  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.
