  Given a critical quantum spin chain, we show how universal information about
its quantum critical point can be extracted from wavefunction overlaps. More
specifically, we consider overlap between low-energy eigenstates of the spin
chain Hamiltonian with different boundary conditions, namely periodic boundary
conditions and open boundary conditions. We show that such overlaps decay
polynomially with the system size, where the exponent only depends on the
central charge. Furthermore, the bulk-to-boundary operator product expansion
(OPE) coefficients can be extracted from the overlaps involving excited states.
We illustrate the proposal with the Ising model and the three-state Potts
model.
