In the end, the packets in the elongated atomic cloud were lined up in their places in the optical lattice like a string of pearls hanging down. The researchers photographed the particles directly under the microscope and measured their frequencies using spectroscopy. Along the gravity axis, the top values of the one millimeter small cloud were shifted slightly to the blue, the bottom to the red. The observed differences correspond exactly to what would be expected mathematically according to the theory of relativity from two clocks that are placed at correspondingly different heights in the earth’s gravitational field. This was only possible because the background noise from all other frequency-smearing influences was reduced to previously unattainable levels. Thus, the clock in Jun Ye’s lab is not only the first to demonstrate the effects of time dilation, it is also the most precise in the world to date.
© Spectrum of Science / Mike Zeitz (detail)
Atomic clock with time dilation | An optical lattice created by superimposed laser beams captures a cloud of strontium atoms. Their oscillation changes with the gravitational field of the earth. Time passes faster for higher-lying atoms.
At the same time, a competing team of researchers at the University of Wisconsin-Madison has been working to improve the performance of optical clocks using a slightly different principle. There, Xin Zheng and his colleagues from Shimon Kolkowitz’s working group arranged several clouds, each consisting of a few hundred strontium atoms, at a distance of almost one centimeter one above the other. This system, which the scientists call Multiplex, consists of several spatially separated clocks in a single experiment. Zheng was able to measure and compare up to six of them in parallel.
The concept improves timing precision by compensating for local laser instabilities by averaging over the entire multiplex. In principle, it would also make it possible to put the beats of the top and bottom clocks in relation. At least mathematically, this should provide the difference predicted by the theory of relativity, because the accuracy of the multiplex is roughly ten times lower than that of Bothwell, but the distance between the individual clocks is ten times greater. However, for their publication, which appeared in the same issue of the journal Nature in February 2022, the authors did not attempt to put Einstein to the test. In any case, the watch from Kolkowitz’ group is also a remarkable achievement. Without the simultaneous coup by Ye’s team, she would now be the record holder.
Advances in high-precision timekeeping are rapid. As early as 2018, research teams, including one led by Jun Ye, demonstrated a precision with atomic clocks that, at least in principle, could be used to determine whether the instrument was lifted by less than a centimeter. Of course, given the complex overall construction, that would not be an easy task. It is therefore an exciting next step that differences can now be registered that arise within an ongoing experiment and also within one and the same atomic cloud. It no longer needs two devices to match, so this is far more than just another record breaking news.