Only recently has it become clear that quantum fluctuations do not des
troy long-range order in the ground state of the 2D S = 1/2 square-lat
tice Heisenberg antiferromagnet. We report a neutron scattering study
of the static structure factor of Sr2CuO2Cl2, which is an optimal expe
rimental realization of this model. Excellent agreement for the magnet
ic correlation length is found to exist between theory, Monte Carlo re
sults and our experiment.