The quantum world is a very wild world, where the seemingly impossible happens all the time: Teensy objects separated by miles are bound together, and particles can even be in two places at once. But one of the most confusing quantum superpowers is the movement of particles through seemingly inescapable 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 tunnel would last from start to finish – from the moment a particle enters the barrier, passes through tunnels and exits the other side, they report online July 22 in the magazine Nature.
Quantum tunneling is a phenomenon where a atom if 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 meter (3 meter) wall that extends as far as the eye can see. Without a ladder or Spider-Man climbing skill, the wall would make it impossible for you to continue.
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In the quantum world, however, it is rare, but possible, for an atom or electron to simply “appear” on the other side, as if a tunnel were through the wall grooves. “Quantum tunneling is one of the most astonishing of quantum phenomena,” said co-author Aephraim Steinberg of the study, co-director of the Quantum Information Science Program at Canadian Institute for Advanced Research. “And it’s fantastic that we can really study it this way now.”
Quantum tunneling is not new to physicists. It forms the basis of many modern technologies, such as electronic chips, called tunnel diodes, which allow the movement of electricity through a circuit in one direction but not the other. Tunnel microscopy (STM) scans also use tunneling to literally show individual atoms on the surface of a solid. Shortly after the first STM was invented, researchers investigated IBM reported using the device to write down the letters IBM with 35 xenon atoms on a nickel substrate.
While the laws of quantum mechanics allow quantum tunneling, researchers do not yet know exactly what happens while a subatomic particle undergoes the tunneling process. Indeed, some researchers thought that the particle immediately appeared on the other side of the barrier as if it were teleporting immediately, Sci-News.com reported.
Researchers have previously tried to measure the amount of time for tunneling to occur, with varying results. One of the difficulties in earlier versions of this type of experiment is identifying the moment when tunneling begins and stops. To simplify the methodology, the researchers used magnets to create a new kind of “clock” that would only tick when the particle tunneled.
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 in it is bathed Magnetic field. Knowing this, the Toronto group uses a magnetic field to form their barrier. If particles are in the shed, they are pursued. Beyond that, they do not. Thus measuring how long the particles preceded, the researchers told how long it took those atoms to tunnel through the barrier.
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“The experiment is a breathtaking technical achievement,” said Drew Alton, a professor of nature at Augustana University in South Dakota.
The researchers prepared about 8,000 rubidium atoms, cooling them to a billionth of a degree above absolute zero. The atoms had to be this temperature, otherwise they would have run randomly at high speeds, instead of staying in a small clump. The scientists used a laser to create the magnetic barrier; they aimed the laser so that the barrier was 1.3 micrometers (microns) thick, as the thickness of about 2,500 rubidium atoms. (So if you were a foot thick, from front to back, this barrier would be the equivalent of about half a mile thick.) Using another laser, the scientists pierced the rubidium atom to the barrier, they move about 0.15 inches per second (4 millimeters / s).
As expected, most rubidium atoms jumped off the shell. By quantum tunneling, however, about 3% of the atoms passed through the shell and appeared on the other side. Based on the advance of these atoms, it took them about 0.6 milliseconds to cross the barrier.
Chad Orzel, an associate professor of physics at Union College New York who was not part of the study, applauded the experiment, “Their experiment is ingeniously built to make it difficult to interpret anything other than what they say,” said Orzel, author of “How to teach quantum mechanics to your dog“(Scribner, 2010) It’s one of the best examples you’ll see of a thought experiment really made,” he added.
Experiments investigating 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 equipment to not only determine the duration of the tunneling process, but also to see if they can learn anything about the speed of the atoms at various points within the barrier. “We’re working on a new measurement where we make the shed thicker and then determine the amount of progress at different depths,” Steinberg said. “It will be very interesting to see if the speed of the atoms is constant or not.”
In many interpretations of quantum mechanics it is – even in principle – impossible to determine the trajectory of a subatomic particle. Such a measurement could lead to insight into the confusing world of quantum theory. The quantum world is very different from the world we are familiar with. Experiments like these will help make it a little less mysterious.
Originally published on Live Science.
