  We propose a variational method for identifying lattice operators in a
critical quantum spin chain with scaling operators in the underlying conformal
field theory (CFT). In particular, this allows us to build a lattice version of
the primary operators of the CFT, from which we can numerically estimate the
operator product expansion coefficients $C_{\alpha\beta\gamma}^{\textrm{
CFT}}$. We demonstrate the approach with the critical Ising quantum spin chain.
