A compass needle trembles when an invisible current passes nearby. Physicists built a tiny compass from a single defect in diamond and brought it close to the edge of a superconductor to listen to the electric currents running there. These currents are unlike ordinary ones: they flow only along the boundary, like water along a riverbank, and do not scatter off obstacles. Until now, their existence in the twisted cuprate superconductor BSCCO had been predicted only theoretically, but no one had seen them directly.

The experiment is like listening to footsteps in the next room. A defect in diamond, called an NV center, senses a magnetic field like a compass needle. When currents run along the edge of the superconductor, they create magnetic noise that makes the spin of the NV center lose its orientation faster. By measuring this rate, physicists saw two bright peaks exactly at the edges of the device at a temperature of 50 K. This is direct proof that edge currents exist.

The dependence of the signal on the distance to the edge turned out to be inversely proportional to the square of the distance, which matches the prediction of theory. From temperature measurements, the magnitude of the topological gap, the energy gap that protects the edge currents, was extracted. At zero temperature, it is about 4 meV, which corresponds to a temperature of about 50 K. This means that topological protection works only at sufficiently low temperatures.

The twist angle turned out to be a critical parameter. At 45°, the topological transition occurs at the same temperature as the ordinary transition to the superconducting state, about 86 K. At 43°, this temperature drops to 70 K, at 40° to 30 K, and at 37° and 24°, the edge modes disappear altogether. This is like tuning a musical instrument: the slightest rotation of the layers changes the entire melody of the edge currents.

A magnetic field adds a new note. Applying a field of 20 G creates a periodic pattern of alternating domains with opposite chirality, that is, currents in neighboring regions run in different directions. This points to the formation of a lattice of Josephson vortices, quantum vortices of magnetic flux trapped between the layers. Such a pattern resembles a chessboard, where black and white squares correspond to currents running in opposite directions.

So a single defect in diamond became a compass that not only pointed the direction but also discerned the fine structure of the edge currents. Previously, the topology of twisted superconductors was judged by indirect transport measurements or theoretical models. Now a direct method has emerged that can be applied to other twisted materials, for example, bilayer graphene or transition metal dichalcogenides. This is a step toward creating high-temperature topological superconductors that could become the basis for error-resistant quantum computing.