  Symmetry protected topological states (SPTs) have the same symmetry and the
phase transition between them are beyond Landau's symmetry breaking formalism.
In this paper we study (1) the critical theory of phase transition between
trivial and non-trivial SPTs, and (2) the relation between such critical theory
and the gapless boundary theory of SPTs. Based on examples of SO(3) and SU(2)
SPTs, we propose that under appropriate boundary condition the critical theory
contains the delocalized version of the boundary excitations. In addition, we
prove that the boundary theory is the critical theory spatially confined
between two SPTs. We expect these conclusions to hold in general and, in
particular, for discrete symmetry groups as well.
