ADMX-VERA

The Axion Dark Matter eXperiment - Volume Enhanced Resonator for Axions (ADMX-VERA) Program

The axion is a compelling wave-like dark matter candidate which also cleans up the strong CP problem. While dark matter by definition interacts weakly with the standard model, axions immersed in a strong magnetic field convert into a photon with frequency equal to the axion mass (in natural units) via the inverse Primakoff effect. Detectors using this effect to probe axions in our galactic halo are called haloscopes. Invented in 1983 by Pierre Sikivie, they are now gaining in popularity with advancements in quantum microwave technology as well as the proliferation of cryogenic facilities.

The predicted axion-generated signal in a haloscope is around 10-24 W. We must use every technology at our disposal to render it more detectable compared to the electronic static, or “noise,” in all detectors. This can be achieved by amplifying it using superconducting quantum amplifiers, for example Josephson parametric amplifiers (JPAs), which add the least noise physically allowed by the Heisenberg uncertainty principle. We’re also interested in qubit-based single photon detectors, which at frequencies above ~10 GHz can become more sensitive than amplifiers. We can also reduce the thermal background noise by cooling the detector down as much as possible, typically to around 100 milli-Kelvin, in a dilution refrigerator.

The technology that makes haloscopes feasible, though, is the resonator. Resonators are essentially boxes that hold photons. If more photons come in from converting axions than go out as radiation or heat, our power reading goes up, and the axion is easier to find. Because the dark matter halo is all around us, all it takes to catch more axions is a bigger box. The tricky thing is that photons only go in boxes that perfectly fit – if you want to hold higher-frequency photons, you have to make a smaller box. This presents a problem – high frequency resonators make bad axion detectors. Here are some concrete numbers to get a sense for when this scaling becomes a problem. The main ADMX haloscope is currently searching for axion signals at around 1 GHz (4 μeV axion mass), with planned experiments up to 4 GHz (16 μeV). ADMX-VERA is currently prototyping resonators from 5 to 8 GHz, with plans stretching into the 10’s of GHz, where ‘post-inflationary’ theories predict the axion will be found.

The key to resonating at higher frequencies without losing out on total volume is a clever choice of resonator geometry. In general, making one of the resonator’s dimensions small sets the frequency, and you’re free to scale up as much as you like in the other two. Because one dimension is much smaller than the others, the resonator is very sensitive to misalignment in the parts and non-idealities in the machining, which we mitigate with an automated alignment algorithm and by hand-lapping the parts. The first of our designs, the “single-wedge” (left diagram and photo in the figure) is currently being used to search for dark photons, a dark matter candidate similar to axions that converts without the need for an external magnetic field. A further iteration, the “triple-wedge” (center in the figure) further increases volume, and is undergoing metrology. Finally, the “beehive” cavity (right in the figure) acts as a collection of smaller resonators which are coupled to each other, encouraging them to resonate together.

Further Reading
-  The high-volume haloscope concept (2019): https://arxiv.org/abs/1910.04156
-  Characterization of the single-wedge (2024): https://arxiv.org/abs/2402.01060 
-  The beehive concept (2024): https://arxiv.org/abs/2404.06627