Ultraprecise atomic clock poised for new physics discoveries

Date:
February 16, 2022
Source:
University of Wisconsin-Madison
Summary:
Physicists have made one of the highest performance atomic clocks
ever.

­ 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.



FULL STORY ========================================================================== 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." ========================================================================== Story Source: Materials provided by
University_of_Wisconsin-Madison. Original written by Sarah Perdue. Note: Content may be edited for style and length.


========================================================================== Related Multimedia:
* The_creation_of_optical_atomic_clocks ========================================================================== Journal Reference:
1. Zheng, X., Dolde, J., Lochab, V. et al. Differential clock
comparisons
with a multiplexed optical lattice clock. Nature, 2022 DOI:
10.1038/ s41586-021-04344-y ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2022/02/220216112210.htm

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