Magnetic quantum objects in a ‘nano egg-box’

Magnetic quantum objects in superconductors, so-called “fluxons”, are particularly suitable for the storage and processing of data bits. Computer circuits based on fluxons could be operated with significantly higher speed and, at the same time, produce much less heat dissipation. Physicists around Wolfgang Lang at the University of Vienna and their colleagues at the Johannes-Kepler-University Linz have now succeeded in producing a “quantum egg-box” with a novel and simple method. They realized a stable and regular arrangement of hundreds of thousands of fluxons – a groundbreaking progress for circuits based on fluxons. The results appear in the new journal Physical Review Applied of the renowned “American Physical Society”.

Speeding up data processing in computers goes hand in hand with a greater heat generation, which limits the performance of fast computers. Researchers have therefore long been trying to develop digital circuits based on superconductors – those puzzling materials that can transport electricity completely without loss when cooled below a certain critical temperature.

Magnetic quantum objects in superconductors

Inside a superconductor, a magnetic field can exist only in small quantized pieces, the fluxons. These are particularly suitable for the storage and processing of data bits. In a homogeneous superconductor, the fluxons are arranged in a hexagonal lattice. Using modern nanotechnology, researchers at the University of Vienna and the Johannes-Kepler-University Linz have now succeeded in building artificial traps for fluxons. By means of these traps the fluxons are forced into a predefined formation.

The importance of the non-equilibrium

Until now, the fluxons could only be observed in a thermodynamic equilibrium, i.e., in a uniform arrangement. “If we try to stack two eggs on top of each other in an egg-box and leave the adjacent pit empty, the egg would quickly roll down and we end up in the equilibrium state with exactly one egg in each pit,” explains Wolfgang Lang from the University of Vienna. From the viewpoint of data processing, however, the fully-filled egg-box contains little information and is therefore useless. It would be much more useful to place the eggs in a predefined pattern. In such a way, for example, the QR code, recognized by smartphones, could be realized in an egg-box – obviously a large amount of information.

At the nanoscale, the researchers have now made a major step forward in this direction by demonstrating for the first time a stable non-equilibrium state of fluxons in an array of more than 180,000 artificial traps. Depending on the external magnetic field, the fluxons arrange themselves in terraced zones, in which each trap either captures no fluxon, exactly one, or even several fluxons. “Even after days, we have observed precisely the same arrangement of fluxons – a long-term stability that is rather surprising for a quantum system,” says Georg Zechner of the University of Vienna, the lead author of the study.

Nanopatterning of superconductors by ion beams

These research results were enabled by a new method, developed by the physicists in Linz and Vienna together with the Vienna-based high-tech company IMS Nanofabrication AG. “Masked ion-beam irradiation allows for the fabrication of nanostructures in superconductors in a single step. It can be applied time-efficiently to large areas, can be ramped-up to an industrial scale and does not require any chemical processes,” emphasizes Johannes D. Pedarnig of the Institute of Applied Physics at the Johannes-Kepler-University Linz. Depending on the mask used, virtually any desired structure can be patterned into the superconductor. The scientists are now planning further experiments on more sophisticated nanostructures, which should demonstrate the systematic transfer of fluxons from one trap to the next. This could be another pioneering step towards the development of fast computer circuits based on fluxons.