New precision in optical atomic clock New optical atomic clock allows even more precise measurement of the second

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Researchers at JILA, a joint facility of the National Institute of Standards and Technology (NIST) and the University of Colorado Boulder in the USA, have succeeded in building an atomic clock that is significantly more precise than its predecessors because it mitigates the previous weaknesses of optical atomic clocks.

If the passage of time were according to each person's feeling, there would probably be billions of different time measurements in the world. Depending on how busy one is—or not—the passage of time is perceived differently. Early civilizations have tried to give time a measure, but it wasn't until the invention of the atomic clock in the 20th century that a second could be distinctly defined as a unit. In 1967, the second was officially designated as the 9,192,631,770-fold period of the radiation emitted during the transition between two hyperfine structure levels of the ground state of caesium-133 atoms.

By now, clocks have been developed that are not based on caesium atoms and are noticeably more precise. Therefore, it is assumed that the unit of measure second could be redefined in a few years. A second is therefore measured with an atomic clock. This is used for highly precise time measurement and is essential for the synchronization of communication networks such as the internet and GPS, as well as for scientific experiments and financial transactions.

From the JILA (Joint Institute for Laboratory Astrophysics), a joint institution of the National Institute of Standards and Technology (NIST) and the University of Colorado Boulder, came the news in early July 2024 of a new atomic clock that is even more precise than previous ones.

A gentle net of laser light

The new type of clock illuminates atoms with visible light waves, which have a much higher frequency than the old-fashioned microwaves. This allows for an even more precise counting of the second. Compared to the currently used microwave clocks, the optical clocks are expected to offer much higher accuracy for international time measurement—they could lose only one second every 30 billion years.

Alexander Aeppli, Kyungtae Kim, William Warfield, Marianna S. Safronova and Jun Ye published their study "Clock with 8×10^-19 Systematic Uncertainty" in Physics Review Letters on July 10, 2024, which can be viewed there. A systematic uncertainty of 8 × 10^-19 implies a very small deviation in the clock's time measurement. More specifically, the clock would only deviate by 8 parts in 10^19 (that's 8 in 10 trillion) from the true time value.

To achieve this performance, the JILA researchers used a flatter, gentler "net" of laser light to trap the atoms. This significantly reduced two major sources of error that could occur in previous optical atomic clocks: the effects of the laser light that traps the atoms, and the atoms colliding when they are packed too closely. "This clock is so precise that it can detect tiny effects that are predicted by theories such as the General Theory of Relativity, even on a microscopic level," said Jun Ye. "It pushes the boundaries of what is possible with time measurement."

The bending of time and space

The General Theory of Relativity is Einstein's theory that describes how gravity is caused by the warping of space and time. One of the most important predictions of the General Theory of Relativity is that time itself is affected by gravity - the stronger the gravitational field, the slower time passes. There is all sorts of science fiction material that picks up on this part of the General Theory of Relativity. But can this also be demonstrated in reality?

With the presented construction method of an atomic clock, according to the JILA researchers, relativistic effects on time measurement can be demonstrated in the submillimeter range, which corresponds to the thickness of a single human hair. Raising or lowering the clock by this tiny distance is enough for the researchers to detect a tiny change in the flow of time caused by the effects of gravity.

This newly gained precision in the measurement of the second allows a much more accurate navigation, not only on Earth, but also in space; at places where the slightest errors in time measurement could lead to serious problems in navigation. Likewise, the new type of time measurement can lead to progress and breakthroughs elsewhere.

"The same methods used to trap and control the atoms could also lead to a breakthrough in quantum computing. Quantum computers need to be able to precisely manipulate the internal properties of individual atoms or molecules to perform calculations. Advances in controlling and measuring microscopic quantum systems have significantly advanced this endeavor," the JILA researchers explain. (sb)