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."
Reference:
Shimon Kolkowitz, Differential clock comparisons with a multiplexed optical
lattice clock, Nature (2022).
DOI: 10.1038/s41586-021-04344-y.
Related: Tobias Bothwell, Resolving the gravitational redshift in a
millimetre-scale atomic sample, Nature (2022).
DOI: 10.1038/s41586-021-04349-7.
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Physics