Physicists have long been fascinated by the magnetic monopole, a hypothetical particle carrying only a single magnetic pole. Paul Dirac first proposed it in 1931, and despite decades of searching, no such particle has ever been found in nature. Yet monopole-like objects can emerge as singular points in the energy landscapes of certain materials and artificial systems, offering a window into how topology — the mathematics of properties that survive continuous deformation — governs physical behavior.
Now a team at the Beijing Institute of Technology has taken this concept into a new dimension, literally. Hao Yuan, Weixuan Zhang, Na Sun and Xiangdong Zhang have demonstrated that tensor monopoles — singular points in a four-dimensional band structure — can be created, moved and annihilated on demand. Their work, published in Reports on Progress in Physics, marks a significant advance in the experimental control of topological phenomena.
Tensor monopoles are not particles in the conventional sense. They exist in a four-dimensional momentum space, where the fourth dimension is not an extra spatial direction but an additional coordinate represented experimentally by a controllable circuit parameter. Until now, such synthetic monopoles have been essentially fixed in place, observable but immovable. The Beijing team changed that by applying a periodic drive to their system, bringing the monopoles into existence and then shifting their positions simply by turning the drive frequency up or down.
Their calculations showed that the monopoles are created in oppositely charged pairs, which then move through the four-dimensional momentum space before eventually annihilating one another. The researchers confirmed these predictions through voltage measurements on the circuit, which revealed the expected band crossings. They also observed Fermi arc surface states linking monopoles of opposite charge, a hallmark of their topological nature.
The ability to manipulate these exotic objects opens a new avenue for exploring how topology behaves in driven systems far from equilibrium. Such systems are notoriously difficult to study, and having a controllable testbed could allow physicists to probe fundamental questions about the interplay between topology and dynamics.
Beyond fundamental physics, the work points toward practical applications. 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 while continuing to operate reliably in imperfect conditions. This kind of reconfigurability could be valuable in communications, sensing and computing, where robustness and adaptability are increasingly important.
The experiment is only a proof of principle, and any real-world device remains years away. But it represents an early step toward that possibility, demonstrating that tensor monopoles can be controlled with a simple periodic drive rather than being passive curiosities. The team's approach relies on standard circuit components and well-understood driving techniques, making it potentially accessible to other laboratories.
The work also highlights how abstract mathematical concepts can become tangible in the laboratory. Topology has already reshaped our understanding of materials, from topological insulators to quantum Hall systems, and the ability to manipulate topological defects in higher dimensions suggests that even richer phenomena may be within reach. For now, the Beijing experiment offers a rare glimpse of four-dimensional physics in action — not in a particle accelerator or a cosmic setting, but on a tabletop circuit.
15
