Condensed-matter physics: magnetic fields without magnetic fields.

Abstract

Exquisite control of quantum systems has allowed researchers to connect reality to ideas of how an exotic form of particle transport known as the quantum Hall effect can occur in the absence of a magnetic field. See Letters p.237 & p.241 The quantum Hall effect leads to topologically protected edge states, and for a long time was thought to exclusively emerge in the presence of an external magnetic field. But in 1988, Duncan Haldane proposed a model in which this exotic electronics structure arises without this requirement. He proposed that, in a honeycomb lattice with a staggered flux, the necessary ingredients for a quantum Hall effect would be inherent in the material itself. The principles behind this concept were later recruited to design topological insulators, but in its original expression, the Haldane model has not been observed in the laboratory. In this issue of Nature, two groups report on progress connected to the Haldane model. Gregor Jotzu et al. report the first realization of the Haldane model and Pedram Roushan et al. show how it can be precisely measured. Jotzu et al. use ultracold fermions to realize the breaking of time-reversal and inversion symmetry — the two main requirements of the model — by implementing a circular modulation of the lattice position and an energy offset between neighbouring sites. Roushan et al. use superconducting quantum circuits — a Josephson junction sandwiched between superconducting electrodes — to realize a non-interacting form of the Haldane model with a single qubit and an interacting two-qubit model through a new experimental setup called 'gmon' coupling architecture. Their setup allows them to characterize both cases by measuring the Berry curvature, a feature that all topological structures have in common.