Photonic analogue of quantum spin Hall effect

TL;DR

This paper demonstrates that photonic topological insulators exhibit robust edge states protected by time-reversal symmetry, revealing a fundamental difference from electronic systems and opening new avenues in topological photonics.

Contribution

It establishes that photonic topological edge states are protected by fermionic time-reversal symmetry, not bosonic, clarifying the topological protection mechanism for photons.

Findings

Photonic pseudo-spin-dependent edge states are protected by fermionic time-reversal symmetry.

Topological protection is robust against Tf-invariant impurities.

First demonstration of topological nature of photons in this context.

Abstract

Symmetry-protected photonic topological insulator exhibiting robust pseudo-spin-dependent transportation, analogous to quantum spin Hall (QSH) phases and topological insulators, are of great importance in fundamental physics. Such transportation robustness is protected by time-reversal symmetry. Since electrons (fermion) and photons (boson) obey different statistics rules and associate with different time-reversal operators (i.e., Tf and Tb, respectively), whether photonic counterpart of Kramers degeneracy is topologically protected by bosonic Tb remains unidentified. Here, we construct the degenerate gapless edge states of two photonic pseudo-spins (left/right circular polarizations) in the band gap of a two-dimensional photonic crystal with strong magneto-electric coupling. We further demonstrated that the topological edge states are in fact protected by Tf rather than commonly believed Tb and their pseudo-spin dependent transportation is robust against Tf invariant impurities, discovering for the first time the topological nature of photons. Our results will pave a way towards novel photonic topological insulators and revolutionize our understandings in topological physics of fundamental particles.