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Quantum Tunneling is Not Instantaneous, Physicists Show

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

upstart writes in with an IRC submission for RandomFactor:

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)

Original Submission

 
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  • (Score: 0) by Anonymous Coward on Thursday July 23 2020, @02:59PM (7 children)

    by Anonymous Coward on Thursday July 23 2020, @02:59PM (#1025409)

    i guess this means we can save ourselfs the cost of digging a tunnel thru the mountain but not save ourselfs the travelling time. einstein would be happy not having to take down all the speed limit signs ...

  • (Score: 4, Informative) by c0lo on Thursday July 23 2020, @04:13PM (3 children)

    by c0lo (156) Subscriber Badge on Thursday July 23 2020, @04:13PM (#1025438) Journal

    Ummm... Some TFA quotes

    In 1993, for example, Steinberg, Paul Kwiat and Raymond Chiao, all then at the University of California, Berkeley, detected photons tunneling through an optical barrier (a special piece of glass that reflected 99 percent of the incident photons; 1 percent of them tunneled through). The tunneling photons arrived earlier, on average, than photons that traveled the exact same distance but were unimpeded by a barrier. The tunneling photons seemed to be traveling faster than the speed of light.

    their measurement is based on phase velocity

    But then

    Last year Litvinyuk and his colleagues published results showing that when electrons in hydrogen atoms are confined by an external electric field that acts like a barrier, they occasionally tunnel through it [nature.com]. As the external field oscillates in intensity, so does the number of tunneling electrons, as predicted by theory. The team established that the time delay between when the barrier reaches its minimum and when the maximum number of electrons tunnel through was, at most, 1.8 attoseconds (1.8 x 10–18 second). Even light, which travels at about 300,000 kilometers per second, can only travel over three ten-billionths of a meter, or about the size of a single atom, in one attosecond. “[The time delay] could be zero, or it would be some zeptoseconds [10–21 second],” Litvinyuk says.

    Some media reports controversially claimed that the Griffith University experiment had shown tunneling to be instantaneous. The confusion has a lot to do with theoretical definitions of tunneling time. The type of delay the team measured was certainly almost zero, but that result was not the same as saying the electron spends no time in the barrier. Litvinyuk and his colleagues had not examined that aspect of quantum tunneling.

    Steinberg’s new experiment claims to do just that. His team has measured how long, on average, rubidium atoms spend inside a barrier before they tunnel through it. The time is of the order of a millisecond—nowhere close to instantaneous.
    ...
    They also verified another strange prediction of quantum mechanics: the lower the energy, or slower the movement, of a tunneling particle, the less time it spends in the barrier. This result is counterintuitive, because in our everyday notion of how the world works, a slower particle would be expected to remain in the barrier for a longer stretch of time.

    --
    https://www.youtube.com/watch?v=aoFiw2jMy-0 https://soylentnews.org/~MichaelDavidCrawford

    • (Score: 2) by Muad'Dave on Thursday July 23 2020, @04:45PM (2 children)

      by Muad'Dave (1413) on Thursday July 23 2020, @04:45PM (#1025452)

      The value obtained by this research was nearly 1 mS. That's an eternity! It'll be interesting where the answer lies between 1.8x10-18 and about 10-3 seconds.

      • (Score: 2) by c0lo on Thursday July 23 2020, @05:11PM

        by c0lo (156) Subscriber Badge on Thursday July 23 2020, @05:11PM (#1025467) Journal

        They obtained attosecond (between the min barrier and max number of particle tunneling) using electrons tunneling an electric field barrier - and they didn't want to say anything about the actual tunneling time.

        The 1ms is for rubidium atoms tunneling a weak magnetic field barrier.

        --
        https://www.youtube.com/watch?v=aoFiw2jMy-0 https://soylentnews.org/~MichaelDavidCrawford

      • (Score: 2) by Kitsune008 on Thursday July 23 2020, @05:19PM

        by Kitsune008 (9054) on Thursday July 23 2020, @05:19PM (#1025470)

        Yeah, that 1 mS surprised me also. But I find myself strangely reassured that it appears to not be instantaneous, or sometimes appearing to 'get there before it leaves'. ( no wonder Einstein developed that wild, flyaway hairdo after quantum mechanics popped out!) :-)

        But with most of what I have come to understand(Ha!Ha!) about quantum physics, the answer may actually be a range of values between 1.8x10-18 and about 10-3 seconds; I would expect to see multiple answers, with some more probable than others, instead of one or only several answers.

        I agree that this will be interesting to see results from more research.

  • (Score: 2) by mhajicek on Thursday July 23 2020, @09:37PM (2 children)

    by mhajicek (51) on Thursday July 23 2020, @09:37PM (#1025551)

    If you clear your mind enough while leaning against a wall sometimes you can noclip through. Careful though, you might find yourself in the backrooms.

    --
    The spacelike surfaces of time foliations can have a cusp at the surface of discontinuity. - P. Hajicek

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

      by takyon (881) <takyonNO@SPAMsoylentnews.org> 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.

      --
      [SIG] 10/28/2017: Soylent Upgrade v14 [soylentnews.org]

      • (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.

        --
        The spacelike surfaces of time foliations can have a cusp at the surface of discontinuity. - P. Hajicek