Topological Josephson plasmon modes on honeycomb lattice [Invited]

Topological protection is a promising way to deal with the decoherence issue in quantum systems. To induce topological excitations in Josephson junction arrays (JJA), we propose a scheme where only Josephson critical currents are tuned in JJA on honeycomb lattice, which is easily achieved by changing potential barriers or lengths of insulators between superconducting islands. In the present system, a hexagonal cluster with six superconducting islands forms a unit cell, and the difference between intra- and inter-hexagon Josephson critical currents yields a topologically nontrivial state accompanied by a band inversion. Topological Josephson plasmon modes appear at the interface between topological and trivial domains, carrying pseudospin dominant energy flows, which mimics the spin-momentum locking in the renowned quantum spin Hall effect.