If you throw a ball at a wall, it's going to bounce back at you – that's classical physics at work. But of course, the world of quantum physics is much spookier, so if you did the same with a particle, there's a chance that it will suddenly appear on the other side. This is thanks to a phenomenon known as quantum tunneling, and now a team of physicists has measured just how long that process takes.
The phrase may conjure images of a particle phasing through a barrier, but it's not literally "tunneling" through – it's actually a product of mathematical random chance. Say, for example, you're mapping out the probability of where a particle will go after you throw it at a wall. Odds are extremely high that it will move back away from the wall – but in the quantum world you can never be 100 percent sure of something. There's a tiny, tiny chance that the particle will ignore the wall and continue its journey on the other side.
While it sounds like science fiction, quantum tunneling is a well-documented phenomenon, taking place in everyday items like electron microscopes and transistors. Elementary particles like electrons are known to "tunnel" out of their atoms on a regular basis too, which is a key driver behind radioactive decay.
But one ongoing mystery around the phenomenon is how long it takes for a particle to quantum tunnel through a barrier – or whether it takes any time at all. Some scientists believe it happens instantaneously, but that would mean it travels faster than the speed of light and may violate the principle of causality.
To try to measure tunneling time, researchers from Griffith University and the Australian National University blasted hydrogen atoms with an intense laser that fires 1,000 light pulses per second. The idea is that this sets up the right conditions for the electron to escape from the atom, allowing the team to precisely measure how long tunneling takes.
The result? Quantum tunneling seems to happen instantaneously – or at least, so incredibly quickly that it's essentially instantaneous. According to the researchers, it takes less than 1.8 attoseconds, which is a billionth of a billionth of a second.
"There's a well-defined point where we can start that interaction, and there's a point where we know where that electron should come out if it's instantaneous," says Robert Sang, co-lead author of the study. "So anything that varies from that time we know that it's taken that long to go through the barrier. That's how we can measure how long it takes. It came out to agree with the theory within experimental uncertainty being consistent with instantaneous tunneling."
Interestingly, this is in contrast to previous studies that measured much longer tunnel times. In 2017 researchers at Max Planck found that quantum tunneling takes up to 180 attoseconds, but the team behind the new study says earlier experiments like these were overly-complicated and prone to errors. In the Max Planck study, for example, the team used krypton and argon atoms, which are far more complex than hydrogen, which has only one electron.
"Previous tests elsewhere used more complicated atoms, containing several or many electrons," says Igor Litvinyuk, co-lead author of the study. "To account for the interaction between different electrons they used different approximate models. And out of those models they extracted the times. Our model used no approximations because we didn't have to worry about electron-electron interactions. Also, in one of those experiments they measured the relative time delay between two species of atoms and not the time delay for a single atom."
The researchers say they've solved this long-standing mystery of physics, but in the true spirit of quantum physics, maybe we can never be 100 percent sure.
The research was published in the journal Nature.
Professor Sang discusses the research in the video below.
Source: Griffith University
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