  We calculate the superfluid stiffness of 2D lattice hard-core bosons at
half-filling (equivalent to the S=1/2 XY-model) using the squared winding
number quantum Monte Carlo estimator. For L_x x L_y lattices with aspect ratio
L_x/L_y=R, and L_x,L_y -> infinity, we confirm the recent prediction [N.
Prokof'ev and B.V. Svistunov, Phys. Rev. B 61, 11282 (1999)] that the
finite-temperature stiffness parameters \rho^W_x and \rho^W_y determined from
the winding number differ from each other and from the true superfluid density
\rho_s. Formally, \rho^W_y -> \rho_s in the limit in which L_x -> infinity
first and then L_y -> infinity. In practice we find that \rho^W_y converges
exponentially to \rho_s for R>1. We also confirm that for 3D systems, \rho^W_x
= \rho^W_y = \rho^W_z = \rho_s for any R. In addition, we determine the
Kosterlitz-Thouless transition temperature to be T_KT/J=0.34303(8) for the 2D
model.
