Science - USA (2022-04-29)

SCIENCE science.org 29 APRIL 2022 • VOL 376 ISSUE 6592 451

Instead of a resonating cavity, DM Ra-

dio consists of a radio circuit containing

a charge-storing capacitor and a current-

storing inductor—a carefully designed coil

of wire—both placed in a magnetic field.

Axions could convert to radio waves within

the inductor coil to create a resonating sig-

nal in the circuit at a certain frequency. Re-

searchers can also look for dark photons by

reconfiguring the coil and turning off the

magnetic field.

To read out the signal, Irwin’s scheme plays

on another implication of quantum mechan-

ics, that by measuring a system’s state you

may change it. The researchers couple

their resonating circuit to a second,

higher frequency circuit, so that, much

as in AM radio, any dark matter signal

would make the amplitude of the higher

frequency carrier wave warble. The

stronger the coupling, the bigger the

warbling, and the more prominent the

signal. But stronger coupling also injects

noise that could stymie efforts to mea-

sure dark matter with greater precision.

Again, a quantum trade-off comes

to the rescue. The researchers modify

their carrier wave by injecting a tiny

warble at the frequency they hope to

probe. Just by random chance, that in-

put warble and any dark matter signal

will likely be somewhat out of sync, or

phase. But the dark matter wave can

be thought of as the sum of two com-

ponents: one that’s exactly in sync with

the added signal and one that’s exactly

out of sync with it—much as any di-

rection on a map is a combination of

north-south and east-west. The experi-

ment is designed to measure the in-

sync component with greater precision

while injecting all the disturbance into

the out-of-sync component, making the

measurement more sensitive and ac-

celerating the rate at which the experi-

ment can scan different frequencies.

Irwin and colleagues have already

run a small prototype of the experiment.

They are now building a larger version,

and ultimately they plan one with a coil that

has a volume of 1 cubic meter. Implementing

the quantum sensing is essential, Irwin says,

as without it, scanning the entire frequency

range would take thousands of years.

SOME DARK MATTER HUNTERS are explicitly

borrowing hardware from quantum com-

puting. For example, Fermilab’s Chou and

colleagues have used a superconducting

qubit—the same kind Google and IBM use

in their quantum computers—to perform

a proof-of-principle search for dark pho-

tons in a very narrow energy range. Like

a smaller version of ADMX or HAYSTAC,

their experiment centers on a resonating

cavity, this one drilled into the edge of

an aluminum plate. There a dark photon

could convert into radio waves, although at

a higher frequency than in ADMX or HAY-

STAC. Ordinarily, experimenters would

bleed the radio waves out through a hole

in the cavity and measure them with a low-

noise amplifier. However, the tiny cavity

would generate a signal so faint it would

drown in noise from the amplifier itself.

The qubit sidesteps that problem. Like

any other qubit, the tiny superconducting

circuit can act like a clock, cycling between

different combinations of 0 and 1 at a rate

that depends on the difference in energy

between the circuit’s 0 and 1 states. That

difference in turn depends on whether

there are any radio photons in the cavity.

Even one is enough to speed up the clock,

Chou says. “We’re going to stick this artifi-

cial atomic clock in the cavity and see if it

still keeps good time.”

The measurement probes only the am-

plitude of the radio waves and not their

phase, obtaining greater precision in the

former in exchange for greater uncertainty

in the latter, the team reported last year in

Physical Review Letters. It might speed up

dark photon searches by as much as a fac-
tor of 1300, Chou says, and it could be ex-
tended to search for axions, if researchers
could apply a magnetic field to the cavity
while shielding the sensitive qubit.
One group has invented a scheme to
search for WIMPs using another candidate
qubit: a so-called nitrogen vacancy (NV)
center within a diamond crystal. In an NV,
a nitrogen atom replaces a carbon atom in
the crystal lattice and creates an adjacent,
empty site that collects a pair of electrons
that can serve as qubit. A WIMP passing
through a diamond can bump carbon at-
oms out of the way, leaving a trail of
NVs roughly 100 nanometers long, says
Ronald Walsworth, an experimental
physicist at the University of Mary-
land, College Park. The NVs will absorb
and emit light of specific wavelengths,
so the track can be spotted clearly with
fluorescence microscopy.
That scheme has little to do with
quantum computing, but it would ad-
dress a looming problem for WIMP
searches. If current liquid xenon detec-
tors get much bigger, they should start
to see well-known particles called neu-
trinos, which stream from the Sun. To
tell a WIMP from a neutrino, physicists
would need to know where a particle
came from, as WIMPs should come
from the plane of the Galaxy rather
than the Sun. A liquid xenon detector
can’t determine the direction of a par-
ticle that caused a signal. A detector
made of diamonds could.
Walsworth envisions a detector
formed of millions of millimeter-size
synthetic diamonds. A diamond would
flash when pierced by a neutrino or
WIMP, and an automated system would
remove it and scan it for an NV track,
using the time of the flash to determine
the track’s orientation relative to the
Sun and the Galaxy, the team explained
last year in Quantum Science and Tech-
nology. Walsworth hopes to build a
prototype detector in a few years. “I ab-
solutely do not want to claim that our idea
would work or that it’s better than other ap-
proaches,” he says. “But I think it’s promising
enough to go forward.”
Physicists have proposed many other
ideas for using quantum sensors to search
for dark matter, and the influx of money
should help transform them into new tech-
nologies, Zurek says. “Things can move
faster when you’re funded,” she says. As
tool builders, dark matter hunters embrace
that push. “They have a great hammer, so
they started looking for nails,” Walsworth
says. Perhaps they’ll bang out a discovery
PHOTOS: (TOP TO BOTTOM ROGER ROMANI/UNIVERSITY OF CALIFORNIA, BERKELEY; KARL VAN BIBBER of cosmic proportions. j

A chip that could sense dark photons (top) and an axion detector,

HAYSTAC, could fit on a tabletop despite their high sensitivity.