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Author(s): M. Eric Tai [1]; Alexander Lukin [1]; Matthew Rispoli [1]; Robert Schittko [1]; Tim Menke [1]; Dan Borgnia [1]; Philipp M. Preiss [1]; Fabian Grusdt [1]; Adam M. Kaufman [...]

Author(s): Anton Mazurenko [1]; Christie S. Chiu [1]; Geoffrey Ji [1]; Maxwell F. Parsons [1]; Mrton Kansz-Nagy [1]; Richard Schmidt [1]; Fabian Grusdt [1]; Eugene Demler [1]; Daniel Greif [1]; [...]

Polarons--electronic charge carriers 'dressed' by a local polarization of the background environment--are among the most fundamental quasiparticles in interacting many-body systems, and emerge even at the level of a single dopant.sup.1. In the context of the two-dimensional Fermi-Hubbard model, polarons are predicted to form around charged dopants in an antiferromagnetic background in the low-doping regime, close to the Mott insulating state.sup.2-7; this prediction is supported by macroscopic transport and spectroscopy measurements in materials related to high-temperature superconductivity.sup.8. Nonetheless, a direct experimental observation of the internal structure of magnetic polarons is lacking. Here we report the microscopic real-space characterization of magnetic polarons in a doped Fermi-Hubbard system, enabled by the single-site spin and density resolution of our ultracold-atom quantum simulator. We reveal the dressing of doublons by a local reduction--and even sign reversal--of magnetic correlations, which originates from the competition between kinetic and magnetic energy in the system. The experimentally observed polaron signatures are found to be consistent with an effective string model at finite temperature.sup.7. We demonstrate that delocalization of the doublon is a necessary condition for polaron formation, by comparing this setting with a scenario in which a doublon is pinned to a lattice site. Our work could facilitate the study of interactions between polarons, which may lead to collective behaviour, such as stripe formation, as well as the microscopic exploration of the fate of polarons in the pseudogap and 'bad metal' phases. Magnetic polarons are imaged with single-site spin and density resolution in the low-doping regime of the atomic Fermi-Hubbard model, showing that mobile delocalized doublons are necessary for polaron formation., Author(s): Joannis Koepsell [sup.1] , Jayadev Vijayan [sup.1] , Pimonpan Sompet [sup.1] , Fabian Grusdt [sup.2] [sup.3] , Timon A. Hilker [sup.1] [sup.5] , Eugene Demler [sup.2] , Guillaume Salomon [...]

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 ., (© 2023. The Author(s), under exclusive licence to Springer Nature Limited.)

Conventional superconductivity emerges from pairing of charge carriers-electrons or holes-mediated by phonons 1 . In many unconventional superconductors, the pairing mechanism is conjectured to be mediated by magnetic correlations 2 , as captured by models of mobile charges in doped antiferromagnets 3 . However, a precise understanding of the underlying mechanism in real materials is still lacking and has been driving experimental and theoretical research for the past 40 years. Early theoretical studies predicted magnetic-mediated pairing of dopants in ladder systems 4-8 , in which idealized theoretical toy models explained how pairing can emerge despite repulsive interactions 9 . Here we experimentally observe this long-standing theoretical prediction, reporting hole pairing due to magnetic correlations in a quantum gas of ultracold atoms. By engineering doped antiferromagnetic ladders with mixed-dimensional couplings 10 , we suppress Pauli blocking of holes at short length scales. This results in a marked increase in binding energy and decrease in pair size, enabling us to observe pairs of holes predominantly occupying the same rung of the ladder. We find a hole-hole binding energy of the order of the superexchange energy and, upon increased doping, we observe spatial structures in the pair distribution, indicating repulsion between bound hole pairs. By engineering a configuration in which binding is strongly enhanced, we delineate a strategy to increase the critical temperature for superconductivity., (© 2023. The Author(s).)