Expected Use: new studies in physics
From: University of Wisconsin--Madison
By Sarah Perdue
February
16, 2022 – University of Wisconsin–Madison physicists have made one of the
highest performance atomic clocks ever, they announced Feb. 16 in the journal
Nature.
Their instrument, known as an optical
lattice atomic clock, can measure differences in time to a precision equivalent
to losing just one second every 300 billion years and is the first example of a
“multiplexed” optical clock, where six separate clocks can exist in the same
environment. Its design allows the team to test ways to search for
gravitational waves, attempt to detect dark matter, and discover new physics
with clocks.
“Optical lattice clocks are already the
best clocks in the world, and here we get this level of performance that no one
has seen before,” says Shimon
Kolkowitz, a UW–Madison physics professor and senior author of the study.
“We’re working to both improve their performance and to develop emerging
applications that are enabled by this improved performance.”
Atomic clocks are so precise because
they take advantage of a fundamental property of atoms: when an electron
changes energy levels, it absorbs or emits light with a frequency that is
identical for all atoms of a particular element. Optical atomic clocks keep
time by using a laser that is tuned to precisely match this frequency, and they
require some of the world’s most sophisticated lasers to keep accurate time.
By comparison, Kolkowitz’s group has “a
relatively lousy laser,” he says, so they knew that any clock they built would
not be the most accurate or precise on its own. But they also knew that many
downstream applications of optical clocks will require portable, commercially
available lasers like theirs. Designing a clock that could use average lasers
would be a boon.
In their new study, they created a
multiplexed clock, where strontium atoms can be separated into multiple clocks
arranged in a line in the same vacuum chamber. Using just one atomic clock, the
team found that their laser was only reliably able to excite electrons in the
same number of atoms for one-tenth of a second.
However, when they shined the laser on
two clocks in the chamber at the same time and compared them, the number of
atoms with excited electrons stayed the same between the two clocks for up to
26 seconds. Their results meant they could run meaningful experiments for much
longer than their laser would allow in a normal optical clock.
“Normally, our laser would limit the
performance of these clocks,” Kolkowitz says. “But because the clocks are in
the same environment and experience the exact same laser light, the effect of
the laser drops out completely.”
The group next asked how precisely they
could measure differences between the clocks. Two groups of atoms that are in
slightly different environments will tick at slightly different rates,
depending on gravity, magnetic fields, or other conditions.
They ran their experiment over a
thousand times, measuring the difference in the ticking frequency of their two
clocks for a total of around three hours. As expected, because the clocks were
in two slightly different locations, the ticking was slightly different. The
team demonstrated that as they took more and more measurements, they were
better able to measure those differences.
Ultimately, the researchers could detect
a difference in ticking rate between the two clocks that would correspond to
them disagreeing with each other by only one second every 300 billion years — a
measurement of precision timekeeping that sets a world record for two spatially
separated clocks.
It would have also been a world record
for the overall most precise frequency difference if not for another paper,
published in the same issue of Nature. That study was led by a group at JILA, a
research institute in Colorado. The JILA group detected a frequency difference
between the top and bottom of a dispersed cloud of atoms about 10 times better
than the UW–Madison group.
Their results, obtained at one
millimeter separation, also represent the shortest distance to date at which
Einstein’s theory of general relativity has been tested with clocks.
Kolkowitz’s group expects to perform a similar test soon.
“The amazing thing is that we
demonstrated similar performance as the JILA group despite the fact that we’re
using an orders of magnitude worse laser,” Kolkowitz says. “That’s really
significant for a lot of real-world applications, where our laser looks a lot
more like what you would take out into the field.”
To demonstrate the potential
applications of their clocks, Kolkowitz’s team compared the frequency changes
between each pair of six multiplexed clocks in a loop. They found that the
differences add up to zero when they return to the first clock in the loop,
confirming the consistency of their measurements and setting up the possibility
that they could detect tiny frequency changes within that network.
“Imagine a cloud of dark matter passes
through a network of clocks — are there ways that I can see that dark matter in
these comparisons?” Kolkowitz asks. “That’s an experiment we can do now that
you just couldn’t do in any previous experimental system.”
This work was supported in part by the
NIST Precision Measurements Grants program, the Northwestern University Center
for Fundamental Physics and the John Templeton Foundation through a Fundamental
Physics grant, the Wisconsin Alumni Research Foundation, the Army Research
Office (W911NF-21-1-0012), and a Packard Fellowship for Science and
Engineering.
https://news.wisc.edu/ultraprecise-atomic-clock-poised-for-new-physics-discoveries/
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