  Line excitations in topological phases are a subject of particular interest
because their mutual linking structures encode robust topological information
of matter. It has been recently shown that the linking and winding of complex
eigenenergy strings can classify one-dimensional non-Hermitian topological
matter. However, in higher dimensions, bundles of linked strings can emerge
such that every string is mutually linked with all the other strings.
Interestingly, despite being an unconventional topological structure, a
non-Hermitian Hopf bundle has not been experimentally clarified. Here, we make
the first attempt to explore the non-Hermitian Hopf bundle by visualizing the
global linking structure of spinor strings in the momentum space of a
two-dimensional electric circuit. By exploiting the flexibility of
reconfigurable couplings between circuit nodes, we can study the non-Hermitian
topological phase transition and gain insight into the intricate structure of
the Hopf bundle. Furthermore, we find that the emergence of a higher-order skin
effect in real space is accompanied by the linking of spinor strings in
momentum space, revealing a bulk-boundary correspondence between the two
domains. The proposed non-Hermitian Hopf bundle platform and visualization
methodology pave the way to design new topologically robust non-Hermitian
phases of matter.
