Scientists from the Niels Bohr Institute at the University of Copenhagen have, for the first time, succeeded in producing, controlling and understanding complex quantum states based on two electron spins connected to a superconductor. The samples, according to the study published in Nature Communications, were mounted in a dilution refrigerator with a base temperature near 30 mK and measured with standard techniques.

Quantum technology is based on understanding and controlling quantum states in nanoelectronic devices with components at the nanoscale, according to the research team. The control could be via electrical signals, like in the components of a computer. The devices are just significantly more complex, when researchers deal with quantum components at nanoscale, and the scientists are still examining and attempting to understand the phenomena that arise on this tiny scale. In this case, it is about the quantum states in nanoelectronic devices made from semiconductor nanowires and superconducting material, requiring the understanding of two fundamental phenomena in modern physics: magnetism and superconductivity.

The research team defined microscopic magnets electrically along a semiconductor nanowire, accomplishing this by placing an electron spin close to a superconductor and then observing how it changes the quantum states. By placing two microscopic magnets rather than one, as had been done before, the possibilities for observing new quantum states arise. In this way, the scientists accumulated knowledge by adding more and more complexity to the systems. “It is a bit like playing with building blocks,” says Kasper Grove-Rasmussen. “Initially we control one single electron spin, then we expand to two, we can modify the coupling between them, tune the magnetic properties, etc. Somewhat like building a house with each additional brick increasing our knowledge of these quantum states.”

According to the team, It is all about categorizing the different quantum states and their relation to one another in order to achieve an overview of how the individual parts interact. During the 1960s, the theoretical foundation for this work was done when three physicists (L. Yu, H. Shiba and A.I. Rusinov) published three independent theoretical works on how magnetic impurities on the surface of the superconductor can cause new types of quantum states. The states, now achieved experimentally by the scientists at the Niels Bohr Institute, are named after the physicists—Yu-Shiba-Rusinov states—but they are significantly more complex than states with a single spin previously achieved. The team says this could be a step on the way to more complex structures that would enhance our understanding of potential quantum computer components based on semiconductor-superconductor materials.