Tunneling, a key feature of quantum mechanics, is when a particle that encounters a seemingly insurmountable barrier passes through it, ending up on the other side. A series of experiments carried out by physicists from Griffith University, Lanzhou University, the Australian National University, Drake University and Korea’s Institute for Basic Science has definitively determined the tunneling delay, which is also the time it takes for an electron to get out or ionize from a hydrogen atom.
Satya Sainadh et al put an upper limit of 1.8 attoseconds on any tunneling delay, in agreement with theoretical findings and ruling out the interpretation of all commonly used ‘tunneling times’ as time spent by an electron under the potential barrier. Image credit: Griffith University.
“In the classical world, Newton’s laws of physics are what large physical bodies obey,” said co-lead author Professor Robert Sang, from the Centre for Quantum Dynamics at Griffith University.
“If you lean on a wall, that wall pushes back in force so that you don’t go through it. But when you go down to the microscopic level, things behave quite differently. This is where the laws of physics change from classical to quantum.”
Professor Sang and colleagues conducted experiments at the Australian Attosecond Science Facility over three years that measured how long it takes for a particle to go through that wall.
“We use the simplest atom, atomic hydrogen, and we’ve found that there’s no delay in what we can measure,” Professor Sang said.
The researchers set up an experiment where they used one of the properties of light and turned it into a ‘clock’ called an attoclock.
By sending a pulse of light to interact with a hydrogen atom, it sets up the conditions so that the lone electron from that atom can tunnel through a barrier.
“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,” Professor Sang said.
“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.”
The tunneling time the team measured was found to be no more than 1.8 attoseconds, much smaller than some theories had predicted.
“We now know the tunneling time must be less than 1.8 attoseconds — which is a billionth of a billionth of a second,” said co-lead author Dr. Igor Litvinyuk, also from Griffith University’s Centre for Quantum Dynamics.
“It’s hard to appreciate how short that is, but it takes an electron about a hundred attoseconds to orbit a nucleus in an atom.”
“Previous tests elsewhere used more complicated atoms, containing several or many electrons,” he explained.
“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 findings appear in the journal Nature.
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U. Satya Sainadh et al. Attosecond angular streaking and tunnelling time in atomic hydrogen. Nature, published online March 18, 2019; doi: 10.1038/s41586-019-1028-3
