In the dead of night at a Colorado laboratory, a discovery was made that could reshape our understanding of time and the universe itself. A signal had appeared on a computer screen, marking a subtle shift in the energy state of a thorium-229 nucleus. And this was no ordinary shift. It represented the “tick” of the world’s first rudimentary nuclear clock.
Chuankun Zhang, the graduate student who first saw the signal, quickly shared the news with his colleagues. Over the next few hours, they confirmed their findings, knowing they had achieved something extraordinary.
“We spent the entire night doing all the tests to check if this is actually really the signal that we were looking for,” Zhang told Nature, adding it “felt amazing” to be part of this discovery.
The breakthrough is the culmination of nearly five decades of research. But for physicists, it’s just the beginning of a new journey — one that could pave the way not only for a novel generation of ultra-precise clocks, but could also redefine our grasp of fundamental forces.
The Promise of the Nuclear Clock
Physicists at JILA, a joint institute of the National Institute of Standards and Technology (NIST) and the University of Colorado Boulder, have demonstrated the critical components of what could become the most precise clock in existence: a nuclear clock. Unlike atomic clocks, which measure the ticks of time by the movements of electrons around an atom, nuclear clocks would track the subtler, faster energy shifts within an atom’s nucleus.
The team, led by NIST and JILA physicist Jun Ye, measured the frequency of ultraviolet light that induces energy jumps in the nuclei of thorium-229 atoms embedded in a crystal. This frequency, effectively the “tick” of the nuclear clock, was synchronized with one of the world’s most accurate atomic clocks. The result? A measurement 100,000 times more precise than previous attempts, signaling a major leap toward building a fully functional nuclear clock.
Why a Nuclear Clock?
Atomic clocks, currently set the international standard for time. They rely on the vibrations of atoms to keep time with astonishing accuracy. At their cores, these clocks use the cesium-133 atom, whose electrons oscillate between two energy states at a highly consistent frequency when exposed to microwaves. This frequency, a natural “tick,” is counted to measure the passage of time. By tuning lasers and microwaves to match the atom’s inherent oscillations, atomic clocks achieve an extraordinary level of precision. So precise, in fact, that they lose only a second after tens of billions of years.
So, if we can already measure time this precisely, why the need for nuclear clocks? The particles in a nucleus, unlike electrons, are largely immune to external disturbances such as electromagnetic fields. This stability could make nuclear clocks not only more precise but also more robust and portable than existing atomic clocks.
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The nuclear clock breakthrough hinges on thorium-229, a rare isotope with a unique feature: its nucleus can undergo a low-energy transition that can be excited by lasers. This property was first suggested in the 1970s when scientists studying byproducts of nuclear weapon research noticed an unusually low-energy state in thorium-229 nuclei. Decades later, researchers finally managed to harness this peculiarity to construct the world’s first nuclear clock prototype.
Building a nuclear clock is no small feat. The researchers embedded trillions of thorium-229 atoms in a crystal and used a specialized device known as a frequency comb. This device emits a spectrum of laser frequencies, allowing researchers to probe multiple energy states simultaneously and find the precise frequency needed to excite the thorium-229 nucleus.
Beyond its applications in timekeeping, the nuclear clock represents a new tool to explore fundamental physics. Because the clock’s frequency is set by the forces holding the nucleus together, slight changes in these forces — potentially caused by exotic particles like dark matter — could be detected with unprecedented sensitivity.
The clock’s high sensitivity to these forces could make it 100 million times more sensitive than atomic clocks to certain types of dark matter, the researchers claim in their new study. This could help physicists probe whether the constants of nature, like the strength of the nuclear force or the speed of light, remain truly constant over time.
Challenges Ahead
Despite the exciting potential, significant work remains before nuclear clocks can surpass atomic ones. The JILA team’s prototype, while novel, has not yet been refined to measure time continuously. There is also an ongoing debate about the best way to implement the clock. Should thorium-229 remain embedded in a crystal, as it is now, or would trapping individual atoms yield even better results?
The laser technology used to induce the energy shifts also needs further development. However, the current setup serves only serves as a prototype for a more advanced system that could eventually form the basis of a practical nuclear clock. The rewards are worth it too. A more precise clock could improve GPS technology, enable new tests of the laws of physics, and perhaps even detect elusive dark matter particles.
The next steps will involve refining the nuclear clock’s precision and testing its limits. Researchers hope to use this new technology to examine the underlying fabric of the universe more closely than ever before. Could the fundamental constants of physics vary over time? Is there more to the forces that bind atoms and nuclei together than we currently understand?
With the advent of the nuclear clock, physicists are poised to tackle these questions head-on. And while the journey to a fully functional nuclear clock is just beginning, the road ahead promises new insights into the nature of time, matter, and the cosmos itself.
In the words of Jun Ye, “Imagine a wristwatch that wouldn’t lose a second even if you left it running for billions of years. While we’re not quite there yet, this research brings us closer to that level of precision.”
The findings were reported in the journal Nature.
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