The first experimental realisation of Floquet tensor monopoles provides a controllable platform for investigating non-equilibrium topology in four dimensions
Ever since Paul Dirac proposed the magnetic monopole in 1931, physicists have searched for particles carrying a single magnetic pole, so far without success. But monopole-like objects can also emerge as unusual points in the energy landscapes of materials and artificial systems.
These objects help reveal how topology, the mathematics of properties that survive continuous deformation, shapes physical behaviour. Recent experiments have pushed this idea beyond familiar three-dimensional settings by creating tensor monopoles.
Tensor monopoles are singular points in a four-dimensional band structure. The fourth dimension is not an extra direction in space, but an additional coordinate in the system’s four-dimensional momentum space, represented experimentally by a controllable circuit parameter.
Until now, however, these synthetic monopoles have been essentially fixed in place. Being able to create, move and annihilate them on demand would open a new way to explore how topology behaves in driven systems far from equilibrium, while providing a testbed for wave-based devices whose properties could eventually be reconfigured in real time.
This is exactly what Hao Yuan, Weixuan Zhang, Na Sun and Xiangdong Zhang at the Beijing Institute of Technology have now demonstrated.
Rather than simply observing tensor monopoles at fixed points, the researchers used a periodic drive to bring them into existence and then shifted their positions by turning the frequency up or down. Their calculations showed the monopoles being created in oppositely charged pairs, moving through the system’s four-dimensional momentum space and eventually annihilating.
Voltage measurements from the circuit revealed the predicted band crossings, while the observation of Fermi arc surface states linking monopoles of opposite charge confirmed their topological properties.
The long-term promise is technology that can change what it does without changing what it is: circuits and wave-guiding devices whose signal paths could be rearranged electronically and keep working in imperfect conditions. Although this experiment is only a proof-of-principle, it is an early step towards that possibility.