Nature Commun. 7, 11994 (2016)

Many situations in physics can be formulated as a many-body problem — and for systems displaying spontaneous symmetry breaking, such as superconductors, superfluids and ferromagnets, the many-body description involves local order parameters. For some other systems, however, such parameters do not exist — they need to be characterized by nonlocal topological invariants. The integer and fractional quantum Hall effects, for example, are described by topological order parameters.

Experimentally probing the topological order of quantum phases is challenging. But Fabian Grusdt and colleagues have now proposed an approach whereby impurity atoms, deliberately introduced into a many-body system, act as probes for the topological invariants. Importantly, the method would enable probing the bulk of the system. They extended the concept of the 'topological polaron' — an impurity bound to a topological excitation — and developed a scheme for interferometric measurements of topological invariants. Grusdt et al. applied their ideas to quantum Hall systems and Chern insulators, and argue that cold-atom ensembles would be ideal platforms for realizing topological polaron interferometry.