A new generation of optical atomic clocks, some accurate enough to lose less than a second over the entire age of the universe, is driving an international push to replace the cesium-based definition of the second that has stood since 1967. Recent results from laboratories across Europe, the United States, and Asia show that multiple competing clock designs now far exceed the precision of cesium, and the metrology community is targeting a formal redefinition vote by 2030.
Why Cesium No Longer Cuts It
The second is currently defined by the frequency of microwave radiation absorbed by cesium-133 atoms, a standard adopted nearly six decades ago. Optical clocks operate on a fundamentally different principle: they probe atomic transitions at visible or ultraviolet light frequencies, which oscillate roughly 100,000 times faster than cesium’s microwave tick. That higher frequency acts like a finer ruler, allowing each measurement cycle to slice time into far thinner intervals. As explained in a NIST overview, optical clocks now reliably produce seconds that are orders of magnitude more accurate than the cesium devices still used to define the unit.
The gap between what optical clocks can do and what the official definition permits has become a practical problem. Researchers who want to compare two optical clocks against each other must still route their measurements through cesium-referenced international timescales, introducing noise from the less precise standard. That bottleneck limits applications ranging from satellite navigation to tests of whether fundamental constants drift over time and complicates efforts to build a seamless global network of next-generation timekeepers.
Metrologists have therefore begun mapping out how and when to move beyond cesium. A dedicated program on the future of the second describes a roadmap in which optical standards are vetted, compared, and gradually integrated into international timekeeping, with the goal of a consensus redefinition once technical and institutional benchmarks are met.
The Coulomb Crystal Clock and Its Rivals
Among the strongest recent results is a mixed-species ion clock that pairs indium-115 and ytterbium-172 ions in a shared electromagnetic trap, forming what physicists call a Coulomb crystal. In work reported in Physical Review Letters, the device achieved a relative systematic uncertainty of 2.5 × 10−18 and produced optical frequency ratio measurements described as among the most accurate ever recorded. “We use indium ions as they have favorable properties to achieve high accuracy,” researcher Tanja Mehlstäubler of Germany’s PTB said, noting that ytterbium ions are added for efficient cooling and to enable cascaded interrogation of several ensembles.
A separate strontium-88 single-ion clock, accepted for publication in Physical Review Applied, reported an estimated fractional systematic uncertainty of 7.9 × 10−19 and was compared against International Atomic Time (TAI) over a 10‑month campaign with high uptime. Its absolute frequency measurement carried a fractional uncertainty of 9.8 × 10−17, limited not by the clock itself but by the cesium-referenced timescale it was measured against. That distinction matters: it illustrates exactly why the current definition constrains the technology rather than the other way around, and why metrologists argue that the official second needs to catch up with the best available hardware.
NIST researchers, meanwhile, built what they describe as the most accurate clock in the world using an aluminum ion paired with a magnesium ion, setting a new record that contributes directly to the redefinition effort. This clock’s performance pushes systematic uncertainties into the low 10−19 range, reinforcing the message that several independent ion-based platforms are now vying to serve as primary optical references once the second is redefined.
A Six-Country Clock Network
Individual lab records, no matter how impressive, are not enough to justify changing a global standard. The metrology community also needs to show that different clocks in different countries agree with one another at extreme precision. A large-scale experiment published in the journal Optica tackled that requirement head-on: ten optical clocks in six countries were compared simultaneously using fiber and satellite links. The campaign evaluated a subset of 38 optical frequency ratios, measured four of those ratios directly for the first time, and improved uncertainties on several others.
This kind of coordinated comparison addresses a critique that has slowed the redefinition debate for years. Skeptics have argued that isolated record-setting clocks do not prove the technology is mature enough for a universal standard. By demonstrating agreement across borders and across different atomic species, the Optica results weaken that objection considerably. The network showed that national metrology institutes can operate optical clocks reliably over long periods, link them with robust infrastructure, and maintain consistent frequency ratios at levels far beyond what cesium can support.
Such international campaigns depend heavily on preprint archives and open collaboration. The work on the six-country network, like many other optical clock advances, first appeared on arXiv, which is maintained by a consortium of institutional member organizations and supported in part by community donations. That rapid dissemination lets laboratories coordinate measurements, cross-check results, and refine techniques in time to influence formal decisions about redefining the second.
Entanglement and the Lattice Approach
Not all optical clocks rely on single trapped ions. Lattice clocks take a different approach by trapping thousands of atoms in grids of laser light and averaging their signals, which can dramatically improve measurement speed. One design at JILA, the joint institute of NIST and the University of Colorado, uses approximately 30,000 strontium atoms held in a two-dimensional optical lattice. Researchers there boosted the clock’s precision through quantum entanglement, linking the atoms so their collective measurement beats the statistical limits that constrain unentangled ensembles.
The tension between single-ion and lattice designs is productive rather than destructive. Single-ion clocks like the indium‑ytterbium Coulomb crystal or the strontium‑88 system tend to achieve the lowest systematic uncertainties because every environmental effect on the lone ion can be carefully characterized and controlled. Lattice clocks, by contrast, trade a slightly higher floor on systematic error for dramatically better short-term stability, thanks to the huge number of atoms measured in parallel. In practice, metrologists expect that both architectures will play roles in the post-cesium era: a handful of ultra-clean ion clocks may define the second, while lattice clocks disseminate that definition quickly and robustly across networks.
From Laboratory Records to a New SI Second
Turning these laboratory achievements into a new unit of time requires more than physics. International working groups must agree on which atomic transitions are mature enough to serve as primary standards, how to weight different clock types in a global average, and how to maintain continuity with the existing cesium-based second so that navigation systems, telecommunications, and financial networks are not disrupted.
The current strategy is incremental. Optical clocks are already being used to realize “secondary representations” of the second, meaning they provide highly accurate time and frequency services while still being indirectly tied to cesium. Over the next several years, as comparisons like the six-country network expand and more independent laboratories demonstrate long-term reliability, those secondary representations are expected to gain enough trust to become the primary reference.
By the time metrologists gather to vote on redefining the second, likely near the end of this decade, they will have in hand a portfolio of proven optical technologies: mixed-species ion traps, single-ion systems based on aluminum, strontium, or indium, and large-scale lattice clocks enhanced by entanglement. They will also have a track record of international coordination, built on shared data, common analysis methods, and dense webs of fiber and satellite links. If the recent pace of progress continues, the question will no longer be whether optical clocks are good enough, but how best to formalize what the world’s leading laboratories are already doing in practice.
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*This article was researched with the help of AI, with human editors creating the final content.
