A compass needle trembles when a current passes nearby. Physicists built a compass out of a single defect in diamond and brought it close to the edge of a superconductor. There, like water flowing along a shore, electric currents run. Their existence had only been assumed before, but now they have been seen directly.
The defect in diamond, called an NV center, works like a compass needle. When currents run along the edge of the superconductor, they create magnetic noise that makes the needle tremble. By measuring this trembling, physicists saw a bright signal exactly at the edges of the device. This proved that edge currents exist.
The twist angle between the layers turned out to be critical. At 45°, edge currents appear at a temperature of about 86 K; at 43°, at 70 K; and at 40°, only at 30 K. If the layers are rotated even further, the currents disappear entirely. So the tiny compass helped tune the superconductor like a musical instrument, where the slightest turn changes the whole melody.
An NV center in diamond is an atom-sized defect capable of sensing the magnetic field from a single electron spin at distances of tens of nanometers. In this work it played the role of a "quantum ear," picking up the noise of quasiparticles racing along the edge of a superconductor.
The idea of topologically protected edge states echoes the concept of "insulating walls" in science fiction, for example the protective fields in Greg Egan's novel Quarantine, where information is stored in topological defects of spacetime.