Realization of a fractional quantum Hall state with ultracold atoms.

Strongly interacting topological matter 1 exhibits fundamentally new phenomena with potential applications in quantum information technology 2,3 . Emblematic instances are fractional quantum Hall (FQH) states 4 , in which the interplay of a magnetic field and strong interactions gives rise to fractionally charged quasi-particles, long-ranged entanglement and anyonic exchange statistics. Progress in engineering synthetic magnetic fields 5-21 has raised the hope to create these exotic states in controlled quantum systems. However, except for a recent Laughlin state of light 22 , preparing FQH states in engineered systems remains elusive. Here we realize a FQH state with ultracold atoms in an optical lattice. The state is a lattice version of a bosonic ν = 1/2 Laughlin state 4,23 with two particles on 16 sites. This minimal system already captures many hallmark features of Laughlin-type FQH states 24-28 : we observe a suppression of two-body interactions, we find a distinctive vortex structure in the density correlations and we measure a fractional Hall conductivity of σ H /σ 0  = 0.6(2) by means of the bulk response to a magnetic perturbation. Furthermore, by tuning the magnetic field, we map out the transition point between the normal and the FQH regime through a spectroscopic investigation of the many-body gap. Our work provides a starting point for exploring highly entangled topological matter with ultracold atoms 29-33 .