sending additional rubidium atoms through the cavity allowed the researchers to measure the photon repeatedly without destroying it or, as the French would say, "Avoir le beurre et l'argent du beurre" ("Getting the butter and money out of it at the same time"). This technique can allow physicists to study the behavior of a photon during its natural lifespan; it can potentially allow researchers to entangle (Update 414) an arbitrary number of atoms and build quantum logic gates (Update 250). (Nogues et al., Nature, 15 July; see also Scientific American, April 1993; figure at Physics News Graphics.)