An optical contortionist

Topological Optics The development of gauge fields is fundamental to our theoretical understanding of interactions in physical systems. There are two kinds of fields: Abelian, in which the measured effects on an observable parameter are commutative; and non-Abelian (noncommutative), where the sequence in which the field is applied matters. The latter are more difficult to realize in solid-state systems, but recent theoretical work has suggested that these could be synthesized optically. Yang et al. generated non-Abelian gauge fields by cascading multiple nonreciprocal optical elements and verified this accomplishment by the observed interference patterns in a Sagnac interferometer. Having a system that is tunable between Abelian and non-Abelian regimes will be important for studying complex topological states in photonic platforms. Science , this issue p. [1021][1] [1]: /lookup/doi/10.1126/science.aay3183