Developing robust qubits through understanding exotic quasiparticles. Copyright Genevieve Martin, ORNL.A splash of neutrons has helped demonstrate novel behavior in a 2D solid state material, providing a better understanding of exotic quasiparticles that could be significant for future approaches to quantum computing. The findings from a new study show that the signatures of fractionalization – the tendency of quantum systems to behave different from the sum of their parts – exist in some materials, and can be measured directly and understood physically if the right approach is used.
The work, by scientists from the DOE’s Oak Ridge National Laboratory and the University of Tennessee in the US, in collaboration with the Max Planck Institute in Germany and Cambridge University in the UK, was reported in Nature Materials [Banerjee et al. Nat. Mater. (2016) DOI: 10.1038/nmat4604]. It expanded upon the work of Alexei Kitaev, who a decade ago produced a theoretical model of microscopic magnets that interact in a way that leads to a disordered state called a quantum spin liquid, which exhibits remarkable properties and supports magnetic excitations equivalent to Majorana fermions, or “quasiparticles”. These are particles that are their own antiparticles, and could become the basis for robust qubits that resolve the problem of quantum decoherence, making them possible building blocks for quantum computers.
“The concept of Majorana fermion originated in fundamental high energy particle physics, but we saw their signatures in a solid state material at modest temperatures”Arnab Banerjee
Developing robust qubits through understanding exotic quasiparticles. Copyright Genevieve Martin, ORNL.Kitaev’s model proposes that certain honeycomb lattice materials display bond-dependent anisotropic magnetic interactions, offering a new type of quantum spin liquid ground state that exhibits these fractionalized Majorana fermions as one of the excitations. The researchers found that one way to observe spin liquid physics in such a material is to “splash” or excite the spins using neutron scattering, using the technique on pure samples of alpha-ruthenium trichloride, a layered material comparable to graphene. The “splash” helped the team to see them, as well as directly measure the resulting magnetic excitations.
When neutrons shine onto and scatter from the material, they can leave small amounts of energy that create magnetic excitations. The form of magnetic excitations created in the material was found to be different than from spin waves seen in ordinary magnets, but extremely similar to the spectrum predicted for the Majorana fermions in the quantum spin liquid. As lead author Arnab Banerjee said “The concept of Majorana fermion originated in fundamental high energy particle physics, but we saw their signatures in a solid state material at modest temperatures”.
The magnetic honeycomb semiconductor achieved this as it meets the required conditions of low-spin ground state and high degree of quantum fluctuations. However, this is only an initial step towards an understanding of such observed and tuned materials’ properties, and the team hopes to enhance its applicability by performing thin-film measurements on these materials using similar exfoliation techniques as have been carried out with graphene.