Picture a radio receiver with a single tuning knob that moves all of its internal coils at once, like one living organism. That is exactly how a new instrument for the search for the axion works, a particle that may fill galaxies with invisible mass. Inside a copper cylinder the height of a coffee can sit rods of aluminum oxide. Turning the shaft makes them move together or apart in sync, changing the frequency to which the resonator is tuned. The mechanics is inspired by kirigami, the art of cutting and folding paper, where a single cut sets the whole structure moving as one.
A conventional resonator searches for the axion on the fundamental mode: the frequency rises if you shrink the cavity. But then the volume drops, and with it the chance of catching a particle. Higher modes solve this problem: the frequency rises without losing volume, yet the electric field inside begins to cancel itself out and the signal weakens.
This is where dielectric rods come to the rescue: they redistribute the field so that it once again interacts efficiently with the magnetic field. The form factor, that is, a measure of this overlap, climbs from nearly zero to 0.55 of the ideal.
But higher modes are temperamental: as the frequency is tuned, the symmetry of the field is easily broken and the signal vanishes. An auxetic mechanism solves this problem too. Auxetics are materials that get thicker, not thinner, when stretched. Here the property is used in reverse: turning the central shaft makes the rods move apart strictly radially, preserving the shape of the field. As a result, the resonator can be tuned smoothly, like a radio receiver, without losing sensitivity.
The setup was cooled to 40 millikelvin, colder than interstellar space, and placed in a magnetic field of 9.8 tesla, hundreds of thousands of times stronger than Earth's. The signal was amplified by a quantum amplifier based on a Josephson junction, which adds a minimum of noise.
Two new scans covered the range 4.98-5.07 gigahertz, which corresponds to an axion mass of 20.61-20.95 microelectronvolts. Together with previous measurements, this yielded continuous coverage of almost 300 megahertz, a first for a single resonator on a higher mode.
Sensitivity reached 1.8 of the KSVZ benchmark, a theoretical model that predicts the strength of the axion-photon coupling. This means that couplings roughly twice as strong as the prediction have been ruled out. No candidate survived the rescan, so upper limits were set at 90% confidence. The work did not catch the axion, but it showed that higher modes are no longer a dead end: they have become a practical tool.
Now this chameleon resonator is ready for new hunts. The principle can be carried over to even higher modes, where the frequency rises by a factor of 3.6, and dielectrics can restore the signal to 0.45 of the ideal. Perhaps that is how we will finally hear the whisper of dark matter in the noise of the cosmos.
Auxetic materials get thicker, not thinner, when stretched, unlike ordinary ones. The tuning mechanism uses this property in reverse: turning the shaft makes the rods move apart or together in sync, changing the resonator frequency the way you tune a radio.
In Liu Cixin's novel The Three-Body Problem, dark matter remains a mystery that humanity tries to crack with giant accelerators. Axion haloscopes like the one described here offer a quieter path: listening to the cosmos in the microwave range, hoping to catch the whisper of an invisible field.