Ultrahigh Performance Atomic Clock Could Pave the Way to Novel Physics Discoveries

Physicists at the University of Wisconsin–Madison have created one of the maximum performance atomic clocks ever. They report their development in the February 16^th issue of the journal Nature.

Their instrument, called an optical lattice atomic clock, can compute variances in time to a precision corresponding to losing just one second every 300 billion years and is the first model of a “multiplexed” optical clock, where six individual clocks can be present in the same environment. Its design enables the researchers to test ways to hunt for gravitational waves, attempt to identify dark matter and determine 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. We’re working to both improve their performance and to develop emerging applications that are enabled by this improved performance.

Shimon Kolkowitz, Senior Study Author and Professor of Physics, University of Wisconsin–Madison

Atomic clocks are so exact because they exploit the fundamental property of atoms: when an electron alters energy levels, it absorbs or releases light with a frequency that is the same for all atoms of a specific element. Optical atomic clocks maintain time by using a laser that is adjusted to exactly match this frequency, and they necessitate some of the world’s most advanced lasers to maintain accurate time.

Relatively speaking, Kolkowitz’s group has “a relatively lousy laser,” he says, so they were aware that any clock they constructed would not be the most exact or precise on its own. But they were also aware that a number of downstream applications of optical clocks will necessitate portable, commercially available lasers like theirs. Developing a clock that could use average lasers would be a bonus.

In their new research, they built a multiplexed clock, where strontium atoms can be divided into multiple clocks oriented in a line in the same vacuum chamber. Using a single atomic clock, the team learned that their laser could only reliably excite electrons in the same number of atoms for one-tenth of a second.

However, when they irradiated the laser on two clocks in the chamber simultaneously and compared them, the number of atoms with excited electrons remained the same between the two clocks for up to 26 seconds. Their results meant they could try out crucial experiments for a lot longer than their laser would permit in a regular 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.

Shimon Kolkowitz, Senior Study Author and Professor of Physics, University of Wisconsin–Madison

The team was keen to know how precisely they could compute variances between the clocks. Two groups of atoms that are in slightly different environments will tick at marginally different rates, based on magnetic fields, gravity or other conditions.

They performed their experiment over a thousand times, measuring the variance in the ticking frequency of their two clocks for a total of roughly three hours. As estimated, because the clocks were in two slightly different environments, the ticking was marginally different. The researchers showed that as they computed more and more measurements, they were able to measure those variances better.

Eventually, the scientists could detect a variance in ticking rate between the two clocks that would correspond to them conflicting with each other by just one second every 300 billion years — a measurement of exact timekeeping that sets a world record for two clocks that are spatially separated.

There would have been another world record for the overall most precise frequency difference if not for another research article, published in the same issue of Nature. That research was directed by a group at JILA, a research institute in Colorado. The JILA group sensed a frequency difference between the top and bottom of a dispersed cloud of atoms approximately 10 times better than the UW–Madison team.

Their outcomes, acquired at one-millimeter separation, also signify the shortest distance to date at which Einstein’s theory of general relativity has been verified with clocks. Kolkowitz’s group hopes to carry out 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. 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.

Shimon Kolkowitz, Senior Study Author and Professor of Physics, University of Wisconsin–Madison

To show the possible applications of their clocks, Kolkowitz’s team compared the frequency variations between each pair of six multiplexed clocks in a loop. They learned that the variances add up to zero when they come back to the first clock in the loop, verifying the consistency of their measurements and setting up the option that they could sense minute frequency variations 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? That’s an experiment we can do now that you just couldn’t do in any previous experimental system.

Shimon Kolkowitz, Senior Study Author and Professor of Physics, University of Wisconsin–Madison

This study received support in part from the NIST Precision Measurements Grants program, the Northwestern University Center for Fundamental Physics, and the John Templeton Foundation through a Fundamental Physics grant, the Wisconsin Alumni Research Foundation, the Army Research Office (W911NF-21-1-0012), and a Packard Fellowship for Science and Engineering.

Journal Reference:

Zheng, X., et al. (2022) Differential clock comparisons with a multiplexed optical lattice clock. Nature. doi.org/10.1038/s41586-021-04344-y.