  We report on the realization of long-range Ising interactions in a cold gas
of cesium atoms by Rydberg dressing. The interactions are enhanced by coupling
to Rydberg states in the vicinity of a F\"{o}rster resonance. We characterize
the interactions by measuring the mean-field shift of the clock transition via
Ramsey spectroscopy, observing one-axis twisting dynamics. We furthermore
emulate a transverse-field Ising model by periodic application of a microwave
field and detect dynamical signatures of the paramagnetic-ferromagnetic phase
transition. Our results highlight the power of optical addressing for achieving
local and dynamical control of interactions, enabling prospects ranging from
investigating Floquet quantum criticality to producing tunable-range spin
squeezing.
