The advice that a watched pot won't boil may ring true in your kitchen, but in the quantum realm, anything goes. Recent experiments by Mark Raizen and his colleagues at the University of Texas have demonstrated that frequent measurements can slow down or speed up the rate at which particles in a quantum system decay from high- to low-energy states. The work therefore confirms two contradictory predictions made 20 years apart. In 1977, E.C. George Sidarshan and Baidyanaith Misra of the University of Texas proposed that continuous measurements would gum up quantum decay, an effect they named after Zeno, the 5th-century philosopher famous for his paradoxes of motion. Just last year, Abraham Kofman and Gershon Kurizki of the Weizmann Institute argued for an opposite, anti-Zeno effect.
To explore these predictions in the lab, Raizen's team trapped sodium atoms in a light wave. This system, left to its own devices, slowly decays as individual atoms escape by way of tunneling through what should be an insurmountable energy barrier. But the researchers noticed that the tunneling rate slowed dramatically when they measured the system every millionth of a second. When they then measured the system every five millionths of a a second, the tunneling rate increased. "The Zeno effect has until now only been seen in very simple systems, and the anti-Zeno effect has never been seen," Kofman told Nature Science Update. "It's an outstanding experiment, in the spirit of the original proposal of the Zeno effect."