  We study a Heisenberg S=1/2 ring-exchange antiferromagnet which exhibits a
quantum phase transition from a spontaneously dimerized (valence bond solid)
phase to a magnetically ordered (Neel) phase. We argue that the quantum
transition is of unconventional nature; both singlet and triplet modes of high
density condense as the transition is approached from the dimer side, signaling
restoration of lattice symmetry. These features are consistent with "deconfined
quantum criticality", of which the present model is believed to be the only
example so far.
