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Physicists make 'quantum sparks' visible – light-first effect allows direct observation of tunneling electrons

Physicists make 'quantum sparks' visible – light-first effect allows direct observation of tunneling electrons

Spark Leap at the Atomic Scale: Physicists have observed for the first time how a tiny spark of light jumps from one atom to another — in less than a trillionth of a second. This is the first time they have succeeded in demonstrating a quantum version of Heinrich Hertz's spark experiment – capturing electrons while tunneling. This opens new opportunities to breakthrough scales that were previously unresolved in microscopy and atomic observation, the team reports in the journal Nature.

Almost 150 years ago, Heinrich Hertz observed how a spark jumped between two charged metal balls and sparked a second spark between two unconnected metal balls a short distance away – apparently there was a wireless transmission of electromagnetic waves. This revolution revolutionized communications technology and provided the basis for radio and radio.

Heinrich Hertz's experiment: A spark generated in an electrical circuit between two metal balls generates electromagnetic waves that also cause two metal balls far away to ignite. © DamageWaltieri/ CC-BY-SA 3.0

A very similar process also occurs at the atomic level: There, too, interactions between electrons can cause a kind of spark that jumps between two atoms, according to the theory. But observing such a short, extremely small quantum spark is not easy. “Detecting the Hertzian emission of a handful of electrons in each oscillatory cycle of light initially seemed like an impossible task,” says lead author Tom Seday from the University of Regensburg.

With micro-atom tip and terahertz pulses

In order to achieve this, you need a method with very high accuracy in both space and time. For this purpose, physicists working with Siday have developed a special microscope that combines the fine spatial resolution of a scanning probe microscope with the measurement of a purely optical signal. “Electronics are exceptionally sensitive, but too slow to directly track the quantum spark driven by a light wave. That's why you have to directly observe the oscillations of the emitted light,” explains senior author Robert Huber from the University of Regensburg.

Specifically, the process called “NOTE” — near-field optical tunneling emission — consists of an atomically precise metal tip, similar to a scanning tunneling microscope. This is approximated by the atoms on the surface of the sample. The gap between them, just a few atoms wide, is then bombarded with very short pulses of terahertz radiation. “The optical interaction between the tip and the sample is detected in the amplitude, wavelength and phase of the scattered light,” the team explains.

A non-classical signal with overlapping orbits

When physicists brought the tip of the microscope closer to the sample so much that the electron orbitals of the atoms on either side began to overlap, something unusual appeared. “Then a non-classical signal appears,” Seday and Huber report. “This comes from an emission process caused by tunneling of electrons between the tip and the sample.”

In other words: their quantum physical wave nature allows electrons to change their location probability from the tip to the sample atom and vice versa. If this were translated into our world, it would be like having someone standing on both sides of a door at the same time. This tunneling in turn leads to an optical interaction – the non-classical “quantum spark”.

“Quantum Spark” makes tunneling visible

Tunneling of electrons changes the oscillating electric field of the emitted terahertz radiation – creating a kind of quantum version of a Hertzian spark. “An observation was born when we were able to show that incoming and outgoing light waves were shifted in time by a quarter of the period of oscillation – in our experiment only a quarter of a trillionth of a second,” explains Sidai's colleague Johannes Heis.

Physicists explained that thanks to this discovery, matter waves can be seen flowing in slow motion on atomic length scales. The observation thus opens up entirely new insights into the quantum motion of electrons – a communication channel to the nanoverse, so to speak. For the first time, this process makes it possible to directly observe processes optically on the length scale of individual atoms and on time scales of less than a trillionth of a second.

Long term meaning

“This experimental approach represents a major advance in observing hypervelocity dynamics at the atomic level, and is likely to have far-reaching implications,” says physicist Shunsuke Sato of the University of Tsukuba, who was not involved in the study. Observation can clarify many open questions in basic research, but it also becomes useful for future data processing and storage. (Nature, 2024; doi: 10.1038/s41586-024-07355-7)

Source: University of Regensburg

May 13, 2024 – Nadja Podbrigar