  The helicity modulus of the S=1/2 XY model is precisely estimated through a
world line quantum Monte Carlo method enhanced by a cluster update algorithm.
The obtained estimates for various system sizes and temperatures are well
fitted by a scaling form with L replaced by \log(L/L_0), which is inferred from
the solution of the Kosterlitz renormalization group equation. The validity of
the Kosterlitz-Thouless theory for this model is confirmed.
