Discovery of quantum states predicted more than 50 years ago

Researchers at the University of Hamburg's Department of Physics have observed a quantum state that had been theoretically predicted by Japanese theorists more than 50 years ago, but remained undetected.

By customizing an artificial atom on the surface of a superconductor, the researchers succeeded in pairing the electrons of the quantum dots to induce the smallest version of the superconductor. the findings were published on August 16 in the journal Nature.

"Proximity superconductivity in atom-by-atom crafted quantum dots."

Typically, electrons repel each other due to their negative charge. This phenomenon has a huge impact on the properties of many materials, such as electrical resistance. The situation changes dramatically when electrons are "stuck" together in boson pairs. Boson pairs do not avoid each other as individual electrons do, but many boson pairs can reside in the same position or move in the same way.

One of the most intriguing properties of materials with such electron pairs is superconductivity, which allows electric current to flow through the material without resistance. Over the years, superconductivity has been used in many important technological applications, including magnetic resonance imaging or highly sensitive magnetic field detectors.

The continuous miniaturization of electronic devices has also largely led to research into how superconductivity can be induced into even smaller structures on the nanoscale.

Now, researchers from the Department of Physics at the University of Hamburg have now realized the pairing of electrons in an artificial atom called a quantum dot - which is the smallest building block of nanostructured electronic devices.

To do this, the researchers, led by Dr. Jens Wiebe of the Institute for Nanostructures and Solid State Physics, locked the electrons one by one in tiny "cages" made of silver atoms. By coupling the locked electrons to an elemental superconductor, the electrons inherit the superconductor's tendency to pair up.

Quantum dots built of individual atoms coupled to a superconducting substrate

Particle-vacancy mixtures in bandgap states

Together with a team of theoretical physicists led by Dr. Thore Posske, the researchers linked an experimental feature: spectral peaks at very low energies, to a quantum state predicted by Kazushige Machida and Fumiaki Shibata in the early 1970s.

Although this state has so far remained undetectable directly by experimental methods, recent studies by researchers in the Netherlands and Denmark have shown that it facilitates the suppression of unwanted noise in TRANSMED quantum bits, an essential component of modern quantum computers.

After the results were published, Kazushige Machida wrote to Dr. Lucas Schneider, the paper's first author, "I thank you for discovering my old paper from half a century ago. For a long time, I have thought that a transition metal nonmagnetic impurity gives rise to an in-gap state, but it is located so close to the superconducting gap edge that its existence could not be proved. Through your ingenious method, it has finally been confirmed experimentally."

Reference link:

[1] https://phys.org/news/2023-08-pairing-electrons-artificial-atoms-quantum.html

[2] https://www.nature.com/articles/s41586-023-06312-0