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 ? = 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 s(H)/s(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).