Quantum tunnelling is instantaneous, researchers find

Physicists establish that electrons waste no time bashing through a barrier. Alan Duffy reports.

A diagrammatic representation of quantum tunnelling.

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Researchers have found that electrons passing through solid matter in a quantum process known as “tunnelling” do so instantaneously.

The finding, led by scientists from Australia’s Griffith University, contradicts previous experiments that suggested a degree of time elapses between the start and finish of a tunnelling event.

The work is detailed in a paper in the journal Nature.

Quantum tunnelling is one of the more bizarre differences between our everyday, classical world and the surprising realm of quantum mechanics.

“If you lean on a wall, that wall pushes back in force so that you don’t go through it,” co-author Robert Sang says.

“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.”

A particle in the quantum world actually can pass through that wall. The experimental question was, how long does it take to transition through a given obstacle – in this case, the electric barrier potential of a hydrogen atom.

"We use the simplest atom, atomic hydrogen, and we’ve found that there’s no delay in what we can measure,” says Sang.

The Nature paper is the culmination of a three-year international project, in which the team shot a hydrogen atom and its lone electron with an enormously powerful, ultra-fast laser contained in Griffiths’ Australian Attosecond Science Facility. The laser was circularly polarised, meaning that it imparted a rotation to an emitted electron.

That resulting rotation in the electron’s "phase" could then be measured as if it were a clock hand ticking around – or in this case, more precisely, an atto-clock.

"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,” explains Sang.

“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 tunnelling.”

The precision of the clock to measure the tunnelling event was driven by the ultra-fast pulse of light in the attosecond laser – just a billionth of a billionth of a second long. The energy emitted by the laser during such a tiny amount of time is greater than that of the entire US power grid.

Sang notes about the attosecond timescale that “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”.

Tunnelling may be an unfamiliar effect in our everyday lives, yet common devices from electron microscopes to computer transistors rely on it.

“One limitation you might think of is how fast can I make a transistor work – the ultimate limit will be partly about how quickly quantum particles can tunnel,” says Sang.

“For a classical computer, it implies a limit as to how quickly you can switch a transistor.”

As we explore the realms, and limits, of these strange quantum mechanical processes, there may be a speed boost for personal computers, too.

The researchers have demonstrated that the electron spends no measurable time “under the potential” as it tunnels through the barrier, but noted that these events “are only as ‘instantaneous’ as the electron wave-function collapse that orthodox interpretations of quantum mechanics” predicts.

This, Sang adds, offers a tantalising possibility of future zeptosecond lasers – which would operate for a period of time a thousand times shorter than an attosecond – “obtaining information on the dynamics of the wave-function collapse itself”. Such a measurement would explore that most fundamental difference of the quantum to the classical world, where common sense expectations break down in the face of wave-functions describing particles.

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Alan Duffy is an astrophysicist at Swinburne University of Technology, Melbourne. Twitter | @astroduff
  1. https://www.nature.com/articles/nature11025
  2. https://www.nature.com/articles/s41586-019-1028-3
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