Pairing of electrons in an artificial atom leads to a breakthrough

The state, known as the Machida-Shibata state, involves the pairing of electrons in an artificial atom on the surface of a superconductor.
Rizwan Choudhury
Close up illustration of atomic particle.
Close up illustration of atomic particle.

Credits: EzumeImages/iStock 

A team of physicists from Hamburg University has made a breakthrough in the field of quantum physics by observing a rare state of matter that was predicted by Japanese theorists more than half a century ago. 

Machida-Shibata state

The state, known as the Machida-Shibata state, involves the pairing of electrons in an artificial atom on the surface of a superconductor. The discovery, published in the journal Nature, could have implications for the development of nanostructured electronic devices and quantum computers.

Electrons are negatively charged particles that usually repel each other. This affects the properties of many materials, such as their electrical resistance. However, under certain conditions, electrons can form pairs and behave like bosons, which are particles that can occupy the same space or have the same motion. 

When this happens, the material can exhibit superconductivity, which means that it can conduct electricity without any resistance. Superconductivity has many important applications in technology, such as magnetic resonance imaging or highly sensitive detectors for magnetic fields.

Superconductivity in the smallest possible unit

The researchers from Hamburg University managed to induce superconductivity in the smallest possible unit: a quantum dot, which is an artificial atom that can trap electrons in a tiny space. They did this by creating quantum dots from silver atoms and coupling them to a superconductor made of lead. By tuning the number of electrons in the quantum dots, they were able to observe the formation of electron pairs and measure their energy spectrum.

The experimental results matched the theoretical predictions made by Kazushige Machida and Fumiaki Shibata in the early 1970s. They proposed that electron pairs in quantum dots coupled to superconductors would have a special state with a very low energy peak. This state had never been directly detected before, despite its relevance for understanding quantum phenomena at the nanoscale.

The researchers also found out that the Machida-Shibata state could be useful for reducing noise in transmon qubits, which are essential components of modern quantum computers. This was confirmed by recent work from researchers from the Netherlands and Denmark.

In an email to the first author of the paper, Dr. Lucas Schneider, Kazushige Machida expressed his gratitude for “discovering” his old paper and verifying it experimentally after 50 years. He wrote: “I thought for a long time that transition metal non-magnetic impurities produce the in-gap state, but the location of it is so near the superconducting gap edge, thus it is impossible to prove its existence. But by your ingenious method, you have finally checked it to be true experimentally”.

The study was published in the journal Nature

Study abstract:

Gapless materials in electronic contact with superconductors acquire proximity-induced superconductivity in a region near the interface. Numerous proposals build on this addition of electron pairing to originally non-superconducting systems and predict intriguing phases of matter, including topological, odd-frequency, nodal-point, or Fulde–Ferrell–Larkin–Ovchinnikov superconductivity. Here we investigate the most miniature example of the proximity effect on only a single spin-degenerate quantum level of a surface state confined in a quantum corral11 on a superconducting substrate, built atom by atom by a scanning tunneling microscope. Whenever an eigenmode of the corral is pitched close to the Fermi energy by adjusting the size of the corral, a pair of particle–hole symmetric states enter the gap of the superconductor. We identify these as spin-degenerate Andreev bound states theoretically predicted 50 years ago by Machida and Shibata, which had—so far—eluded detection by tunnel spectroscopy but were recently shown to be relevant for transmon qubit devices. We further find that the observed anticrossings of the in-gap states are a measure of proximity-induced pairing in the eigenmodes of the quantum corral. Our results have direct consequences on the interpretation of impurity-induced in-gap states in superconductors, corroborate concepts to induce superconductivity into surface states, and further pave the way towards superconducting artificial lattices.

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