# Heidelberg 2015 – wissenschaftliches Programm

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# Q: Fachverband Quantenoptik und Photonik

## Q 15: Poster: Quantum Optics and Photonics I

### Q 15.47: Poster

### Montag, 23. März 2015, 17:00–19:00, C/Foyer

**Measuring the Chern number of Hofstadter bands with ultracold bosonic atoms** — •Christian Schweizer^{1,2}, Monika Aidelsburger^{1,2}, Michael Lohse^{1,2}, Marcos Atala^{1,2}, Julio Barreiro^{1,2}, Sylvain Nascimbène^{3}, Nigel Cooper^{4}, Immanuel Bloch^{1,2}, and Nathan Goldman^{3,5} — ^{1}Fakultät für Physik, LMU München, Germany — ^{2}MPQ Garching, Germany — ^{3}Collège de France & LKB, CNRS, UPMC, ENS, Paris, France — ^{4}T.C.M. Group, Cavendish Laboratory, Cambridge, UK — ^{5}CENOLI, Faculté des Sciences, Université Libre de Bruxelles, Belgium

Sixty years ago, Karplus and Luttinger pointed out that quantum particles moving on a lattice could acquire an anomalous transverse velocity in response to a force, providing an explanation for the unusual Hall effect in ferromagnetic metals. A striking manifestation of this transverse transport was then revealed in the quantum Hall effect, where the plateaus depicted by the Hall conductivity were attributed to a topological invariant characterizing Bloch bands: the Chern number. Until now, topological transport associated with non-zero Chern numbers has only been revealed in electronic systems. Here we use studies of an atomic cloud’s transverse deflection in response to an optical gradient, in combination with the determination of the band populations to measure the Chern number ν of artificially generated Hofstadter bands; for the lowest band we obtain an experimental value of ν_{exp}=0.99(5). This result, which constitutes the first Chern-number measurement in an atomic system, is facilitated by an all-optical artificial gauge field scheme, generating uniform flux in optical superlattices.