Written by Adrian Cho
Kent Irwin has a vision: He aims to build a glorified radio that will reveal the nature of dark matter, the invisible stuff that makes up 85% of all matter. For decades, physicists have struggled to figure out what the stuff is, stalking one hypothetical particle after another, only to come up empty. However, if dark matter consists of certain nearly massless particles, then in the right setting it might generate faint, unquenchable radio waves. Irwin, a quantum physicist at Stanford University, plans to tune in to that signal in an experiment called Dark Matter Radio (DM Radio).
No ordinary radio will do. To make the experiment practical, Irwin’s team plans to transform it into a quantum sensor—one that exploits the strange rules of quantum mechanics. Quantum sensors are a hot topic, having received $1.275 billion in funding in the 2018 U.S. National Quantum Initiative. Some scientists are employing them as microscopes and gravimeters. But because of the devices’ unparalleled sensitivity, Irwin says, “dark matter is a killer app for quantum sensing.”
DM Radio is just one of many new efforts to use quantum sensors to hunt the stuff. Some approaches detect the granularity of the subatomic realm, in which matter and energy come in tiny packets called quanta. Others exploit the trade-offs implicit in the famous Heisenberg uncertainty principle. Still others borrow technologies being developed for quantum computing. Physicists don’t agree on the definition of a quantum sensor, and none of the concepts is entirely new. “I would argue that quantum sensing has been happening in one form or another for a century,” says Peter Abbamonte, a condensed matter physicist and leader of the Center on Quantum Sensing and Quantum Materials at the University of Illinois, Urbana-Champaign (UIUC).
ASTROPHYSICAL EVIDENCE for dark matter has accreted for decades. For example, the stars in spiral galaxies appear to whirl so fast that their own gravity shouldn’t keep them from flying into space. The observation implies that the stars circulate within a vast cloud of dark matter that provides the additional gravity needed to rein them in. Physicists assume it consists of swarms of some as-yet-unknown fundamental particle.
In the 1980s, theorists hypothesized what soon became the leading contender: weakly interacting massive particles (WIMPs). Emerging in the hot soup of particles after the big bang, WIMPs would interact with ordinary matter only through gravity and the weak nuclear force, which produces a kind of radioactive decay. Like the particles that convey the weak force, the W and Z bosons, WIMPs would weigh roughly 100 times as much as a proton. And just enough WIMPs would naturally linger—a few thousand per cubic meter near Earth—to account for dark matter.
Occasionally a WIMP should crash into an atomic nucleus and blast it out of its atom. So, to spot WIMPs, experimenters need only look for recoiling nuclei in detectors built deep underground to protect them from extraneous radiation. But no signs of WIMPs have appeared, even as detectors have grown bigger and more sensitive. Fifteen years ago, WIMP detectors weighed kilograms; now, the biggest contain several tons of frigid liquid xenon.
The second most popular candidate—and one DM Radio targets—is the axion. Far lighter than WIMPs, axions are predicted by a theory that explains a certain symmetry of the strong nuclear force, which binds quarks into trios to make protons and neutrons. Axions would also emerge in the early universe, and theorists originally estimated they could account for dark matter if the axion has a mass between one-quadrillionth and 100-quadrillionths of a proton.
In a strong magnetic field, an axion should sometimes turn into a radio photon whose frequency depends on the axion’s mass. To amplify the faint signal, physicists place in the field an ultracold cylindrical metal cavity designed to resonate with radio waves just as an organ pipe rings with sound. The Axion Dark Matter Experiment (ADMX) at the University of Washington, Seattle, scans the low end of the mass range, and an experiment called the Haloscope at Yale Sensitive to Axion CDM (HAYSTAC) at Yale University probes the high end. But no axions have shown up yet.