Universal conductivity in the two-dimensional boson Hubbard model

We use quantum Monte Carlo methods to evaluate the conductivity {σ} of the two-dimensional disordered boson Hubbard model at the superfluid--Bose-glass phase boundary. At the critical point for particle density {ρ}=0.5, we find σc=(0.45±{}0.07)σQ, where σQ=e*²/h from a finite-size scaling analysis of the superfluid density. We obtain σc=(0.47±{}0.08)σQ from a direct calculation of the current-current correlation function. Simulations at the critical points for other particle densities, {ρ}=0.75 and 1.0, give similar values for {σ}. We discuss possible origins of the difference in this value from that recently obtained by other numerical approaches.