Researchers are developing a new generation of atomic clocks that could redefine the fundamental unit of time. These instruments, known as strontium optical lattice clocks, have achieved a level of precision where they would not lose or gain a single second over the entire age of the universe. This leap in accuracy promises to revolutionize fields ranging from fundamental physics and geoscience to telecommunications and satellite navigation.
The new clocks harness the properties of strontium atoms cooled to near absolute zero and trapped in a crystal-like structure made of laser light. Unlike the current international standard for timekeeping, which is based on the microwave frequency of cesium atoms, optical clocks use the much higher frequency of visible light. This faster “ticking” allows time to be divided into far smaller intervals, leading to a massive improvement in both stability and accuracy. Several research groups worldwide have now demonstrated clocks with systematic uncertainty better than one part in a billion billion, pushing measurement science to its limits.
A New Standard for the Second
Since 1967, the official definition of a second has been based on the natural oscillations of the cesium-133 atom, which resonates at a frequency of 9,192,631,770 hertz in the microwave spectrum. This standard has been the bedrock of global timekeeping, enabling technologies like GPS and the internet. However, the best cesium fountain clocks are reaching their physical limits, with an accuracy of around one part in 1016, equivalent to losing a second every 30 million years.
Optical lattice clocks represent a new paradigm. They operate using an electronic transition in strontium atoms that oscillates at a frequency of approximately 429 trillion hertz. This frequency, nearly 100,000 times higher than that of cesium, is the key to their superior performance. A higher frequency provides a more finely graduated ruler for measuring time. The remarkable stability and accuracy of these clocks have led scientific bodies to consider them prime candidates for a future redefinition of the SI second.
The Architecture of Atomic Precision
Creating a strontium optical lattice clock is a complex feat of physics and engineering, involving the precise control of atoms with lasers. The process is designed to eliminate the tiny environmental perturbations that can introduce errors into timekeeping.
Cooling and Trapping Atoms
The first step involves cooling a gas of tens of thousands of strontium atoms to temperatures just a few thousandths of a degree above absolute zero. At these ultracold temperatures, atomic motion slows to a near standstill. The atoms are then trapped in an optical lattice, a grid-like potential created by intersecting laser beams. This structure holds the atoms in place, resembling eggs in a carton, which dramatically reduces frequency shifts caused by atomic movement—a major source of error in earlier clock designs.
The ‘Magic Wavelength’ Technique
A crucial innovation in optical clock design is the use of a “magic wavelength” for the lattice laser. The intense light of the laser trap itself can disturb the sensitive energy levels of the atoms, slightly altering their ticking frequency. The magic wavelength technique, first proposed by Hidetoshi Katori of the University of Tokyo in 2001, uses a specific frequency of light that shifts two key energy levels by the exact same amount. This cancels out the disruptive effect of the lattice, allowing the atoms to behave as if they are in a disturbance-free environment. Further refinements, such as finding a “magic lattice depth,” have helped researchers reduce other sources of error, such as those caused by interactions between the atoms themselves.
Pushing the Limits of Measurement
Recent experiments have demonstrated astounding levels of performance. A team at JILA, a joint institute of the University of Colorado Boulder and the National Institute of Standards and Technology (NIST), along with the University of Delaware, has achieved a systematic uncertainty of better than one part in a billion billion. This means the clock is so accurate that if it had started running at the Big Bang, it would have lost or gained less than a single second over the subsequent 13.8 billion years.
This is achieved not only through the high-frequency oscillator but also by averaging the measurements across thousands of atoms simultaneously. Probing a large number of atoms reduces the statistical uncertainty, known as quantum projection noise, allowing the clock to reach its optimal accuracy very quickly. Some of the most stable designs can achieve a precision of a few parts in 1016 after just one second of measurement.
From the Laboratory to the Real World
While once confined to specialized physics laboratories, strontium optical lattice clocks are now being engineered into compact and transportable devices, opening up new applications in both fundamental science and industry.
Testing Fundamental Physics
Portable optical clocks provide a powerful tool for testing Albert Einstein’s theory of general relativity. The theory predicts that time passes more slowly in a stronger gravitational field—a phenomenon called time dilation. With their extreme precision, these clocks can detect the minuscule difference in the passage of time over a height difference of just a few centimeters. Transportable clocks have been used to perform the most precise ground-based measurements of this effect ever recorded.
Next-Generation Technology
The superior timing offered by optical clocks is critical for a range of emerging technologies. Their stability could enhance the synchronization of 5G and future telecommunication networks, improve the accuracy of global navigation satellite systems for applications like autonomous vehicles, and provide new tools for geoscience. For example, networks of these clocks could be used to monitor volcanic activity or changes in sea level by detecting subtle shifts in gravitational potential. To meet this demand, researchers are developing commercial-grade, user-friendly systems designed for long-term, automatic operation outside the lab.
Challenges and the Path Forward
Despite the rapid advances, a significant hurdle remains before the full potential of optical clocks can be realized globally: synchronization. An 18-digit-precise clock is only useful if its time can be shared with others at the same level of precision.
Current time transfer methods, which primarily rely on satellite links using onboard cesium clocks, are limited to a precision of about 16 digits. This creates a bottleneck, as the clocks are more precise than the network used to compare them. While fiber-optic cables can transmit timing signals with much higher fidelity, this solution is currently only practical over medium distances. Developing new methods for synchronizing these next-generation clocks across continents is a key area of ongoing research, paving the way for a new era in global timekeeping.
