YSM Issue 94.2

FOCUS
Physics
Backes’s
group is searching
for axion frequencies at around
four or five gigahertz, but the axion
could be hiding anywhere on the range
of hertz to terahertz—where a frequency of
one hertz is one cycle per second, and one
terahertz is a trillion times that.
Though the range Backes and her
colleagues are investigating is small relative
to the orders of magnitude of potential
frequencies that surround it, it is strongly
grounded in theory. Some groups of
theorists have done large-scale calculations
that indicate that the axion’s location is
likely around the range they are exploring.
Experimentally, this range is favorable
because the low gigahertz range makes
for a nicely sized detector: resonator size
is proportional to desired frequency, so a
much lower frequency would require an
inconveniently large detector. A higher
frequency, conversely, would necessitate an
exceptionally smaller one.
The Coolest Part: Vacuum Squeezing
The vacuums physicists use are nothing
like the loud cleaning appliances with
which most people are familiar. In physics,
a vacuum is the absence of matter and
energy. It is the ground state of all fields
in quantum mechanics, and its energy
fluctuates in quantum fluctuations, creating
temporary random changes of energy
in a point in space. Vacuum squeezing
redistributes these fluctuations, enhancing
or repressing them along different time
intervals. “I think I’m biased, but this is the
coolest part of what I’ve done,” Backes said.
When conducting any experiment,
data is likely to come with noise, which
is a result of random variations that
interfere with the signal. Like radio
static, the electronic noise in axion
experiments
doesn’t
h a v e
any specific frequency or phase
preference. Since that noise is made of
components with different phases, one
could mathematically decompose it into
terms of more traditional sine and cosine
wave patterns. The amplitudes of these
sine- and cosine-like components of these
fluctuations don’t commute, which is why
the noise exists in the first place. “Like
the traditional uncertainty relationship
between position and momentum, you
can't measure all fluctuations at once
without adding noise to your system,”
Backes explained. “That's where these
quantum vacuum fluctuations come in.”
However, it does not matter whether all
the phases can be precisely measured in the
detector—the way that axion signals are
measured does not necessitate it. Instead,
physicists squeeze the noise into one
“quadrature,” like position and momentum
in the previous example, meaning that the
sine- and cosine-like fluctuations would be
two measurement quadratures.
If we were to think of noise as a malleable
ball, squeezing it would involve taking a
round noise state that lacks phase preference
and processing it with an amplifier so that
it is squeezed into an oblong blob of a noise
state—one that has a phase preference. The
resonator’s power has no phase preference,
so when the newly squeezed state is guided
into its cavity, the axion is measured until
it no longer has a phase preference either.
Essentially, one axion arrives, displacing
the squeezed state in one direction, and
another follows, displacing it in a different
direction. This process molds the squeezed
state, and the pattern continues until it is
broadened by the displacement.
Once the squeezed state is slightly
fattened, it is read out of the microwave
cavity and squeezed in the opposite
direction. Flattening the squeezed state
amplifies the hypothetical axion signal
IMAGE COURTESY OF LAUREN CHONG
The upper plates of the experiment's dilution
refrigerator at the Lamoreaux Group.
along the newly shaped quadrature,
which is the same quadrature that
originally had the squeezed noise.
Ultimately, this allows Backes and her
team to measure an amplified signal
against subquantum limited noise,
making it easier to detect axions.
This work is revolutionizing the field,
which should make the search for axions
more efficient. “I think the big impact of
this specific paper and this work is it shows
for the first time that quantum squeezing
can be used as a tool to speed up a fullscale
fundamental particle search,” Backes
said. As the first experiment to show that
one can look for new fundamental particles
against a noise background that is below
the standard quantum limit, this work is at
the forefront of the search for dark matter.
Dark matter has eluded physicists for
decades, but Backes and her team might
have unearthed a faster path to find it
with their novel approach to detecting
the axion. They have not had luck
yet, but their research takes time.
For now, they can only continue
flipping through galactic radio
channels, hoping to find the
station at the end of the
universe. ■
ABOUT THE AUTHOR BRIANNA FERNANDEZ
BRIANNA FERNANDEZ is a sophomore in Pierson College studying astrophysics. In addition
to writing for YSM, she is one of the magazine’s copy editors. Outside of YSM, she researches
exoplanets with Professor Debra Fischer and advocates for free prison phone calls with the
Yale Undergraduate Prison Project.
THE AUTHOR WOULD LIKE TO THANK Kelly Backes for her time and enthusiasm to share her research.
FURTHER READING
Backes, K. M., Palken, D. A., Al Kenany, S., Brubaker, B. M., Cahn, S. B., Droster, A., ... & Wang, H. (2021). A
quantum enhanced search for dark matter axions. Nature, 590(7845), 238-242.e
24 Yale Scientific Magazine May 2021 www.yalescientific.org