In his famous ’arrow’ paradox, the greek philosopher Zeno negated motion, since it has to be a succession of instantaneous rests. A couple millenia later, we consider this paradox with amusement, as a testimony of times when derivatives and infinitesimal quantities were still to be discovered. However, Zeno is back when it comes to quantum mechanics ! A quantum measurement, even an ideal one, strongly affects the measured system’s state, as is described by the projection postulates. In the quantum world, a watched arrow cannot move, a watched kettle cannot boil. Experiments have confirmed this strange effect and demonstrated for instance that a watched spin cannot precess. Now, a team at ENS has observed a new and spectacular manifestation of the “Quantum Zeno Effect”. Repeated ideal measurements of the number of photons in a `photon box’, a very high quality superconducting cavity, inhibit the injection of a coherent field by a classical source. Quantum measurements stop a runaway process in which the field energy, if unimpeded, would grow quadratically with time towards mesoscopic values. This experiment sheds light on the quantum measurement process and on the back action of an energy measurement onto the field phase.
J. Bernu et al Phys. Rev. Lett. 101, 180402 (2008)