A quantum leap in physics brings the nuclear clock closer to reality.

Imagine a clock that, even after one billion years of operation, ticks so precisely that it never misses a second. According to recent research, scientists are now more likely than ever to achieve that degree of timekeeping accuracy.

Such a device would be far more accurate than current state-of-the-art atomic clocks, which measure time by defining a second through carefully timed energy jumps in atoms' electrons. The signals that excite the atoms in atomic clocks oscillate at a billion times per second.

A new method that researchers have created could improve this accuracy by producing and monitoring oscillations in an even more difficult target: the nucleus of an atom. To create this nuclear clock, the researchers excited the nuclear particles in a solid-crystal thorium-229 atom using ultraviolet light. Then, using an instrument known as an optical frequency comb, they counted the waves in the UV signal to determine the frequency of the energy pulses that were impacting the nucleus, which is comparable to a pendulum in a conventional clock.

A far higher frequency signal is needed to cause energy jumps in a nucleus than is necessary for atomic clocks. This method should yield more precise time measurements since it has more wave cycles per second.

Although their nuclear clock is still in development, if it is successful, it has the potential to revolutionize not only timekeeping but also physics research and even the way scientists probe the structure of the cosmos. According to research that was published on September 4 in the journal Nature, the prototype is already as accurate as an atomic clock, and future iterations are anticipated to be even more accurate and stable.

As the scientists have demonstrated that these signals can be generated and measured, "there are a lot of things that we can push to further improve the accuracy," according to Chuankun Zhang, the lead study author and graduate student at JILA, a collaborative research center supported by the National Institute of Standards and Technology and the University of Colorado Boulder.

Zhang explained that among the modifications might be adjusting the lasers' alignment and frequency to better ping the nucleus.

Dr. Olga Kocharovskaya, a distinguished professor of physics at Texas A&M University who was not involved in the research, stated, "This work truly marks the dawn of a nuclear clock."

In 2023, scandium-45 atom nuclei were examined by Kocharovskaya and associates as potential candidates for a nuclear clock. The energy transition and detectable pulse those atoms produced at the time were the strongest ever seen in a nucleus, but Kocharovskaya informed via email that the new findings from thorium-229 produced a stronger signal and were more stable.

 "The confidence this paper provides in the reality of the nuclear clock is the broader significance," the speaker stated. "It is unequivocal that the construction of such a clock is possible and imminent."

Repeatedly

Atomic clocks work by pinging an atom's electrons at particular frequencies with electromagnetic radiation. The electrons are excited by energy bursts, which cause them to move into a higher orbit around the atom. According to NASA, oscillations that cause electrons to change states indicate the passage of time.

Compared to common clocks that measure seconds in the vibrations of quartz crystals, which are prone to going out of sync, atomic clocks are far more reliable. Atomic clocks have been used for international timekeeping, space exploration, and GPS technologies for many years.

Nevertheless, sync loss can also affect atomic clocks. According to Zhang, electromagnetic disturbances have the potential to disrupt excited electrons and compromise timekeeping accuracy.

On the other hand, nucleus particles in an atom are more difficult to disturb than electrons. The strongest of all the fundamental forces, the strong nuclear force, firmly holds protons and neutrons together. According to the researchers, more accurate time measurements are made possible by wavelengths that oscillate at higher frequencies and can cause a nucleus transition.

Prior to this study, numerous significant advancements in the field of nuclear clock development had occurred. The first came about in 1976 when it was discovered that the thorium nucleus was "uniquely low-energy" and that vacuum ultraviolet, or VUV, laser light could be used to force it into an excited state. According to the study, by 2003, scientists were suggesting that the isotope thorium-229 would be a good candidate for nuclear clocks because thorium required less energy to excite its nucleus than most other types of atoms did.

Scientists developed a novel technique in 2023 to embed thorium-229 into crystals. This solid state system suppressed nuclear decay signals, facilitating the tracking of desired signals. The wavelength of VUV light necessary to excite the nucleus in thorium-229 was measured earlier this year by other researchers.

Zhang stated, "Our work builds on top of that." "This crystal and our frequency comb light source allowed us to excite the various transition energies and the nuclear transition." Zhang said that their findings were approximately a million times more accurate than those of earlier measurements.

Dr. Shimon Kolkowitz, an associate professor and physics chair at the University of California, Berkeley, described the paper as a "true tour de force."

Kolkowitz, who was not involved in the research, said, "The quality of the data and the speed with which they achieved the remarkable results in this new manuscript is really amazing." "It marks a significant advancement in the creation of nuclear clocks, a goal pursued by physicists for many years.”

Transforming physics

Scientists have already benefited greatly from the accuracy and stability of atomic clocks in their studies of space-time, gravitational fields, and earthquakes. Nuclear clocks could provide "a major boost" to these fields, according to Kocharovskaya. According to her, nuclear clocks would be more accurate, simpler, and more portable than atomic clocks because they wouldn't need strong magnetic and electric shielding, intense cooling, or high vacuum conditions.

Zhang suggests that nuclear clocks combined with atomic clocks could transform physics education itself. According to Zhang, scientists may be able to determine whether fundamental physics constants are actually as constant as they appear to be or whether they change at previously unmeasureable levels by tracking and comparing frequency ratios in the two kinds of clocks over time.

He continued by saying that the study of dark matter, the enigmatic material that makes up 80% of the universe but has never been directly measured, may benefit greatly from the application of this paired clock technique.

According to some scientists, dark matter interacts with quarks, gluons, and electrons in amounts that are currently undetectable.

Zhang stated that the goal is to determine whether dark matter interacts with the atomic nucleus in a manner that differs slightly from that of the atom's electron orbit. "New physics would be indicated if there was a change over time in the transition frequency ratio between the nuclear clock and the atomic clock."

These results suggest that the day when nuclear clocks outperform atomic clocks and eventually replace them is not too far off, according to Kolkowitz, even though there is still much work to be done in this regard.

"I expect that eventually some of the kinds of experiments we are currently doing in my lab to test relativity and search for new physics with atomic clocks will instead be performed with nuclear clocks as better UV laser sources are developed and as some of the mysteries and tricks of nuclear clocks get worked out," Kolkowitz stated.

Regarding nuclear clocks, what's next? Currently, only time will tell.