Physics

Does quantum tunneling take time or is it instantaneous?

Researchers at the Max Planck Institute have determined how long it takes electrons to quantum tunnel
Researchers at the Max Planck Institute have determined how long it takes electrons to quantum tunnel
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To measure the time it takes an electron to quantum tunnel, the researchers hit atoms with short laser pulses that were rotating. If it took zero time (simple man model), the electron would be unaffected, but if it took any time at all (Wigner model) the trajectory would be skewed
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To measure the time it takes an electron to quantum tunnel, the researchers hit atoms with short laser pulses that were rotating. If it took zero time (simple man model), the electron would be unaffected, but if it took any time at all (Wigner model) the trajectory would be skewed
Researchers at the Max Planck Institute have determined how long it takes electrons to quantum tunnel
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Researchers at the Max Planck Institute have determined how long it takes electrons to quantum tunnel
A diagram demonstrating the ball analogy of quantum tunneling: to climb the hill a ball would need sufficient energy, but with quantum tunneling there's a chance that the ball could randomly "tunnel" through the hill and appear on the other side
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A diagram demonstrating the ball analogy of quantum tunneling: to climb the hill a ball would need sufficient energy, but with quantum tunneling there's a chance that the ball could randomly "tunnel" through the hill and appear on the other side

In the weird world of quantum physics, it's not unusual for particles to tunnel through barriers that under normal circumstances they shouldn't be able to pass through. While this process, called quantum tunneling, is well documented, physicists haven't been able to figure out if it happens instantaneously or takes a given amount of time. Now a team from the Max Planck Institute for Nuclear Physics has an answer.

The most common analogy used to explain this quirky quantum phenomenon is a ball rolling over a hill. Normally, the ball needs a certain amount of energy to push it up and over, otherwise it's stuck at the bottom. It's simple. But in quantum physics, there's a chance that the ball could randomly move to the other side of the hill, through a process called quantum tunneling. This has been well documented for decades: elementary particles escaping from atoms is one of the key drivers behind radioactive decay.

A diagram demonstrating the ball analogy of quantum tunneling: to climb the hill a ball would need sufficient energy, but with quantum tunneling there's a chance that the ball could randomly "tunnel" through the hill and appear on the other side
A diagram demonstrating the ball analogy of quantum tunneling: to climb the hill a ball would need sufficient energy, but with quantum tunneling there's a chance that the ball could randomly "tunnel" through the hill and appear on the other side

The part of the process still up for debate is the timescale involved for the particle to tunnel to freedom. There are two theories: the "simple man" model says that it happens instantly, so the escaping electron will just appear at the exit of the tunnel with no velocity. But in 1955, physicist Eugene Wigner proposed the idea that it takes a finite (albeit short) amount of time for the particle to make the journey.

To investigate, the Max Planck team induced quantum tunneling of electrons in atoms, and then measured the time (if any) it took them to do so. Since it would be happening over an incredibly tiny timescale, the scientists developed a clever little trick that would allow them to see which scenario was happening.

To induce quantum tunneling, the scientists blasted a gas mixture of krypton and argon atoms with short laser pulses. This temporarily weakens the electric field that binds the electrons in place, increasing the probability that one of them will tunnel out. The trajectory of the electron's exit from the nucleus is guided by the laser's electric field, and in this particular experiment, the laser beam is rotating, which causes the "energy pot" containing the electrons to rotate too.

That means that the team can now settle the question by measuring the trajectory the electron takes as it zips away. Essentially, if it takes any time at all for the particle to tunnel, the energy pot will have rotated slightly by the time it exits, sending it off on a different course than if it happened instantaneously.

To measure the time it takes an electron to quantum tunnel, the researchers hit atoms with short laser pulses that were rotating. If it took zero time (simple man model), the electron would be unaffected, but if it took any time at all (Wigner model) the trajectory would be skewed
To measure the time it takes an electron to quantum tunnel, the researchers hit atoms with short laser pulses that were rotating. If it took zero time (simple man model), the electron would be unaffected, but if it took any time at all (Wigner model) the trajectory would be skewed

Using krypton and argon atoms was also key to tracking the difference in trajectory. These atoms have different barrier heights and tunnel lengths, meaning their trajectories should be slightly different if time is a factor. If, however, the tunnel time is zero, they should follow the exact same path.

Sure enough, the researchers found that time does play a role in the process. In their experiments, the electrons took between 80 and 180 attoseconds – a billionth of a billionth of a second – to make the trip, confirming Wigner's long-standing theory.

The research was published in the journal Physical Review Letters.

Source: Max Planck Institute

7 comments
piperTom
It's "between 80 and 180 attoseconds..." That time is, of course, measured in the rest frame of the gas itself. An observer in motion would get a different result; that's according to Special Relativity. If the result had been zero, then some observers would get a negative result, meaning the output electron appeared before the other vanished. That would violate all sorts of conservation (and symmetry) laws. So... good result: physics is saved.
Alex Aricci
How could it happen instantaneously? surely that's transmitting information faster than the speed of light. Not only that but if it exists with zero velocity then surely it's lost energy and thus the gas should get cooler if there were a lot of quantum tunneling events, effectively refrigerating it, but where does the energy go?
DMarcus
So help me out here. What was the "distance" travelled, and how much faster than the speed of light was this?
sk8dad
Dreams of ansible communication...crushed.
TomWatson
"Aye, it's a Worm-Hole Captain. We need more power to keep her afloat".
ColinChambers
Created particle movement has a resisting force , unless it has zero viscosity . Even upon a quantum rail this is not possible . So time will apply . Interesting theory but "instantaneously "you knew the answer before the experiment ? Jacktar .
Jose Gros
Quantum tunnelling takes time, in the order of a minute or so to drive your second attention to the Milky Way centre