In 1927, while In trying to understand how atoms come together to form molecules, the German physicist Friedrich Hund discovered one of the most fascinating aspects of quantum mechanics. He discovered that under certain conditions, atoms, electrons, and other small particles in nature can cross physical barriers that would confuse macroscopic objects, moving like ghosts through walls. Under these rules, a trapped electron could escape confinement without external influence, like a golf ball sitting in the first hole of a course suddenly disappearing and appearing in the second hole without anyone lifting a club. The phenomenon was completely strange, and became known as “quantum tunnel”.
Since then, physicists have discovered that tunnel construction plays a key role in some of nature’s most dramatic phenomena. For example, the quantum tunnel makes the sun shine: it allows the hydrogen nuclei in the nuclei of the stars to curl up close enough to merge into helium. Many radioactive materials, such as uranium-238, break down into smaller elements by ejecting material through tunnels. Physicists have even taken advantage of tunneling to invent the technology used in prototype quantum computers, as well as the so-called scanning tunnel microscope, which is capable of imaging individual atoms.
Still, experts don’t understand the process in detail. Publishing in Nature Today, physicists at the University of Toronto report a new basic measurement about the quantum tunnel: how long it takes. To return to the golf analogy, they essentially timed how long the ball is between the holes. “In the experiment, we asked, ‘How long did a given particle spend in the barrier?'” Says physicist Aephraim Steinberg of the University of Toronto, who led the project.
A “barrier” to an atom is not a material wall or divider. To confine an atom, physicists generally use force fields made of light or perhaps an invisible mechanism like electric attraction or repulsion. In this experiment, the team trapped rubidium atoms on one side of a barrier made of blue laser light. The photons in the laser beam formed a force field, pushing the rubidium to keep it confined in space. They discovered that the atoms spent approximately 0.61 milliseconds on the light barrier before appearing on the other side. The exact amount of time depended on the thickness of the barrier and the speed of the atoms, but its key finding is that “the tunneling time is not zero,” says physicist Ramón Ramos, who was a Steinberg graduate student at the time and He is now a postdoctoral researcher at the Institute of Photonic Sciences in Spain.
This result contradicts an experimental finding from last year, also published in Naturesays physicist Alexandra Landsman of Ohio State University, who was not involved in any of the experiments. In that document, a team led by physicists at Griffith University in Australia presented measurements that suggest the tunnel occurs instantaneously.
So which experiment is correct? Does a tunnel occur instantly or does it take approximately one millisecond? The answer may not be so simple. The discrepancies between the two experiments stem from a simmering disagreement in the quantum physics community about how to keep time at the nanoscale. “In the last 70, 80 years, people have come up with many definitions for time,” says Landsman. “Isolated, many of the definitions make perfect sense, but at the same time they make predictions that contradict each other. That is why there has been so much debate and controversy in the last decade. One group would think one definition makes sense, while another group would think of another. “
The debate becomes mathematical and esoteric, but the bottom line is that physicists disagree on when a quantum process begins or stops. The subtlety is evident when you remember that quantum particles for the most part have no definite properties and exist as probabilities, just like a coin tossed into the air is neither heads nor tails, but has the possibility of being until it lands. You can think of an atom as a wave, scattered in space, where its exact position is undefined: it could have a 50 percent chance of being in one place and a 50 percent chance in another, for example. With these vague properties, it is not obvious what counts as the particle that “enters” or “leaves” the barrier. On top of that, physicists have the additional technical challenge of creating a timing mechanism precise enough to start and stop in unison with the particle’s motion. Steinberg has been fine-tuning this experiment for more than two decades to achieve the necessary level of control, he says.
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