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Watching fermions transition on site
Optical lattices are a promising platform for simulating the many-body physics that occurs in solids. In lattices filled with cold bosonic atoms, “quantum microscopy” makes it possible to watch quantum phase transitions as they unravel. Greif et al. bring a similar capability to lattices filled with fermions, which are trickier to cool but are a closer match to electrons in a solid. Tuning the interaction between the 6Li atoms allowed for the observation of transitions from a metallic phase to a band insulator and then to an interaction-dominated Mott insulator phase.
Science, this issue p. 953
The complexity of quantum many-body systems originates from the interplay of strong interactions, quantum statistics, and the large number of quantum-mechanical degrees of freedom. Probing these systems on a microscopic level with single-site resolution offers important insights. Here we report site-resolved imaging of two-component fermionic Mott insulators, metals, and band insulators, using ultracold atoms in a square lattice. For strong repulsive interactions, we observed two-dimensional Mott insulators containing over 400 atoms. For intermediate interactions, we observed a coexistence of phases. From comparison to theory, we find trap-averaged entropies per particle of 1.0 times the Boltzmann constant (kB). In the band insulator, we find local entropies as low as 0.5 kB. Access to local observables will aid the understanding of fermionic many-body systems in regimes inaccessible by modern theoretical methods.