Braiding Majorana corner modes in a two-layer second-order topological insulator

The recent advances in the field of topological materials have established a novel understanding of material physics. Besides theoretical achievements, a number of proposals for decoherence-protected topological quantum computation were provided. It is, however, a yet unanswered question, what material could be the most feasible candidate in engineering the building blocks of a quantum computer (qubits). Here we propose a possible answer by describing a device based on a two-dimensional second-order topological insulator with particle-hole symmetry (PHS). This material has one-dimensional boundaries, but exhibits two zero-dimensional Majorana quasiparticles localized at the corners of a square-shaped sample. The two states reside at zero energy as long as PHS is conserved, whereas their corner-localization can be adjusted by in-plane magnetic fields. We consider an adiabatic cycle performed on the degenerate ground-state manifold and show that it realizes the braiding of the two zero-energy corner modes. We find that each zero-mode accumulates a non-trivial statistical phase π within a cycle, which confirms that, indeed, PHS ensures non-Abelian Majorana excitation braiding in the proposed device. The fractional statistics of the corner states opens the possibility to perform logical operations and, ultimately, might enable building robust qubits for large scale implementations. We also suggest possible paths for experimental realizations of this proposal