Graphene magnets, key to building quantum computers

A group of international researchers from Switzerland, Portugal, Spain and Germany, led by scientists from EMPA (Zurich) and the International Iberian Nanotechnology Laboratory (INL), and including the University of Alicante, have just discovered a change in the behavior of magnetic molecules when interactingin an experiment in which they have managed to build strings of quantum magnets made of nanographenes.

This change, whicha priori it is counterintuitive”, as Joaquín Fernández Rossier, research professor in the Department of Applied Physics of the UA and the INL points out, supposes the verification of what the English physicist Duncan Haldane already predicted in the eighties when capturing the essence of one of the central models of quantum magnetismproposed in 1983 by FDM Haldane, work that earned him the Nobel Prize in 2016. The discovery has been published in the journal Nature on October 13, 2021 in the article Observation of fractional edge excitations in nanographene spin chains.

The result obtained in this international collaboration opens the door to the possibility of building quantum computers using this type of molecules.

Differences arise in interaction: the “sociology of particles”

International collaboration has left the creation of magnetic molecules in the hands of scientists at the University of Dresden; EMPA, in Switzerland, has been commissioned to carry out the experiments; and at the University of Alicante Gonçalo Catarina, Ricardo Ortiz and Joaquín Fernández Rossier have carried out the theoretical part of the collaboration, analyzing the data obtained and concluding that what Haldane predicted has occurred. In the 1980s, the English physicist Duncan Haldane built a mathematical model for spin 1 particles in which the fractionation of the spins occurred.

To conclude this result, researchers have created magnetic molecules. These molecules are in the shape of a triangle and are each a small magnet, like a compass. Unlike normal compasses, a quantum compass can only point in a few directions. Thus, the compasses of the electrons can only point in two directions, which in the jargon of theoretical physicists is expressed by saying that electrons have “spin ½”. The compasses of the molecules in the experiment can only point in three directions, and it is then said that they have spin 1. In the attached image (number 4), taken with a scanning tunneling microscope (STM), which allows to visualize and study atoms and molecules , you can see an image taken with a scanning tunneling microscope with chains of molecules or chains of magnets, formed by each magnetic molecule in a triangular shape.

The surprising result, predicted by Haldane, is that when spin 1 molecules come together to form a chain, their magnetism disappears throughout the chain except for the first and last units. In these units the system corresponds as if it had a spin ½, that is, the quantum magnetism of a single electron. Thus, the result of combining magnetic molecules of three states, or spin 1, gives rise to the emergence of magnetic molecules of two states, or spin ½.

The importance of this result is “almost philosophical”, points out the UA and INL researcher. “Particle physics focuses on studying elementary particles and their basic properties, such as their charge, spin, and mass.” These types of experiments show that “the properties of a system are very different from the parts with which the system is made. The behavior is very different, ”he concludes. “This is called ‘the sociology of particles’ or of electrons in this case.”

Joaquín Fernández Rossier affirms that this work “shows the potential to use nanographenes to form two-dimensional networks of nanomagnets that make it possible to verify predictions similar to those of Haldane, such as the existence of quantum states that would allow measurement-based quantum computing.”

quantum computing

In conventional quantum computing it is necessary to put many pairs of quantum bits in communication (qubits), initially independent, to achieve their quantum entanglement. An alternative scheme is to start from a situation in which the quantum bits are already entangled, that is, they are not independent “With this you simplify the task when making this type of computer work.”

Physicists are considering the possibility of making a two-dimensional crystal with these molecules, “it is possible that these triangles (molecules) had this property to do measurement-based quantum computing.” This is the next job that they are going to carry out: the creation of this type of crystal and check if it works. “It’s going from one dimension to two dimensions.” Achieving this property in the crystal would reduce the number of operations in quantum computers, “since these computers lose coherence, a quantum property that allows them to be in two states at the same time and which is essential for the execution of quantum algorithms.”

Bibliographic reference:

S Mishra, G Catarina, F Wu, R Ortiz, D Jacob, K Eimre, J Ma, CA Pignedoli, X Feng, P Ruffieux, J Fernández-Rossier, R Fasel; Observation of fractional edge excitations in nanographene spin chains. Nature 598, 287–292 (2021).

DOI: 10.1038/s41586-021-03842-3

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