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Physicists watch quantum particles tunnel through solid barriers. Here's what they found.

Abstract image of quantum tunneling or space-time wormholes.
(Image: © Shutterstock)

The quantum world is a pretty wild one, where the seemingly impossible happens all the time: Teensy objects separated by miles are tied to one another, and particles can even be in two places at once. But one of the most perplexing quantum superpowers is the movement of particles through seemingly impenetrable barriers.

Now, a team of physicists has devised a simple way to measure the duration of this bizarre phenomenon, called quantum tunneling. And they figured out how long the tunneling takes from start to finish — from the moment a particle enters the barrier, tunnels through and comes out the other side, they reported online July 22 in the journal Nature

Quantum tunneling is a phenomenon where an atom or a subatomic particle can appear on the opposite side of a barrier that should be impossible for the particle to penetrate. It's as if you were walking and encountered a 10-foot-tall (3 meters) wall extending as far as the eye can see. Without a ladder or Spider-man climbing skills, the wall would make it impossible for you to continue. 

Related: The 18 biggest unsolved mysteries in physics

However, in the quantum world, it is rare, but possible, for an atom or electron to simply "appear" on the other side, as if a tunnel had been dug through the wall. "Quantum tunneling is one of the most puzzling of quantum phenomena," said study co-author Aephraim Steinberg, co-director of the Quantum Information Science Program at Canadian Institute for Advanced Research. "And it is fantastic that we're now able to actually study it in this way."

Quantum tunneling is not new to physicists. It forms the basis of many modern technologies such as electronic chips, called tunnel diodes, which allow for the movement of electricity through a circuit in one direction but not the other. Scanning tunneling microscopes (STM) also use tunneling to literally show individual atoms on the surface of a solid. Shortly after the first STM was invented, researchers at IBM reported using the device to spell out the letters IBM using 35 xenon atoms on a nickel substrate.

While the laws of quantum mechanics allow for quantum tunneling, researchers still don't know exactly what happens while a subatomic particle is undergoing the tunneling process. Indeed, some researchers thought that the particle appears instantaneously on the other side of the barrier as if it instantaneously teleported there, Sci-News.com reported

Researchers had previously tried to measure the amount of time it takes for tunneling to occur, with varying results. One of the difficulties in earlier versions of this type of experiment is identifying the moment tunneling starts and stops. To simplify the methodology, the researchers used magnets to create a new kind of "clock" that would tick only while the particle was tunneling.

Subatomic particles all have magnetic properties and when magnets are in an external magnetic field, they rotate like a spinning top. The amount of rotation (also called precession) depends on how long the particle is bathed in that magnetic field. Knowing that, the Toronto group used a magnetic field to form their barrier. When particles are inside the barrier, they precess. Outside it, they don't. So measuring how long the particles precess told the researchers how long those atoms took to tunnel through the barrier.

Related: 18 times quantum particles blew our minds

"The experiment is a breathtaking technical achievement," said Drew Alton, physics professor at Augustana University, in South Dakota. 

The researchers prepared approximately 8,000 rubidium atoms, cooled them to a billionth of a degree above absolute zero. The atoms needed to be this temperature, otherwise they would have moved around randomly at high speeds, rather than staying in a small clump. The scientists used a laser to create the magnetic barrier; they focused the laser so that the barrier was 1.3 micrometers (microns) thick, or the thickness of about 2,500 rubidium atoms. (So if you were a foot thick, front to back, this barrier would be the equivalent of about half a mile thick.) Using another laser, the scientists nudged the rubidium atoms toward the barrier, moving them about 0.15 inches per second (4 millimeters/s).

As expected, most of the rubidium atoms bounced off the barrier. However, due to quantum tunneling, about 3% of the atoms penetrated the barrier and appeared on the other side. Based on the precession of those atoms, it took them about 0.6 milliseconds to traverse the barrier.

Chad Orzel, an associate professor of physics at Union College in New York, who was not part of the study, applauded the experiment, "Their experiment is ingeniously constructed to make it difficult to interpret as anything other than what they say," said Orzel, author of "How to Teach Quantum Mechanics to Your Dog" (Scribner, 2010) It "is one of the best examples you'll see of a thought experiment made real," he added.

Experiments exploring quantum tunneling are difficult and further research is needed to understand the implications of this study. The Toronto group is already considering improvements to their apparatus to not only determine the duration of the tunneling process, but to also see if they can learn anything about velocity of the atoms at different points inside the barrier. "We're working on a new measurement where we make the barrier thicker and then determine the amount of precession at different depths," Steinberg said. "It will be very interesting to see if the atoms' speed is constant or not."

In many interpretations of quantum mechanics, it is impossible — even in principle — to determine a subatomic particle's trajectory. Such a measurement could lead to insights into the confusing world of quantum theory. The quantum world is very different from the world we're familiar with. Experiments like these will help make it a little less mysterious. 

Originally published on Live Science.

  • tommytoes
    Trying to wrap my head around the math here and was hoping someone could help me out.

    moving them about 0.15 inches per second (4 millimeters/s)
    barrier was 1.3 micrometers (microns) thick
    it took them about 0.6 milliseconds to traverse the barrier

    Was there a rate of acceleration or deceleration here upon encountering the barrier? It seems slow versus the initial observations that it's instant or near instant.
    Reply
  • David F Walter
    tommytoes said:
    Trying to wrap my head around the math here and was hoping someone could help me out.

    moving them about 0.15 inches per second (4 millimeters/s)
    barrier was 1.3 micrometers (microns) thick
    it took them about 0.6 milliseconds to traverse the barrier

    Was there a rate of acceleration or deceleration here upon encountering the barrier? It seems slow versus the initial observations that it's instant or near instant.
    RESPONSE:
    At a velocity of 4 mm/sec, the rubidium atom would need 0.33 msec to traverse 1.3 microns. This is 1.3 * 10^-6 / 4*10^-3 seconds. With the observed time of 0.6 msec to cross the barrier, the implication is that the barrier induced a 0.27 msec delay.

    It would be interesting to know the exit velocity from the barrier. This might indicate whether the tunneling changed the momentum of the particle reversibly or irreversibly inside the barrier. Or otherwise stated, does the particle return to its original entrance velocity or not as it exits the barrier.

    David F Walter
    Reply
  • EMN
    I believe David's calculations are correct. However there is a round-off error in the article.

    .15 inch/sec = 3.81 mm/sec. This means it lost a little less speed , but it still lost some.

    David's question is what I am curious about as well. What was the speed after it exited and what direction was it in compared to the initial direction.

    -Eric
    Reply
  • Valentine Michael Smith
    Fnord.


    David F Walter said:
    ...the implication is that the barrier induced a 0.27 msec delay.
    Reply
  • SteveMDFP
    "The barrier induced a 0.27 msec delay."
    Where *exactly* was the atom during that 0.27 msec? I'm guessing it was in superposition, on both sides of the barrier simultaneously.
    Reply
  • cashme
    admin said:
    A team of physicists has devised a simple way to measure the of a bizarre phenomenon called quantum tunneling.

    Physicists watch quantum particles tunnel through solid barriers. Here's what they found. : Read more
    "Fundamental particles such as an electron are real, extended particles each of size equal to its De Broglie wavelength, rather than a point-particle-probability-wave. Potential energy is gained as the particle traverses the barrier that is cleared; even though its initial kinetic energy was less than the barrier height. Energy conservation is obeyed at all times. Tunnelling arises from physical laws.":
    This understanding was known since 1990, by derivations realized without any assumptions of waves, without the use of any of the 41 free parameters or postulates that are required to try and make quantum mechanics work. The first principles are :

    The first principles used were:
    1 ) Haus re-derivation of Goedecke’s non radiation condition in a new way(1986 at MIT):
    Haus, H. A. (1986). "On the radiation from point charges". American Journal of Physics. 54 (12): 1126–1129. Bibcode:1986AmJPh..54.1126H2 ) Maxwell’s/Heavysides’s electro-magnetic formulas
    3 ) Einstein’s Special Theory of Relativity
    4 ) The Stern-Gerlach Experiment
    5 ) The DeBroglie Matter-Wave formulations

    These first principles were first used in 1986, by Herman Haus, Institute professor of electronic engineering at MIT, to answer the question, "What is the quantum level mechanism at work in the Free Electron Laser?" This was asked by the USA Department of Defence. The answer, derived by Haus, was a purely classical mechanism and which answer was accepted by the DoD in 1986 and then by the academic physics community in 2019. Using those same first principles, Randell Mills with the help of Hermann Haus, started the derivation of the Grand Unified Theory-Classical Physics (GUT-CP) and later with the help of John J. Farrell, a Chemistry professor at Franklin and Marshall College, published in 1990, the fuller thesis of this theory.
    By using GUT-CP as a guide to guide their development, at least four items were developed. Since not one items has been successfully developed using accepted Standard Quantum Mechanics then, the case for GUT-CP is much greater than that for SQM.

    Do consider this in a serious way, and do not just try to ban me from this site. There are many others who will continue to spread the word.
    Reply
  • TorbjornLarsson
    Very cool (literary) experiment! While simple in principle and complicated in practice, they can tease apart the effects both experimentally and theoretically. The measurement is weak, so they have to integrate out the result - the ground state of the tunneling require simulation over an imaginary time axis - and they get that "about 40% of the measured time for the lowest incident energy comes from the time spent in the classically forbidden region."

    And of course there is the traversal time estimate: "At the lowest incident velocity (4.1 mm s^−1), we observe a transmission probability of 3%. Given the energy dependence of the transmission, we calculate that the transmitted atoms have a velocity distribution with a peak at 4.8 mm s^−1, corresponding to κd ≈ 3. About three-quarters of this distribution cor-responds to energies below the barrier height. The measured traversal time τ_y is 0.61(7) ms."

    David F Walter said:

    It would be interesting to know the exit velocity from the barrier. This might indicate whether the tunneling changed the momentum of the particle reversibly or irreversibly inside the barrier. Or otherwise stated, does the particle return to its original entrance velocity or not as it exits the barrier.

    Unfortunately the measurement is complicated and the particle behavior also depends on a measurement backaction.

    "Considering incident particles polarized in the x direction and a magnetic field along z, one would expect the spin to precess by an angle θ = ω_Lτ, where ω_L is the Larmor frequency and τ is the time spent in the barrier. By working in the limit of a weak magnetic field (ω_L →0), this time can be measured without substantially perturbing the tunnelling particle. Büttiker15 noted that even in this limit, measurement backaction cannot be neglected, and it results in preferential transmission of atoms aligned with the magnetic field. This leads to two spin rotation angles: a precession in the plane orthogonal to the applied magnetic field, θ_y, as well as an alignment along the direction of the field, θ_z. He defined times associated with the spin projections: τ_z, τ_y and τ_x=sqrt(t_y^2+t_s^2); the latter is often known as the ‘Büttiker time’. It turns out that combinations of two such quantities appear in other theoretical treatments as a single complex number33,34, but researchers were hesitant to accept complex-valued times without a clear interpretation. Later, further studies11,12 associated τ_y and τ_z with the real and imaginary parts of the ‘weak value’13 of a dwell-time operator, thereby providing them with distinct interpretations as the inherent tunnelling time and the measurement backaction, respectively."

    "We investigate the two Larmor times by performing full-spin tomography of the transmitted spin-½ particles. Rotations after the scattering event enable us to measure the spin components along the x, y and z axes of the Bloch sphere (Fig. 4b). From the different projections, we find the traversal time τ_y and the time τ_z associated with the backaction of the measurement (Fig. 4c). At the lowest incident velocity (4.1 mm s^−1), we observe a transmission probability of 3%. Given the energy dependence of the transmission, we calculate that the transmitted atoms have a velocity distribution with a peak at 4.8 mm s^−1, corresponding to κd ≈ 3. About three-quarters of this distribution corresponds to energies below the barrier height. The measured traversal time τy is 0.61(7) ms."

    "The total duration of the simulations is set such that all the atoms have finished interacting with the barrier. By setting the scattering lengths to zero, we go from the Gross–Pitaevskii equation, also known as the nonlinear Schrödinger equation, to the Schrödinger equation. We find no major differences between the interacting and the non-interacting cases (see Extended Data Fig. 2)."

    The passing of the barrier is a complicated situation, especially in this setup.

    SteveMDFP said:

    Where *exactly* was the atom during that 0.27 msec? I'm guessing it was in superposition, on both sides of the barrier simultaneously.

    Good question!

    It is easier to think of the elementary particles as particles of their quantum fields at first. The field penetrates the barrier but there is no probability current inside the barrier, there is no observable particle inside the volume https://en.wikipedia.org/wiki/Probability_current ] but it is its wave function (describing the particle probability amplitude) that has been delocalized over the barrier https://en.wikipedia.org/wiki/Quantum_tunnelling ].

    Pulling that back to the atom, if it tunnels as a coherent system - as we can see it does - I doubt you can say it existed - was observable - in the common sense definition during the tunneling time. Quantum fields are funny things, they answer the classical question "particle and/or wave", but not always in the way we would think.
    Reply
  • whoo_whoo
    1292510467773136896
    Reply
  • Valentine Michael Smith
    If ulimately only fields exist, why do people continue to think in terms of particles, rather than self-propogating wakes?
    Reply
  • cashme
    Valentine Michael Smith said:
    If ulimately only fields exist, why do people continue to think in terms of particles, rather than self-propogating wakes?
    If so then yes, why think in terms of particles. What seems to be happening in quantum physics, is that there are only a few choices, waves or fields at base of all phenomena. What if one opens those choices to include particles as consisting of waves where, the waves are very different from the kind that oscillate in terms of their charge varying across space as is commonly depicted in graphic form.

    But,instead what if the particles do exist and exactly as predicted by Randell Mills' theory, the Grand Unified Theory-Classical Physics. The waves in that model consist of, resonating in place, circular standing charges of many such circles of charge combining to form a solid sphere of charge or the resulting particle. That is the kind of model that was used at MIT by Hermann Haus to answer a question posed by the USA Department of Defence , "What quantum level mechanism explains how the Free Electron Laser works?"

    Besides the DoD, I also find that theory and the way particles are modelled, to work much more accurately to describe all phenomena than, when using the kind of waves used under accepted QM. for explaining phenomena.

    Those statement may be taken as, being very heretical by, very many who do physics at the quantum level. The waves, as currently depicted, provide a substantial substrate that seems to work very accurately by which, very many phenomena can be explained. The reason most would find a different model of waves or field /cum waves, to be a difficult concept to entertain is, because nearly everyone uses the current model, as a common language for representing what, is happening at the quantum level. Getting away from that, would seem to require that common language having to be relearned almost from the ground up.




    I have studied GUT-CP for about 7-8 years and it seems to work much better, by a factor of at least 100 times, more accurately than, the kind of quantum mechanics that uses waves or fields. This works that much better everywhere, from quarks up to the whole of the universe, plus more. Using QM, with its types of waves, nothing was ever made to work. Under GUT-CP there are currently at least 4 items that were fully developed and do work in a way that QM has no way of explaining.
    Reply