By Taj Dyson

The Universe has an invisible skeleton made of particles we can’t describe in our current theories, known only as dark matter. This skeleton is not small — dark matter comprises 85% of the matter in existence, and its strong gravity pulled everything else together inside it to form stars, galaxies, and eventually us. Discovering what dark matter is made of would fill an enormous gap in theories of subatomic physics and allow us to better understand how the Universe came to be.

The axion, a theoretical particle proposed to solve another unrelated mystery, could be the answer. The search for axion dark matter is well underway with the Axion Dark Matter eXperiment (ADMX), and our group at Stanford is expanding on that search with Volume Enhanced Resonators for Axions (VERA) by developing new axion detectors.

How to look for axion dark matter

Axions are incredibly light particles, at most one billionth of the mass of a proton, making them invisible to most detection methods even though they may be passing through and around you right now, as the galaxy’s dark matter halo. Luckily, axions have a quirk: when immersed in a magnetic field, they convert into electromagnetic waves — light (see Figure 1), and we know how to detect those.

To catch an axion, you need to build a haloscope: find the biggest, baddest magnet possible (often the superconducting kind used in MRI machines), then do everything you can to make the tiny axion-made light signal brighter. A great tool for enhancing the signal is a resonator, which works like the body of a string instrument (here with light waves rather than sound waves). The waves crash between the walls of the resonator, building upon each other each time. Generally, bigger resonators have lower resonant frequency (the frequency they’re best at building up); think of the pitch of a violin as compared to a double-bass. We don’t know what frequency the axion signal has, so we have to tune our resonant frequency across a wide range. Searching for axions at relatively low frequencies is easier, because low-frequency resonators are bigger and thus naturally catch more dark matter from the halo passing through them. The axion signal doesn’t care about what’s easy, though.

We want our resonator to be sensitive to uncharted higher-frequency signals without sacrificing size — how do you make something the size of a double-bass sound like a violin? We’re working on many answers to this question, but our first and simplest is the single wedge (seen in Figure 2a, cut in half). The narrow gap (indicated by the green arrows) sets the resonant frequency while the total volume is set by the height and width (indicated by the blue arrows), which can be as big as we want.

Making a resonator for ADMX-VERA

Now that we’ve designed our haloscope’s resonator (Figure 2a), we need to get it fabricated out of aluminum and make sure it resonates as well as we expect. Computer simulations tell us that, even assuming it’s perfectly made, only 62% of the volume can hold a resonance (Figure 2b). Furthermore, machining is never perfect, and after precise metrology while immersed in liquid nitrogen (Figure 2c) we find deviations from perfect flatness of a few times the width of a hair. Simulating the resonator’s initial actual shape tells us the resonance occupies only 42% of the volume (Figure 2d). We continually flatten the parts, measure, and simulate to get as close to the ideal resonance as possible.

More ways to enhance the axion signal

The resonator is the core of a haloscope, but there are many other tricks to play to make the axion signal easier to detect. For one, you can make the haloscope colder, so it glows less (all objects glow, but in our daily lives, this glow is mostly in the infrared, so we don’t see it). The detector will be cooled to below 0.2 °F above absolute zero, or about 4 °F colder than space. Since our system will be cold anyway, we can also use a “traveling wave parametric amplifier” shown in Figure 3. These superconducting devices can amplify the signal while adding the least amount of background noise allowed by the Heisenberg uncertainty principle.

Future Developments

We are currently running a dark matter search at room temperature using the resonator discussed throughout this article (see Figure 4a). We don’t yet have a powerful magnet, so we’re searching for dark photons, another theorized dark matter candidate exactly like axions except they would convert into light on their own, without a magnetic field. Next year, we’ll do the same in a grad-student-designed cryogenic setup, and finally look for axions proper in years following, using the main ADMX magnet at the University of Washington.

Looking for axions where no one has before invites a variety of resonator designs, such as the “beehive” (Figure 4b) and the “triple wedge” (Figure 4c), each led by a graduate student in our group. Our small group is backed up by years of expertise in the broader ADMX collaboration. With the combination of novel resonator designs and quantum devices, we aim to make ADMX-VERA an important contributor to the search for axions.

Edited by Josephine Wong, Lori White, Xinnan Du, and Jack Dinsmore