Hidden-symmetry-protected quantum pseudo–spin Hall effect in optical lattices

We propose a scheme to realize a Z₂ topological insulator in a square optical lattice. Different from the conventional topological insulator protected by the time-reversal symmetry, here the optical lattice possesses a hidden symmetry, which is responsible for the present Z₂ topological order. With a properly defined pseudospin, such a topological insulator is characterized by the helical edge states that exhibits pseudo--spin-momentum locking, so it can be considered as a quantum pseudo--spin Hall insulator. The Z₂ topological invariant is derived and its experimental detection is discussed as well.