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posted by Fnord666 on Thursday July 23 2020, @01:56PM   Printer-friendly
from the apparently-you-can-get-there-from-here dept.

sorry takyon

Quantum Tunneling Is Not Instantaneous, Physicists Show:

Although it would not get you past a brick wall and onto Platform 9¾ 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.

Such weirdness is best understood by thinking of the particle in terms of its wave function, a mathematical representation of its quantum state. The wave function evolves and spreads. And its amplitude at any point in time and space lets you calculate the probability of finding the particle then and there—should you make a measurement. By definition, this probability can be nonzero in many places at once.

If the particle confronts an energy barrier, this encounter modifies the spread of the wave function, which starts to exponentially decay inside the barrier. Even so, some of it leaks through, and its amplitude does not go to zero on the barrier's far side. Thus, there remains a finite probability, however small, of detecting the particle beyond the barrier.

[...] Steinberg admits that his team's interpretation will be questioned by some quantum physicists, particularly those who think weak measurements are themselves suspect. Nevertheless, he thinks the experiment says something unequivocal about tunneling times. "If you use the right definitions, it's not really instantaneous. It may be remarkably fast," he says. "I think that's still an important distinction."

Journal Reference:
Ramón Ramos, David Spierings, Isabelle Racicot, et al. Measurement of the time spent by a tunnelling atom within the barrier region, Nature (DOI: 10.1038/s41586-020-2490-7)


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  • (Score: 2) by takyon on Thursday July 23 2020, @10:47PM (1 child)

    by takyon (881) <reversethis-{gro ... s} {ta} {noykat}> on Thursday July 23 2020, @10:47PM (#1025571) Journal

    Where'd you get that idea from, Brian Greene in The Elegant Universe [pbs.org]? It sounded like bullshit in 2003, and it still does today.

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  • (Score: 2) by mhajicek on Friday July 24 2020, @06:19AM

    by mhajicek (51) on Friday July 24 2020, @06:19AM (#1025679)

    It's a joke.

    And a copypasta horror genre.

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