Quantum Computing New Measurement Method Paves the Way for Less Disruptive Quantum Characterization

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Hybrids of magnets and superconductors are interesting materials that exhibit sensitive quantum phenomena. Therefore, it is extremely important to measure them with as little interference as possible. Using special techniques, these quantum phenomena can be detected and controlled over greater distances with a scanning tunneling microscope.

The development of quantum electronics faces a fundamental problem: quantum states, i.e., the specific energy states of particles on an atomic level, are very sensitive to external disturbances. Any measurement alters them, similar to tapping a soap bubble, causing it to burst. This makes their characterization and integration into electronic systems more difficult.

Researchers from the University of Hamburg and the University of Illinois Chicago have demonstrated both experimentally and theoretically how these quantum phenomena can be detected and controlled over greater distances using a scanning tunneling microscope.

Researchers from the University of Hamburg and the University of Illinois Chicago have now developed a solution that could be of great significance to developers of quantum sensors and computers.

Yu-Shiba-Rusinov Quasiparticles: Quantum States With Potential

In hybrid materials made of magnets and superconductors, so-called Yu-Shiba-Rusinov quasiparticles are formed. A superconductor is a material that conducts electric current without resistance, but only at very low temperatures. When a magnetic atom is introduced into a superconductor, it locally disrupts the perfect superconducting order and generates special energy states. This is referred to as a Yu-Shiba-Rusinov quasiparticle.

These quasiparticles are not real particles but collective excitations of the entire system—comparable to a wave moving through a pond. They have special properties that make them interesting for quantum technologies.

Until now, these quantum states could only be measured with a scanning tunneling microscope, a device that scans the surface with an atomically sharp tip. The problem: the measuring tip must be positioned directly above the magnetic atom, which disturbs the sensitive quantum system.

The Trick With the Quantum Corral

The Hamburg team led by Dr. Jens Wiebe developed a clever trick: they constructed a quantum corral from 91 precisely positioned silver atoms on a superconducting silver crystal. This corral functions like a waveguide for quantum states.

How the quantum corral works:

1. Precise construction: Individual silver atoms were pushed together into a ring-shaped corral using the tip of the scanning tunneling microscope. This is work on an atomic level.
2. Dimensioning according to the Fermi energy: The size was calculated so that a quantum state of the confined electrons lies exactly at the Fermi energy. This is the energy boundary between occupied and unoccupied electron states in the material.
3. So-called antinodes of the quantum state: Due to the special geometry, several areas of high probability (so-called antinodes) are created, where the electrons preferentially reside.

Theoretical Confirmation Through Simulations

In parallel with the experiments, the researchers conducted computer simulations using a tight-binding model. This is a mathematical model that describes how electrons jump between neighboring atoms. The simulations confirmed that the measured state consists of both Cooper pairs (the bound electron pairs in the superconductor) in the bulk and on the surface of the crystal.

The technique makes it possible to measure fragile quantum states of magnet-superconductor hybrids with a local probe while simultaneously minimizing the disruptive influence of the probe. The researchers now hope to apply this technique in the future to Majorana quasiparticles, which have great potential for the development of novel, topological quantum computers. Furthermore, quantum cages could be used in the future to precisely control interactions between the quasiparticles of multiple magnet-superconductor hybrids. (heh)