Researchers from Universidad Complutense de Madrid, MIT and Harvard University have discovered a mechanism that enhances the presence of Majorana particles at the edges of a topological superconductor, thanks to the presence of long-range magnetic interactions. Moreover, they have shown that it is possible to find new topological phases of matter by merging distant Majoranas into a new particle. This great achievement could have future applications for quantum technologies.

In a recent paper published in Physical Review Letters, these researchers explain how they improve the propagation of Majorana particles at the edge of a topological superconductor by exploring the nature of long-range interactions, and transforming the Majoranas into more stable quasiparticles.

The study of topological phases of matter has become a very active field of research, that is revolutionizing our understanding of nature. It has given rise to new materials like topological insulators, Weyl semimetals, and topological superconductors.

Topological superconductors are materials that, besides conducting electric current without dissipating energy as heat, they host unconventional particles known as Majorana fermions. It is unknown whether these particles exist in high energy physics, but remarkably, Majorana fermions appear as low energy excitations (quasiparticles) in certain materials.

Useful to build quantum computers
These particles are very exotic in condensed matter. They behave as their own antiparticle and have been proposed as building blocks of future topological quantum computers. A quantum computer uses certain remarkable properties of quantum physics to solve tasks and processes, that would otherwise be unsurmountable by conventional computers.

Nonetheless, these properties (such as quantum entanglement between tiny particles) are very sensitive to environmental interactions (decoherence). This is the main reason why the construction of quantum computers turns out to be a great challenge that is currently being tackled by research labs and companies all over the world. The Majorana fermions that appear in topological superconductors are much more robust than other conventional particles, which would allow to build such novel computers.

The existence of Majorana fermions had been already proven in earlier works: chains of magnetic impurities placed on top of a superconductor substrate have shown that long-range magnetic interactions between electrons appear very naturally in these materials.

According to the authors of this publication, these interactions are very similar to the one between two magnets that feel attraction or repulsion to one another. In this case, it would be the magnetic moments of the electrons that interact with each other instead of the magnets.

However, it remained unknown what the effect of these magnetic interactions was over the properties of superconducting materials. This is precisely what this research work has solved.

Merging of Majorana particles
The researchers of this collaboration have found cases where the long-range effects of the magnetic interactions were so strong, that two distant Majorana fermions merged into a non-local topological quasiparticle.

This surprising effect could be used to store quantum information in a non-degenerate system (i.e. two-level systems with different energy), but with extra protection against external noise, caused by decoherence, or the loss of quantum effects.

These findings represent a great leap towards the understanding of the role of long-range magnetic interactions in the realm of topological superconductors. Moreover, these results will spark generation of novel topological phases of mater, widening their current applications in spintronics, quantum memories and computers, and other related fields.

Research Report: Chiral Topological Superconductors Enhanced by Long-Range Interactions, O. Viyuela, L. Fu, M. A. Martin-Delgado, Phys. Rev. Lett. 120, 017001 (2018)

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