  We have studied the two-dimensional S=1/2 square-lattice antiferromagnet
$Cu(pz)_2(ClO_4)_2$ using inelastic neutron scattering and series expansion
calculations. We show that the presence of antiferromagnetic next-nearest
neighbor interactions enhances quantum fluctuations associated with resonating
valence bonds. Intermediate magnetic fields lead to a selective tuning of
resonating valence bonds and a spectacular inversion of the zone-boundary
dispersion, providing novel insight into 2D antiferromagnetism in the quantum
limit.
