Looking for dark matter in a HAYSTAC — with some quantum squeezing

The search for dark matter — the invisible glue that binds the cosmos and makes up most of the mass of galaxies — is a bit like looking for a needle in a near-infinite haystack.

For one thing, scientists don’t know exactly what dark matter is. They are only able to infer its existence based on the gravitational pull it has on visible matter.

Identifying dark matter, however, would provide major insights for understanding a fundamental force of the unseen universe. Potential candidates for dark matter’s identity range from sterile neutrinos and weakly interacting massive particles (WIMPs) to axions, hypothetical particles that are thought to balance out a mysterious symmetry in the universe.

For more than a decade, a Yale-led team of scientists has used a HAYSTAC — the Haloscope At Yale Sensitive to Axion CDM — to hunt for axions. New findings, demonstrated in a study in the journal Nature, show that the team has improved the sensitivity of the detector so the search for the axion can proceed at a faster rate.

HAYSTAC — which is a collaboration between Yale, the University of California-Berkeley, and the University of Colorado-Boulder — was started in 2010, with the first results published in 2017.

“Over the ensuing years, there have been exciting developments in quantum-enhanced detection technologies,” said Steve Lamoreaux, a Yale physics professor and principal investigator for HAYSTAC. “To take advantage of these new instrumentation techniques, we rebuilt the detector starting in 2018 and the improved detector was brought back online a year later.”

Based at Yale’s Wright Laboratory, HAYSTAC uses a microwave cavity held at an extremely cold temperature and immersed in a large magnetic field to search for cold dark matter (CDM) axions. HAYSTAC looks for photon signals (photons are particles of light or electromagnetic radiation) produced by axions in a magnetic field.

“Axion parameter space is vast,” said the study’s first author, Yale graduate student Kelly Backes. “Searching through it experimentally is going to take a long time and a lot of effort, and the rate at which we can search is fundamentally limited by the laws of quantum mechanics.”

For this reason, the scientists turned to a technique known as “quantum squeezing,” which is borrowed from quantum physics research. HAYSTAC scientists said this “squeezing” reduces the amount of quantum “noise” the axion signal must compete with, speeding up the search.

“We have set limits on dark matter axions and ruled out a subset of predicted axion models,” said Reina Maruyama, an associate professor of physics at Yale, co-author of the study, and one of the leaders of the HAYSTAC experiment.

“The findings of this paper demonstrate that through the use of quantum measurement techniques it is possible to search for the axion much more quickly than the quantum limited rate,” Backes said. “The tools used in our quantum squeezing setup were initially developed for use in quantum computing labs. The fact that they could also enhance axion detection highlights what is possible when two fields of physics, in this case dark matter detection and quantum information, come together.”

Maruyama noted that HAYSTAC, along with the gravitational wave experiment LIGO, are the only fundamental physics experiments working with noise levels low enough to employ quantum squeezing.

Additional co-authors from Yale were research scientist Sid Cahn, graduate student Sumita Ghosh, and undergraduate students Sukhman Singh and Jean Wang. Former Yale postdoc Danielle Speller, who is now an assistant professor at Johns Hopkins University; former Yale graduate student Benjamin Brubaker, who is now a postdoctoral fellow at the University of Colorado-Boulder; and former Yale undergraduate Cady van Assendelft, who is now a graduate student at Stanford, are also co-authors.

The research was funded, in part, by the National Science Foundation and the Heising-Simons Foundation.