Art courtesy of Alex Dong.
The matter we interact with on a daily basis is known as normal matter. Counterintuitively, this normal matter makes up only about five percent of the universe. Physicists think the rest is composed of two other constituents: dark energy, the force thought to be speeding up the expansion of the universe, and dark matter, an unknown type of matter that only interacts strongly with gravity. Though neither of them can be detected directly, the role that dark energy plays in the rate of the universe’s expansion, as well as the effect that dark matter has on galactic structure, make physicists confident in these hypothesized substances.
Some people wonder how galaxies rotate at a seemingly impossible rate; in theory, they simply could not maintain such high speeds given the masses we detect from Earth. From the perspective of astronomers, galaxies’ low masses could not generate enough gravity to hold them together. Something is giving them extra mass—something undetectable. This is what we have designated dark matter. But what is dark matter, really? What is it made of? Enter the axion.
Researchers at the Yale Department of Physics’s Wright Laboratory have come together in pursuit of the axion. By looking for this hypothetical particle—which is thought to comprise dark matter—they think it could be possible to find dark matter. Led by fifth-year graduate student Kelly Backes, this group recently published a paper in Nature that reports the use of vacuum squeezing to double the search rate for axions. This process enabled them to circumvent the quantum limit that many dark matter searches can barely approach. By breaking through this limit, rather than merely approaching it, they are ushering in an age in which searches for fundamental physics are less hindered by noise, bettering the chances of the axion’s discovery.
Hiding in Plain Sight
This question of missing galactic mass has been pondered by physicists since the 1930s, resulting in a variety of potential candidates for what dark matter could be. “Because there are so many different options for what could be dark matter, a lot of the well-motivated theories are particles or theories that solve multiple problems,” Backes said. “There are very few things in physics that just exist for one weird purpose.” One promising dark matter candidate is the axion—a hypothetical particle that was first proposed in 1977 by Roberto Peccei and Helen Quinn. Named after a laundry detergent, the axion was theorized to solve the strong charge-parity (CP) problem of quantum chromodynamics—a problem within the Standard Model of particle physics, in which two fundamental nuclear forces act differently. Later, researchers Steven Weinberg and Frank Wilczek suggested that the axion could also be what makes up dark matter.
Although axions are theorized to be infinitesimally small and extraordinarily light, they completely pervade the space which they occupy, existing everywhere and all the time. Their omnipresence makes it more convenient to imagine each axion as a sea of particles that oscillate together rather than as individual ones. With this knowledge in hand, the detectors searching for axions aim to sense them as waves.
While we generally think of dark matter in the context of its interactions with gravity, since this is the only way in which we know it exists, dark matter seems to have feeble interactions with other fundamental forces. As a result, some scientists looking for axions do so by detecting their extremely weak interactions with electromagnetism. “If you supply a magnetic field, the axion field and this magnetic field interact and produce a tiny [bit of] excess electric field that is actually then detectable, and that’s the interaction that we center our detector around,” Backes said. Essentially, axions are detected as excess power in a detector. They react with magnetic fields, yielding small traces of electric field. It is Backes’s job to amplify this electric signal and, from there, detect these evasive axions.
The Great Cosmic Radio
Backes’s research group has spent years searching for this dark matter candidate, analyzing excess power that could be produced by axion waves and a large magnetic field in the hopes of finding the answer to this long-held mystery. “Essentially what we’re doing is operating a really, really, really sensitive radio,” Backes said. The interactions they measure occur inside of a microwave cavity, or a resonator, and when the resonator frequency matches that of the axion field, the interaction is enhanced, amplifying the signal.
When you’re flipping through radio stations in the car, you’ll tune to a station, listen to hear if that’s what you want, and tune to the next station until you find what you’re looking for. However, your radio will only pick up a station if it is tuned to the same frequency as the incoming radio waves. In this case, the detector functions similarly to the car radio, and the axion is the station to be detected.
But where exactly is the axion station located? As it turns out, it is hard to tell. “You’re tuning through frequency space, and you’re looking for the only station in the universe, and you have no idea where it is,” Backes said. For now, Backes’s group is searching for axion frequencies at around four or five gigahertz, but the axion could be hiding anywhere on the range of hertz to terahertz—where a frequency of one hertz is one cycle per second, and one terahertz is a trillion times that.
Though the range Backes and her colleagues are investigating is small relative to the orders of magnitude of potential frequencies that surround it, it is strongly grounded in theory. Some groups of theorists have done large-scale calculations that indicate that the axion’s location is likely around the range they are exploring. Experimentally, this range is favorable because the low gigahertz range makes for a nicely sized detector: resonator size is proportional to desired frequency, so a much lower frequency would require an inconveniently large detector. A higher frequency, conversely, would necessitate an exceptionally smaller one.
The Coolest Part: Vacuum Squeezing
The vacuums physicists use are nothing like the loud cleaning appliances with which most people are familiar. In physics, a vacuum is the absence of matter and energy. It is the ground state of all fields in quantum mechanics, and its energy fluctuates in quantum fluctuations, creating temporary random changes of energy in a point in space. Vacuum squeezing redistributes these fluctuations, enhancing or repressing them along different time intervals. “I think I’m biased, but this is the coolest part of what I’ve done,” Backes said.
When conducting any experiment, data is likely to come with noise, which is a result of random variations that interfere with the signal. Like radio static, the electronic noise in axion experiments doesn’t have any specific frequency or phase preference. Since that noise is made of components with different phases, one could mathematically decompose it into terms of more traditional sine and cosine wave patterns. The amplitudes of these sine- and cosine-like components of these fluctuations don’t commute, which is why the noise exists in the first place. “Like the traditional uncertainty relationship between position and momentum, you can’t measure all fluctuations at once without adding noise to your system,” Backes explained. “That’s where these quantum vacuum fluctuations come in.”
However, it does not matter whether all the phases can be precisely measured in the detector—the way that axion signals are measured does not necessitate it. Instead, physicists squeeze the noise into one “quadrature,” like position and momentum in the previous example, meaning that the sine- and cosine-like fluctuations would be two measurement quadratures.
If we were to think of noise as a malleable ball, squeezing it would involve taking a round noise state that lacks phase preference and processing it with an amplifier so that it is squeezed into an oblong blob of a noise state—one that has a phase preference. The resonator’s power has no phase preference, so when the newly squeezed state is guided into its cavity, the axion is measured until it no longer has a phase preference either. Essentially, one axion arrives, displacing the squeezed state in one direction, and another follows, displacing it in a different direction. This process molds the squeezed state, and the pattern continues until it is broadened by the displacement.
Once the squeezed state is slightly fattened, it is read out of the microwave cavity and squeezed in the opposite direction. Flattening the squeezed state amplifies the hypothetical axion signal along the newly shaped quadrature, which is the same quadrature that originally had the squeezed noise. Ultimately, this allows Backes and her team to measure an amplified signal against subquantum limited noise, making it easier to detect axions.
This work is revolutionizing the field, which should make the search for axions more efficient. “I think the big impact of this specific paper and this work is it shows for the first time that quantum squeezing can be used as a tool to speed up a full-scale fundamental particle search,” Backes said. As the first experiment to show that one can look for new fundamental particles against a noise background that is below the standard quantum limit, this work is at the forefront of the search for dark matter.
Dark matter has eluded physicists for decades, but Backes and her team might have unearthed a faster path to find it with their novel approach to detecting the axion. They have not had luck yet, but their research takes time. For now, they can only continue flipping through galactic radio channels, hoping to find the station at the end of the universe.
About the Author
BRIANNA FERNANDEZ is a sophomore in Pierson College studying astrophysics. In addition to writing for YSM, she is one of the magazine’s copy editors. Outside of YSM, she researches exoplanets with Professor Debra Fischer and advocates for free prison phone calls with the Yale Undergraduate Prison Project.
THE AUTHOR WOULD LIKE TO THANK Kelly Backes for her time and enthusiasm to share her research.
Backes, K. M., Palken, D. A., Al Kenany, S., Brubaker, B. M., Cahn, S. B., Droster, A., … & Wang, H. (2021). A quantum enhanced search for dark matter axions. Nature, 590(7845), 238-242.e