MADISON, Wis. — Imagine arriving for a work meeting and your boss informs you that you’re late by about one billionth of a second. This nightmare scenario may come to pass one day thanks to a team of physicists at the University of Wisconsin–Madison. Researchers there created one of the highest performance atomic clocks ever constructed. Putting the potential for increased scrutiny on tardiness aside for a moment, this new clock design is a major scientific boon, allowing scientists to test new methods of searching for gravitational waves, attempt to detect dark matter, and discover new physics via clocks.
Formally known as the optical lattice atomic clock, this incredible device is capable of measuring differences in time to a precision equivalent to losing a single second every 300 billion years. This new clock is the first real-world example of a “multiplexed” optical clock, which refers to six separate clocks all existing within the same environment.
“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 senior study author Shimon Kolkowitz, a UW–Madison physics professor, in a statement. “We’re working to both improve their performance and to develop emerging applications that are enabled by this improved performance.”
The science behind optimal atomic clocks
What makes atomic clocks so precise? When an electron’s energy levels fluctuate, it responds by absorbing or emitting light with an identical frequency that is linked to all atoms of a particular element. Optical atomic clocks keep such precise track of time by utilizing a laser tuned to match those frequencies precisely. As one can imagine, this is an incredibly delicate process requiring the world’s most sophisticated lasers.
Now, Prof. Kolkowitz’s group knew they didn’t have access to those types of lasers. In their own words, their laser was “pretty lousy.” However, they also knew that many downstream optical clock applications require portable, commercially available lasers like theirs. So, if the research team could develop an accurate clock using their average laser, it would be a big deal.
They created a multiplexed clock that allows for strontium atoms to be separated into multiple clocks arranged in a line within a single vacuum chamber. With one atomic clock, they noted their laser was only reliably capable of exciting electrons in the same number of atoms for one-tenth of a second.
But, when that same laser was aimed at two clocks in the chamber at the same time, the amount of atoms with excited electrons stayed identical between the two clocks for as long as 26 seconds! That meant they could run meaningful experiments for much longer than their “lousy” laser would have normally facilitated with a typical optical clock.
“Normally, our laser would limit the performance of these clocks,” Prof. Kolkowitz explains. “But because the clocks are in the same environment and experience the exact same laser light, the effect of the laser drops out completely.”
Next, researchers wanted to ascertain how precisely they could measure differences among the clocks. For reference, two groups of atoms operating within slightly different environments tick at slightly different rates depending on a variety of factors (gravity, magnetic fields, etc).
Study authors ran their experiment, astoundingly, over 1,000 times. Each round they measured ticking frequency differences between the two clocks for roughly three hours. As predicted, ticking was slightly different between the clocks (due to two slightly different locations). As more and more measurements were recorded, the research team was better able to understand and measure these fluctuations.
By the end of this process the researchers were able to narrow down this ticking difference between the two clocks to a rate of them disagreeing with each other by only one second every 300 billion years. Such an exact measurement of precision timekeeping sets a new world record for two spatially separated clocks.
Almost a world record
This work would have set another world record pertaining to the overall most precise frequency difference, had it not been for another study published in the same issue of Nature. That project, led by JILA (a research institute in Colorado), detected a frequency difference between the top and bottom of a dispersed cloud of atoms roughly 10 times more accurate than the UW–Madison group of scientists.
The JILA group’s results, gathered at one millimeter separation, also represent the shortest distance ever by which Albert Einstein’s theory of general relativity has been tested out via clocks. Prof. Kolkowitz’s team wants to conduct similar work in the near future.
“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,” Prof. Kolkowitz comments. “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 better demonstrate all of the possible uses for their clocks, the research team compared frequency changes between each pair of six multiplexed clocks within a loop. This led to the discovery that the differences added up to zero upon returning the first clock to the loop. This observation confirms the measurements are accurate and consistent, opening the door toward the detection of 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?” Prof. Kolkowitz asks. “That’s an experiment we can do now that you just couldn’t do in any previous experimental system.”
The study is published in Nature.