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Phonons are quanta of the vibrations of the lattice in solids. They can carry angular momentum and allow an emergent chirality. This Perspective defines various types of chiral phonon and classifies the previously observed manifestations of them.
Higher-order interactions reveal new aspects of the interplay between topology and dynamics in complex systems. This Perspective describes the emerging field of higher-order topological dynamics and discusses the open research questions in the area.
Free-electron quantum optics is an emerging field that requires a quantum-mechanical description of both the electronic and the optical contributions. This Perspective summarizes recent developments and discusses challenges and opportunities.
