The Mundane Topology

Re-entrant switching currents in Josephson junctions — where the supercurrent first decreases and then increases again as the magnetic field increases — have been reported as signatures of topological phase transitions. The re-entrance is interpreted as evidence that the junction passes through a topological phase boundary, entering a state that supports Majorana bound states before returning to a trivial phase at higher fields. The observation is consistent with theoretical predictions for topological superconductors.

Mudi, Anupam, Mourik, and Frolov (arXiv:2603.28530, March 2026) demonstrate that the same re-entrant switching currents are reproduced by mundane mechanisms that do not involve topology. Mode interference in a disordered junction produces re-entrance through the beating of multiple Andreev bound states whose energies cross as the field changes. Supercurrent interference from a corrugated weak link — geometric roughness at the junction interface — creates field-dependent Fraunhofer-like patterns that mimic re-entrance. Neither mechanism requires Zeeman splitting, spin-orbit coupling, or topological phase transitions.

Multiple non-topological mechanisms fit the data equally well. The re-entrant switching current is not a specific signature of topology — it is a generic signature of multi-mode junctions in magnetic fields. Any system with several current-carrying channels whose field dependences differ will show non-monotonic switching current as a function of field, because the channels can constructively or destructively interfere.

The structural observation: a signature that is consistent with topology is not evidence for topology when non-topological mechanisms produce the same signature. The re-entrant switching current is a necessary consequence of the topological transition but not a sufficient one — it is necessary for many other phenomena too. The measurement does not select between explanations; it is degenerate across them. This is the same measurement degeneracy as "The False Antiferroelectric" (#7002): a macroscopic observable with multiple microscopic origins.