Dark matter doesn't emit or reflect light, yet its gravity holds galaxies together. What it's made of is a mystery. One leading candidate is the axion, an ultra-light particle. Searching for axions is like listening for a very faint radio signal, but the receiver's own noise gets in the way.
Scientists have proposed a new kind of receiver: a single electron in a Penning trap — a device where magnetic and electric fields hold a charged particle suspended in place. The electron absorbs microwave photons, particles of light. Each absorption slightly shifts the phase of its oscillation. This shift can be detected if the signal is amplified in a special way that suppresses thermal noise. This single-photon counter operates at frequencies between 30 and 60 GHz and is suitable for hunting axions with masses from 0.124 to 0.248 meV (millielectronvolts — a unit of energy so tiny it's minuscule even for elementary particles).
The method is compared to an ideal detector. It falls short only slightly, but that's enough to probe, for the first time, a mass range that was previously unreachable. Beyond the hunt for dark matter, the technique could be used for ultra-precise frequency measurements in Penning traps.
A Penning trap can hold a single electron for months without letting it touch the walls, like levitation in an invisible cage of magnetic fields.