For the first time, physicists have seen cold atoms suddenly reverse their motion like a ball spontaneously changing direction mid-flight and returning to the thrower's hand. What's more, one group reports, chaos in the quantum world surrounding the atoms accelerates this weird process almost a thousand-fold.
How different from that standby of predictability, the pendulum. A pendulum bob swings down from left to right, picking up speed as it drops and slowing as it rises to a stopping point. Then it turns around and swings back to the left until it stops again. Classical physics says the pendulum can change direction only at the two end points, and generations of grandfather clocks have agreed. Quantum mechanics, however, predicts that there is a tiny chance that the pendulum can reverse direction at any point, a process physicists call dynamical tunneling.
To see dynamical tunneling in action, two teams--one based at the National Institute of Standards and Technology (NIST) in Gaithersburg, Maryland, the other at the University of Texas, Austin--first used a complicated series of laser beams and magnetic fields to cool atoms of cesium or sodium to a temperature of a few billionths of a degree above absolute zero. Then other light beams maneuvered the atoms onto one of two islands of stability. Each "island" is a set of quantum states for particles at the same point in space but with opposite momentums, like pendulums swinging in different directions. Classical physics says that once a particle is marooned on an island of stability, it can never leave. But both teams saw particles flip from one island to another.
"This is an exciting new quantum effect," says physicist Winfried Hensinger of the University of Queensland in Australia, a member of the NIST team that reported these results in the 6 July issue of Nature.
The second team, led by Mark Raizen of the University of Texas, was surprised to find that the atoms jumped from island to island "a thousand times faster" than theories of dynamical tunneling predicted, says Raizen. They suggest in a report published online by [http://www.sciencemag.org/cgi/content/abstract/1061569v1] Science on 6 July that a sea of nearby chaotic quantum states helps the particles on the journey from one island to another.