Quantum Tunneling is Not Instantaneous, Physicists Show

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Quantum Tunneling Is Not Instantaneous, Physicists Show:

Although it would not get you past a brick wall and onto Platform 93/4 to catch the Hogwarts Express, quantum tunneling-in which a particle "tunnels" through a seemingly insurmountable barrier-remains a confounding, intuition-defying phenomenon. Now Toronto-based experimental physicists using rubidium atoms to study this effect have measured, for the first time, just how long these atoms spend in transit through a barrier. Their findings appeared in Nature on July 22.

The researchers have showed that quantum tunneling is not instantaneous-at least, in one way of thinking about the phenomenon-despite recent headlines that have suggested otherwise. "This is a beautiful experiment," says Igor Litvinyuk of Griffith University in Australia, who works on quantum tunneling but was not part of this demonstration. "Just to do it is a heroic effort."

To appreciate just how bizarre quantum tunneling is, consider a ball rolling on flat ground that encounters a small, rounded hillock. What happens next depends on the speed of the ball. Either it will reach the top and roll down the other side or it will climb partway uphill and slide back down, because it does not have enough energy to get over the top.

This situation, however, does not hold for particles in the quantum world. Even when a particle does not possess enough energy to go over the top of the hillock, sometimes it will still get to the opposite end. "It's as though the particle dug a tunnel under the hill and appeared on the other side," says study co-author Aephraim Steinberg of the University of Toronto.

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