Ultraprecise atomic clock poised for new physics discoveries

 

Physicists at the University of Wisconsin–Madison have created one of the highest-performing atomic clocks ever, according to a paper published in the journal Nature on Feb. 16.

Their device, known as an optical lattice atomic clock, can measure time differences to a precision of one second per 300 billion years and is the first example of a "multiplexed" optical clock, in which six different clocks may coexist in the same space. Its architecture lets the researchers to experiment with different methods for searching for gravitational waves, detecting dark matter, and discovering new physics using clocks.

"Optical lattice clocks are already the best clocks in the world, and here we have this degree of performance that no one has ever seen before," says Shimon Kolkowitz, a physics professor at the University of Wisconsin–Madison and the study's senior author. "We're working to improve their performance as well as build new applications that are made possible by that performance."

Atomic clocks are extremely accurate because they take advantage of a fundamental property of atoms: when an electron changes energy levels, it absorbs or emits light at the same frequency for all atoms of the same element. Optical atomic clocks preserve time by utilising a laser that is precisely tuned to this frequency, and they require some of the world's most advanced lasers to do it.

Kolkowitz's group, on the other hand, has "a relatively terrible laser," so whatever clock they made would not be the most accurate or precise on its own. However, they were well aware that many downstream applications of optical clocks would necessitate portable, commercially available lasers such as theirs. It would be a boon to design a clock that could use common lasers.

They developed a multiplexed clock in which strontium atoms are divided into numerous clocks that are stacked in a line in the same vacuum chamber in their latest work. The scientists discovered that their laser could only dependably excite electrons in the same number of atoms for one-tenth of a second using just one atomic clock.

When scientists flashed the laser on two clocks in the chamber at the same time and compared them, they discovered that the number of atoms with excited electrons remained constant for up to 26 seconds. As a result of their findings, they were able to conduct significant studies for considerably longer than a regular optical clock would allow.

"Normally, the performance of these clocks would be limited by our laser," Kolkowitz explains. "However, because the clocks are in the same environment and are exposed to the same laser light, the laser's effect is completely lost."

The committee then inquired as to how exactly the differences between the clocks could be measured. Depending on gravity, magnetic fields, and other factors, two groups of atoms in slightly different settings will tick at slightly different rates.

They repeated their experiment 1,000 times, monitoring the difference in ticking frequency between their two clocks for a total of three hours. The ticking was slightly varied since the clocks were in two slightly different positions, as expected. The researchers proved that as more measurements were taken, they were able to better measure the changes.

Finally, the researchers were able to identify a variation in ticking rate between the two clocks that corresponded to their differing by only one second every 300 billion years—a world record for precision timekeeping for two spatially separated clocks.

If it hadn't been for another research published in the same issue of Nature, it would have been a world record for the overall most precise frequency difference. A team from JILA, a Colorado-based research institute, led the investigation. The JILA group was nearly 10 times better than the UW–Madison group at detecting a frequency difference between the top and bottom of a dispersed cloud of atoms.

Their findings, which were achieved at a distance of one millimetre, also reflect the shortest distance at which Einstein's theory of general relativity has been tested with clocks to date. Kolkowitz's team plans to conduct a similar experiment soon.

"What's amazing is that we showed identical performance to the JILA group despite employing a laser that's orders of magnitude worse," Kolkowitz says. "That's huge for a lot of real-world applications, where our laser looks a lot more like something you'd take out in the field."

Kolkowitz's team compared the frequency variations between each pair of six multiplexed clocks in a loop to demonstrate the clocks' possible applications. They discovered that when they return to the first clock in the loop, the variances sum up to zero, verifying the constancy of their measurements and allowing them to detect minute frequency changes inside the network.

"Imagine a cloud of dark matter passing across a network of clocks—are there any comparisons where I can detect that dark matter?" Kolkowitz enquires. "That's an experiment we can do now that we couldn't do before in any other experimental setting."

References:

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