In 1927, while trying to understand how atoms bind to form molecules, the German physicist Friedrich Hund discovered one of the most beguiling aspects of quantum mechanics. He found that, under certain conditions, atoms, electrons, and other small particles in nature can cross physical barriers that would confound macroscopic objects, moving like ghosts through walls. By these rules, a trapped electron could escape confinement without outside influence, like a golf ball sitting in the first hole of a course suddenly vanishing and appearing in the second hole without anyone lifting a club. The phenomenon was utterly alien, and it came to be known as “quantum tunneling.”

Since then, physicists have found that tunneling plays a key role in some of nature’s most dramatic phenomena. For example, quantum tunneling makes the sun shine: It enables hydrogen nuclei in stars’ cores to snuggle close enough to fuse into helium. Many radioactive materials, such as uranium-238, decay into smaller elements by ejecting material via tunneling. Physicists have even harnessed tunneling to invent technology used in prototype quantum computers, as well as the so-called scanning tunneling microscope, which is capable of imaging single 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 quantum tunneling: how long it takes. To go back to the golf analogy, they essentially timed how long the ball is in between 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.

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