NON-EXPONENTIAL DECAY of a quantum system has been observed for the first time. Many unstable systems such as a mass of radioactive nuclei will characteristically undergo a process (quantum tunneling) whereby the number of nuclei remaining in an initial state after a time t will be proportional to e^-at, where a is a constant. Quantum mechanics does not expressly forbid non-exponential decay, and physicists at the University of Texas have now devised a scheme---sodium atoms trapped in a web of laser light---in which the rate of atoms escaping (under the auspices of quantum tunneling) is non-exponential, at least over intervals on the order of 10 microseconds. (Nature, 5 June 1997.)