  Recent experiments on several cuprate compounds have identified an enhanced
thermal Hall response in the pseudogap phase. Most strikingly, this enhancement
persists even in the undoped system, which challenges our understanding of the
insulating parent compounds. To explain these surprising observations, we study
the quantum phase transition of a square-lattice antiferromagnet from a
confining N\'eel state to a state with coexisting N\'eel and semion topological
order. The transition is driven by an applied magnetic field and involves no
change in the symmetry of the state. The critical point is described by a
strongly-coupled conformal field theory with an emergent global $SO(3)$
symmetry. The field theory has four different formulations in terms of $SU(2)$
or $U(1)$ gauge theories, which are all related by dualities; we relate all
four theories to the lattice degrees of freedom. We show how proximity of the
confining N\'eel state to the critical point can explain the enhanced thermal
Hall effect seen in experiment.
