Scientists unveil one of the highest performance atomic clocks ever

According to a report published in the journal Nature on Wednesday, University of Wisconsin-Madison physicists have made one of the highest performing atomic clocks ever.

Their optical lattice atomic clock, the next step beyond standard atomic clocks, 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. Six separate clocks can exist in the same environment within a multiplexed optical clock.

The team will now be able to attempt to detect dark matter, discover deeper into physics with clocks and search for gravitational waves.

"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.”

Keeping note of consistent frequencies

An atomic clock uses the resonance frequencies of atoms as its resonator. According to Encyclopedia Britannica, the resonator is "regulated by the frequency of the microwave electromagnetic radiation emitted or absorbed by the quantum transition (energy change) of an atom or molecule." Atoms resonate at consistent frequencies, while the oscillating frequency of a quartz crystal in a quartz clock can vary due to manufacturing tolerances. The consistent frequencies in a cesium atom are what make atomic clocks so precise.

Atomic clocks' ultra-precise accuracy to tell the time was recently harnessed by U.S. government agency DARPA. They had announced the Robust Optical Clock Network (ROCkN) program, which aims to build a super-accurate optical atomic clock compact enough to fit inside a military aircraft or field vehicle. The optical atomic clock, the most accurate type of atomic clock, is so accurate that it wouldn't have lost a second over the universe's entire existence of more than 13 billion years.

Optical atomic clocks keep time by using a laser that is tuned to precisely match this frequency. To keep accurate time, they require some of the world’s most sophisticated lasers.

The right environment

Kolkowitz says that their group has “a relatively lousy laser" by comparison. As a result, they were aware that any clock they built would not be the most accurate or precise on its own. They also knew that the ensuing applications of optical clocks will require portable, commercially available lasers like theirs.

Their new study saw a multiplexed clock in which 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.

But, 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 findings were proof that they could run essential experiments for much longer than their laser would allow in a normal optical clock.

“Normally, our laser would limit the performance of these clocks. But because the clocks are in the same environment and experience the exact same laser light, the effect of the laser drops out completely," says Kolkowitz.

Next, the group wanted to figure out how precisely they could measure differences between the clocks. Depending on gravity, magnetic fields, and other conditions, two groups of atoms in slightly different environments will tick at different rates.

The experiment was run over a thousand times, which measured the difference in the ticking frequency of their two clocks for around three hours. As foreseen, the ticking slightly varied because the clocks were in two slightly different locations. As the team took more and more measurements, they were better able to measure those differences.

Eventually, the researchers were able to detect a difference in ticking rate between the two clocks that would coincide to the differences 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.

Time-consuming, but need of the hour

The result could have easily become a world record for the overall most precise frequency difference if not for another paper, published in the same issue of Nature. A group at JILA, a research institute in Colorado, detected a frequency difference between the top and bottom of a dispersed cloud of atoms about 10 times better than the UW–Madison group.

Obtained at one-millimeter separation, the results also signify 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 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 illustrate 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. This confirmed the consistency of their measurements and established 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.”