A Quantum Sense for Dark Matter

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.

In recent years physicists have begun to consider other possibilities. Maybe axions are either more or less massive than previously estimated. Instead of one type of particle, dark matter might even consist of a hidden “dark sector” of multiple new particles that would interact through gravity but not the three other forces, electromagnetism and the weak and strong nuclear forces. Rather, they would have their own forces, says Kathryn Zurek, a theorist at the California Institute of Technology. So, just as photons convey the electromagnetic force, dark photons might convey a dark electromagnetic force. Dark and ordinary electromagnetism might intertwine so that rarely, a dark photon could morph into an ordinary one.

To spot such quarry, dark matter hunters have turned to quantum sensors—a shift partly inspired by another hot field: quantum computing. A quantum computer flips quantum bits, or qubits, that can be set to 0, 1, or, thanks to the odd rules of quantum mechanics, 0 and 1 at the same time. That may seem irrelevant to hunting dark matter, but such qubits must be carefully controlled and shielded from external interference, exactly what dark matter hunters already do with their detectors, says Aaron Chou, a physicist at Fermi National Accelerator Laboratory (Fermilab) who works on ADMX. “We have to keep these devices very, very well isolated from the environment so that when we see the very, very rare event, we’re more confident that it might be due to the dark matter.”

The interest in quantum sensors also reflects the tinkerer culture of dark matter hunters, says Reina Maruyama, a nuclear and particle physicist at Yale and co-leader of HAYSTAC. The field has long attracted people interested in developing new detectors and in quick, small-scale experiments, she says. “This kind of footloose approach has always been possible in the dark matter field.”

FOR SOME NOVEL SEARCHES, the simplest definition of a quantum sensor may do: It’s any device capable of detecting a single quantum particle, such as a photon or an energetic electron. “I call a quantum sensor something that can detect single quanta in whatever form that takes,” Zurek says. That’s what is needed for hunting particles slightly lighter than WIMPs and plumbing the dark sector, she says.

Such runty particles wouldn’t produce detectable nuclear recoils. A wispy dark sector particle could interact with ordinary matter by emitting a dark photon that morphs into an ordinary photon. But that low-energy photon would barely nudge a nucleus.

In the right semiconductor, however, the same photon could excite an electron and enable it to flow through the material. Kahn and Abbamonte are working on an extremely sensitive photodiode, a device that produces an electrical signal when it absorbs light. Were such a device shielded from light and other forms of radiation and cooled to near absolute zero to reduce noise, a dark matter signal would stand out as a steady pitter-pat of tiny electrical pulses.

Read the full story in Science