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450 29 APRIL 2022 • VOL 376 ISSUE 6592 science.org SCIENCE
steady pitter-pat of tiny electrical pulses.
The trick is to find a semiconductor sen-
sitive to very low-energy photons, Kahn
says. The industrial standard, silicon, re-
leases an electron when it absorbs a pho-
ton with an energy of at least 1.1 electron
volts (eV). To detect dark sector particles
with masses as low as 1/100,000th that of
a proton, the material would need to un-
leash an electron when pinged by a photon
of just 0.03 eV. So Kahn, Abbamonte, and
colleagues at Los Alamos National Labo-
ratory are exploring “narrow bandgap”
semiconductors such as a compound of
europium, indium, and antimony.
Even lighter dark-sector particles would
create photons with too little energy to
liberate an electron in the most sensitive
semiconductor. To hunt for them, Zurek
and Matt Pyle, a detector physicist at the
University of California, Berkeley, are devel-
oping a detector that would sense the infin-
itesimal quantized vibrations set off when
a dark photon creates an ordinary photon
that pings a nucleus. It would “only rattle
that nucleus and produce a bunch of vibra-
tions,” Pyle says. “So the detectors must be
fundamentally different.”
Their detector consists of a single crystal
of material composed of two types of ions
with opposite charges, such as gallium arse-
nide. The feeble photon spawned by a dark
photon would nudge the different ions in op-
posite directions, setting off quantized vibra-
tions called optical phonons. To detect these
vibrations, Zurek and Pyle dot the crystal
with small patches of tungsten and chill it
to temperatures near absolute zero, where
tungsten becomes a superconductor that
carries electricity without resistance. Any
phonons would slightly warm the tungsten,
reducing its superconductivity and leading
to a noticeable spike in its resistance.Within 5 years, the researchers hope to
improve their detector’s sensitivity by a
factor of 10 so that they can sense a single
phonon and hunt dark-sector particles
weighing one-millionth as much as a pro-
ton. To provide the dark matter, such par-
ticles would have to be so numerous that
a detector weighing just a few kilograms
should be able to spot them or rule them
out. And because so few experiments have
probed this mass range, even little proto-
type detectors unshielded from background
radiation can yield interesting data, Pyle
says. “We run just in our lab aboveground,
and we can get world-leading results.”SOME PHYSICISTS ARGUE that true quan-
tum sensors should do something more
subtle. The Heisenberg uncertainty prin-
ciple states that if you simultaneously
measure the position and momentum of
an electron, the product of the uncertain-
ties in those measurements must exceed a
“standard quantum limit.” That means no
measurement can yield a perfectly precise
result, no matter how it’s done. However,
the principle also implies you can swap
greater uncertainty in one measurement
for greater precision in the other. To some
physicists, a quantum sensor is one that
exploits that trade-off.
Physicists are using such schemes to en-
hance axion searches. To make up dark mat-
ter, those lightweight particles would be so
numerous that en masse they’d act like a
wave, just as sunlight acts more like a light
wave than a hail of photons. So with their
metal cavities, ADMX and HAYSTAC re-
searchers are searching for the conversion
of an invisible axion wave into a detectable
radio wave.
Like any wave, the radio wave will have an
amplitude that reveals how strong it is and a
phase that marks its exact synchronization
relative to whatever ultraprecise clock you
might choose. Conventional radio circuits
measure both and run into a limit set by the
uncertainty principle. But axion hunters care
only about the signal’s amplitude—is a wave
there or not?—and quantum mechanics lets
them measure it with greater precision in
exchange for more uncertainty in the phase.
HAYSTAC experimenters exploit that
trade-off to tamp down noise in their ex-
periment. The vacuum—the backdrop for
the measurement—can itself be considered
a wave. Although that vacuum wave has on
average zero amplitude, its amplitude is still
uncertain and fluctuates to create noise. In
HAYSTAC a special amplifier reduces the
vacuum’s amplitude fluctuations while al-
lowing those in the irrelevant phase to grow
bigger, causing any axion signal to stand out
more readily. Last year, HAYSTAC research-
ers reported in Nature that they had searched
for and ruled out axions in a narrow range
around 19-quadrillionths of a proton mass. By
squeezing the noise, they increased the speed
of the search by a factor of 2, Maruyama says,
and validated the principle.
Such “squeezing” has been demonstrated
for decades in laboratory experiments with
lasers and optics. Now, Irwin says, “These
techniques for beating the standard quan-
tum limit [have] been used to actually do
something better, as opposed to do some-
thing in a demonstration.” In the DM Radio
experiment, he hopes to use a related tech-
nique to probe for even lighter axions as well
as dark photons. GRAPHIC: V. ALTOUNIAN/SCIENCEAxion
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Superconducting
tungsten
Gallium
arsenide
Radio signals
Dark photon
Real photon
Arsenic Gallium
Phonon
Magnetic
field
Metal
cavity
Radio
waves
Uncertainty in amplitude and phase
Increase phase uncertainty to decrease
uncertainty in amplitude
Tuning
rod
Particles and waves
Quantum detectors include devices that can detect a single quantum, such as a photon, and devices that exploit
a quantum trade-off to measure one variable more precisely at the cost of greater uncertainty in another.
Just one click
A dark matter candidate
called a dark photon
could morph into
an ordinary photon
that would trigger a
quantized vibration in a
crystal. The vibration,
or phonon, would warm
superconducting heat
sensors on the crystal.
Quantum trade-off
Within a resonating
cavity, a wave of
hypothetical axions
could transform into
faint radio waves,
uncertain in both
amplitude and phase.
Quantum techniques
could reduce the
uncertainty in the
amplitude while
increasing that in
the wave’s
irrelevant phase.