Quantum Tunneling | How Long It Takes For A Particle To Tunnel Through A Barrier?

  • Physicists measure the time an electron takes to pass through a potential barrier. 
  • They performed the experiment with hydrogen atoms. 
  • The process takes no longer than 1.8 attoseconds (10-18 seconds).

One of the key features in quantum mechanics is tunneling of a particle through a potential barrier. In this phenomenon, a subatomic particle passes through a seemingly insurmountable barrier that it can’t surmount under the laws of classical mechanics.

Recently, an international team of physicists measured how long a particle takes to tunnel through a barrier. They have been working on this project for over 3 years.

Since tunneling happens instantaneously, it’s very difficult to record such events. Unlike previous researches that used heavier elements containing several electrons, the new study utilized the simplest atom, hydrogen, to measure the quantum tunneling.

The Experiment

In classical physics, physical objects obey Newton’s law. For example, if you push a wall, it pushes you back with the same force in the opposite direction.

However, if you look at the microscopic scale, things behave differently. This is the phase where laws of quantum mechanics start taking over. In quantum physics, electrons can actually pass through that wall.

The research team led by Griffith University conducted an experiment, in which they used hydrogen atoms and ultrashort pulses of light to determine the tunneling delay – the time an electron takes to ionize or get out from a hydrogen atom.

Reference: Nature | doi:10.1038/s41586-019-1028-3 | Griffith University 

The make electron tunnel through a potential barrier, they sent a light pulse to interact with a hydrogen atom. More specifically, they hit these atoms in rotating electric fields with powerful (30 gigawatts) short bursts of light. There were 1,000 pulses/sec fired at a single atom.

Artistic impression of a pulse striking an electron, enabling quantum tunneling | Credit: Griffith University 

Using an attosecond angular streaking technique, the team found that there is almost no delay in what they can actually measure. The process takes no longer than 1.8 attoseconds (10-18 seconds).

For those who don’t know, an electron takes approximately 100 attoseconds to orbit an atom’s nucleus. To put this into perspective, one attosecond is to a second what a second is to nearly 31.71 billion years.

The outcomes differed from previous studies because physicists used an element with the simplest atomic structure. Since previous experiments involved heavier elements (consisting of several electrons), they used complicated models to account for the interaction between electrons.

How This Is Useful?

The results obtained in this study can be used with other atoms as well to understand more about quantum laws. Since hydrogen atom yields zero delay, other delays associate with multi-electron systems could be calibrated in respect to that.

Quantum tunneling plays a crucial role in numerous physical events and applications, including nuclear fusion, scanning tunneling microscope, quantum computing, tunnel diode. It puts physical limits on the size of transistors (or how quickly a transistor can be switched) used in classical computers.

Read: Scientists Use Quantum Computer To Reverse Time

As per classical physics, light travels at the speed of 299,792 km/s. The is what we know for sure, so the question is whether there is any violation to that in terms of tunneling?

Written by
Varun Kumar

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