With information Agência Fapesp – 01.09.2022
Brazilian researchers are proposing a more practical path toward a type of quantum computer that is even more futuristic than the quantum computers currently in development.
Topological quantum computing exploits phenomena that occur on the surface of materials – the study of topology won the 2016 Nobel Prize in Physics.
Once considered exotic phenomena, topological phenomena are now known to be ubiquitous, although experimentally they continue to be observed in frontier regions, including Majorana fermions, magnetic monopoles, Weyl semimetals, and Mott insulators.
Vitor Dantas and Eric Andrade, of the São Carlos Institute of Physics, have taken a step forward in this nascent field by making more realistic studies involving a material known as spin fluid.
Spin is the intrinsic magnetic moment of a particle, for example an electron. It is as if the particle had a magnet in it that enabled it to interact not only with the spins of other particles, but also with external magnetic fields, even at rest.
By analogy with ordinary liquids, systems in which the spins have no magnetic order, even at temperatures close to absolute zero, are called spin liquids. Following the same kind of nomenclature, a system representing magnetic order would be a solid of spins, since the magnetic order of its spins is analogous to the periodic arrangement of atoms in a crystal.
It is precisely these topological phase transitions that are so interesting not only for understanding the fundamental aspects of matter, but also for a new type of quantum computing.
A honeycomb network with two impurities (vacancies), located in the center of the blue hatched areas.
[Imagem: Eric Andrade/USP]
The model that describes spin fluids was formulated by the Russian physicist Alexei Yurievich Kitaev, born in 1963 and currently a professor at the California Institute of Technology.
“The Kitaev model is a two-dimensional spin-fluid proposal. It’s very interesting, but a bit artificial. You’d expect deviations from the original model to show up in real materials. And that’s what we explored at work,” explained Eric.
Specifically, the duo studied how Kitaev’s model behaves in a honeycomb-like network in the presence of defects and disorders, including a magnetic field and additional interactions. “Since 2009, there have been strong theoretical suggestions that this model can be performed on Mott insulators, which exhibit strong spin-orbit coupling,” says Eric.
Mott’s insulators are complex materials that present themselves as electrical insulators, but still have spin dynamics, that is, they behave like magnetic materials. There is great interest in them because they enable separate access to the charge and spin of the electron. On the other hand, spin-orbit coupling refers to the manifestation of the special theory of relativity that connects the spin of the electron with its movement in three-dimensional space.
The Brazilian duo chose iridium oxide, H3LiIrtwoON6. This Mott insulator has no magnetic order up to 50 milkelvin, making it a promising candidate for the Kitaev spin liquid.
“We were able to explain the contradictions that existed between the predictions of the pristine Kitaev model and the experimental results obtained with H3LiIrtwoON6. We have shown that the presence of a small amount of vacancies is sufficient to explain the experimental data. The empty places correspond to the replacement of the Ir magnetic ion with a magnetic no. As hydrogen is a very light element, these defects are expected to appear in the system during the crystallization process,” explained Eric.
The researcher continues: “In addition to providing a consistent scenario of experiments in terms of a disordered Chinese spin liquid, our work also provided an important prediction: this material must present a topological phase in the presence of an external magnetic field. , this topological phase is characterized by the presence, at the edge of the system, of Majorana fermions which they carry a current of quantized energy. Therefore they carry an electric current.”
The existence of the Majorana fermion is something that is being intensively researched. In a paper from 1937, the Italian physicist Ettore Majorana, who is considered one of the greatest geniuses in the history of science and who disappeared without a trace at the age of 31, put forward a hypothesis about a particle that would have itself as an antiparticle. And he suggested that the neutrino might be that particle.
Current research is focused not only on neutrinos, but also on quasiparticles, or apparent particles, composed of excitations in condensed matter systems. Apart from their interest in fundamental physics, these exotic particles, generally called Majorana frmions, define an important frontier in the field of quantum information and computation, as they could be used, for example, for error correction. The work of the Brazilian duo makes another contribution to their understanding.
Article: Disorder, low-energy excitations and topology in the Chinese spin liquid
Authors: Vitor Dantas, Eric C. Andrade
Journal: Physical Review Letters
Vol.: 129, no. 3
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