NEWSSCIENCE science.org 29 APRIL 2022 • VOL 376 ISSUE 6592 449and paperclips, if it satisfies all cosmological
constraints, it’s fair game.” Quantum sensing
is essential for testing many of those models,
Irwin says. “It can make it possible to do an
experiment in 3 years that would otherwise
take thousands of years.”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 im-
plies that the stars circulate within a vast
cloud of dark matter that provides the addi-
tional gravity needed to rein them in. Physi-
cists 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 par-
ticles after the big bang, WIMPs would in-teract 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 ap-
peared, even as detectors have
grown bigger and more sensi-
tive. Fifteen years ago, WIMP
detectors weighed kilograms;
now, the biggest contain sev-
eral 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 ra-
dio 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 ax-
ions 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 grav-
ity but not the three other forces, electro-
magnetism and the weak and strong nu-
clear 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 darkelectromagnetic 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: quan-
tum 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 re-
flects 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 peo-
ple interested in developing new detectors
and in quick, small-scale experiments, she
says. “This kind of footloose approach has al-
ways 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 sen-
sor 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 de-
tectable 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
A collision of galaxy
clusters separated gas
(pink) from dark matter
(blue), mapped from
subtle gravitational
distortions in the images
of background galaxies.“It can make
it possible to
do an experiment
in 3 years that
would otherwise
take thousands
o f y e a r s .”
Kent Irwin,
Stanford University