From sundials to atomic clocks, humanity’s quest to measure time has been a journey of increasing precision. Now, that journey has reached a new frontier with the creation of the world’s first nuclear clock – a device that promises to revolutionize everything from GPS navigation to our understanding of physics by using the core of an atom itself.
This milestone, reported in the journal Nature, marks the dawn of a new era in precision timekeeping. The development pushes the boundaries of what’s possible in measurement science and brings us one step closer to unraveling the mysteries of the universe.
At the heart of this breakthrough is a peculiar atom called thorium-229. Unlike most atomic nuclei, which require enormous amounts of energy to excite, thorium-229 possesses a nuclear-excited state that can be reached using ultraviolet light. This unique property makes it the perfect candidate for creating a nuclear clock – a timepiece that could potentially outperform even the most advanced atomic clocks available today.
The achievement is the result of a collaboration between researchers from JILA, a joint institute of the University of Colorado Boulder and the National Institute of Standards and Technology (NIST), and scientists from the Vienna University of Technology (TU Wien). Their work combines cutting-edge laser technology with precision atomic clocks and a specially engineered crystal containing thorium atoms.
“With this first prototype, we have proven: Thorium can be used as a timekeeper for ultra-high-precision measurements. All that is left to do is technical development work, with no more major obstacles to be expected,” says Thorsten Schumm of TU Wien, one of the lead researchers on the project, in a media release.
To understand the significance of this achievement, it’s helpful to know how our current best timekeepers work. Today’s most precise clocks, known as atomic clocks, use the oscillations of laser light to count time, much like a very fast-ticking pendulum. These oscillations are kept stable by matching them to the energy transitions of atoms like cesium or strontium.
The nuclear clock takes this concept a step further. Instead of using transitions in the electron shell of an atom, it uses transitions within the atomic nucleus itself. This is a big deal because atomic nuclei are much smaller than atoms and are far less affected by external disturbances like electromagnetic fields. In theory, this means a nuclear clock could be vastly more precise and stable than an atomic clock.
The road to this breakthrough has been long and challenging. For decades, scientists have known that thorium-229 had potential for nuclear clocks, but the exact energy needed to excite its nucleus remained elusive. It wasn’t until earlier this year that Schumm’s team at TU Wien finally succeeded in precisely measuring this energy and using a laser to switch thorium nuclei between two quantum states.
Building on this success, the JILA team, led by Jun Ye, took the next crucial step. They developed a sophisticated system that combines an ultra-stable strontium atomic clock with a special laser setup called a frequency comb. This setup allowed them to produce the precise ultraviolet light needed to excite the thorium nuclei embedded in a crystal.
“Imagine a wristwatch that wouldn’t lose a second even if you left it running for billions of years,” says Ye, an NIST and JILA physicist. “While we’re not quite there yet, this research brings us closer to that level of precision.”
The crystal itself is a marvel of engineering. Developed at TU Wien over several years, it contains thorium-229 nuclei in just the right configuration to interact with the laser light.
“This crystal is the central element of the experiment,” explains Schumm.
When the researchers shone their precisely tuned ultraviolet light on the crystal, they were able to observe the thorium nuclei switching between energy states. By measuring the exact frequency of light that causes this switch, they effectively created the world’s first nuclear clock.
The precision achieved in this experiment is staggering. The team was able to measure the energy difference between the two nuclear states to within a few kilohertz – a million times more precise than previous measurements. This level of accuracy is akin to measuring the distance between New York and Los Angeles to within the width of a human hair.
Moreover, if this clock were to run for the entire age of the universe – about 13.8 billion years – it would only be off by about 0.02 seconds. That’s an accuracy of approximately 99.9999999999999%. This level of precision far surpasses what was possible just a few years ago and brings us tantalizingly close to unlocking new realms of fundamental physics.
