Superconducting interference requires a loop. In a SQUID — superconducting quantum interference device — two Josephson junctions connected in a ring create a loop through which magnetic flux can thread. The critical current oscillates with the enclosed flux, producing the most sensitive magnetometer known. The key is the geometry: you need to fabricate the ring, deposit the junctions, pattern the device.
Yin, Cao, Feng, and collaborators (arXiv 2602.22788, February 2026) observe superconducting interference patterns in unpatterned few-layer NbSe₂ — a thin sheet of superconductor with no junctions, no rings, no deliberately created geometry. Periodic magnetoresistance oscillations appear in the superconducting fluctuation regime, just above the transition where global superconductivity is lost but local superconducting correlations persist.
The mechanism is thermally activated vortices. In a two-dimensional superconductor near the transition, the condensate contains vortices — topological defects around which the superconducting phase winds by 2π. At low temperatures, vortex-antivortex pairs are bound together (the Berezinskii-Kosterlitz-Thouless regime). Near the transition, pairs unbind, and free vortices proliferate. The free vortices destroy global phase coherence — the resistance reappears.
But the loss of coherence is not uniform. Local regions maintain superconducting correlations, and supercurrents circulate in loops defined by the vortex configuration. As vortices move through these loops under thermal activation, the enclosed flux changes. The critical current of each loop oscillates with the flux, producing magnetoresistance oscillations that mirror SQUID behavior — but without any engineered geometry.
The oscillation period corresponds to flux quantization through naturally occurring supercurrent loops whose size is set by the inter-vortex spacing. The amplitude depends on temperature: stronger near the transition where vortex density is optimal (enough vortices to define loops, not so many that the signal averages out), weaker far from the transition in either direction.
The superconducting diode effect — different critical current in the two current directions — also appears, with efficiency that tracks the oscillation amplitude. The diode effect, like the interference, arises from the broken symmetry of vortex motion in the presence of current. No inversion symmetry breaking was designed into the device; the vortex configuration provides it spontaneously.
An unpatterned flake of NbSe₂ — atomically thin, featureless to any lithographic inspection — contains its own interferometer. The geometry is not in the device. It's in the phase.