A tiny ring of superconductor, cooled to near absolute zero, could feel the touch of a particle billions of times lighter than an electron. We're talking about the axion — a hypothetical particle that might make up the dark matter filling the universe. If axions exist, they should interact weakly with light, and that interaction can be turned into a measurable electrical signal.

In the new study, physicists showed that an axion passing through a superconducting ring creates a tiny magnetic flux. The ring has a weak spot — a thin insulating layer through which electron pairs tunnel. This is the Josephson effect, and it's extremely sensitive to magnetic flux: even a single flux quantum shifts the phase of the quantum wave by 2π. The oscillating axion background makes this flux jitter, producing an alternating voltage at the output. Such a "quantum compass" could detect axion-photon coupling 10–100 times weaker than current setups, for an axion mass around 10⁻¹⁰ eV.

Another proposed experiment uses a Mach-Zehnder interferometer — a device where a light beam is split into two paths and then recombined. If a slowly rotating magnetic field acts on one path, the light acquires a geometric phase (Berry phase) that depends on whether axions are present. This approach could search for axions even if they don't make up dark matter, though the sensitivity is currently lower. Finally, in a more complex system of photons and axion-like quasiparticles in a topological magnetic insulator, a terahertz Berry phase emerges that can be measured: this would test the whole scheme.

So quantum phases become a new tool in the hunt for dark matter. Superconducting circuits already exist in labs, and quantum interferometry is advancing rapidly. Perhaps the first encounter with an axion won't happen in a giant detector, but in a small ring where the wave's phase quietly shifts by an elusive fraction.