&Bullet; *physics* 14, p82

A lattice of highly excited atoms can have a topological phase, shows a new theoretical study.

To distinguish a loop of yarn from a single, intertwined thread, you need to trace its entire length until you either find an end or get back to the beginning. Similarly, some materials can adopt configurations that cannot be identified by local measurements; they must be examined as a whole. Theorists predict a multitude of uses for these “topologically ordered states” since topological order has been associated with superconductivity and offers desirable properties for quantum computing. Ruben Verresen from Harvard University and colleagues now theoretically show that such a topological phase of cold atoms can be shown on a constructed lattice [1] . The experimental implementation of their proposal would represent a rare observation of topological phases in a material.

The researchers suggest a phase with so-called

${Z}_{2}$topological order in a 2D array of cold atoms. Each atom would form a vertex in a “ruby” grid made up of triangles, rectangles, and hexagons. The team would excite some of the atoms into an extremely high quantum state known as the Rydberg state. Neighboring atoms cannot be excited into this state at the same time, since a Rydberg atom shifts the energy structure of its neighbor in such a way that the excitation laser becomes out of resonance. This restriction would force the system into an alternating pattern of Rydberg and ground state atoms, the interactions of which are mathematically similar to the theories of topological materials proposed in the 1970s. These topological states could be verified by global measurements of the system, for example by determining the proportion of excited atoms along arbitrarily selected atomic chains. The researchers and their staff have started building this system, and early experiments have already shown some of the signatures their theory predicted.

–Sophia Chen

Sophia Chen is a freelance science writer based in Columbus, Ohio.

## References

- R. Verresen
*et al.*, “Prediction of the topological order of the toric code from the Rydberg blockade”, Phys. Rev. X**11**, 031005 (2021).