QUANTUM LEAP: INSIDE THE ATOMIC NUCLEUS—HOW SCIENTISTS CREATED A NEW KIND OF QUANTUM CLOCK

For decades, atomic clocks have measured time through transitions involving electrons surrounding an atomic nucleus. Scientists have now taken that principle into the nucleus itself. Following landmark experiments in 2024, research published in October 2026 demonstrated operating nuclear clocks based on thorium-229, using precisely controlled ultraviolet light to stabilize frequency against one of nature’s most unusual nuclear transitions. The achievement introduces an entirely new class of quantum timekeeping and creates opportunities to investigate fundamental physics with exceptional sensitivity

The development of quantum mechanics transformed humanity’s understanding of matter by revealing that atoms possess precisely defined energy states and undergo transitions governed by measurable physical laws. These discoveries eventually enabled scientists to build atomic clocks capable of maintaining extraordinarily accurate frequency references through electromagnetic interactions with electrons. For generations, the electronic structure of atoms provided the foundation for the world’s most advanced timekeeping systems, allowing physicists to refine the measurement of time through progressively more precise spectroscopy. Yet another possibility remained largely beyond experimental reach: using transitions occurring inside the atomic nucleus itself as a frequency reference. The nucleus contains protons and neutrons bound through powerful interactions, and its energy structure generally operates on scales far removed from the optical transitions used in conventional atomic clocks. Reaching those nuclear states through controlled laser excitation presented an immense scientific challenge.

That challenge has now entered a new stage. In October 2026, independent scientific teams reported operating nuclear clocks based upon thorium-229, an unusual isotope possessing an exceptionally low-energy nuclear transition accessible through vacuum-ultraviolet radiation. Rather than relying upon an electron changing its state around the nucleus, these instruments use a transition between two quantum configurations of the nucleus itself. The experiments demonstrated that carefully stabilized laser light could interrogate the nuclear resonance and receive feedback capable of maintaining the optical frequency against that reference. The achievement represents a fundamental advance in precision measurement because it establishes that a laser-accessible nuclear transition can function as the controlling reference within an operating clock, rather than remaining exclusively an object of spectroscopic investigation.

The October developments are especially significant when considered alongside the scientific progress achieved during the preceding years. In 2024, an international collaboration led by scientists at JILA, a joint institute of the National Institute of Standards and Technology and the University of Colorado Boulder, established a direct frequency relationship between the thorium nuclear transition and an optical atomic clock based on strontium. Additional experiments demonstrated nuclear excitation in solid materials and examined how the surrounding crystal environment influenced the transition. By 2025, experimental advances had expanded the available methods for detecting nuclear excitation, including techniques based upon electron emission rather than direct observation of ultraviolet fluorescence. These achievements addressed separate technical obstacles involving the generation of appropriate radiation, the detection of extremely weak signals, and the interpretation of nuclear behavior within engineered materials.

The result is a remarkable expansion of quantum control into a region of matter that historically required far more energetic radiation and very different experimental techniques. Thorium-229 provides access to a nuclear excitation with an energy of approximately 8.4 electronvolts, corresponding to radiation near 148 nanometers in wavelength. That energy is extraordinarily small compared with many familiar nuclear excitations, which commonly involve energies thousands or millions of times greater. Its unusual position within the nuclear spectrum creates an opportunity to investigate nuclear quantum states using specialized optical methods, bringing the precision of modern laser spectroscopy into contact with the internal structure of the nucleus.

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Why Thorium-229 Is Different From Other Atomic Nuclei

The atomic nucleus occupies only a tiny fraction of an atom’s overall spatial extent, yet it contains nearly all of the atom’s mass. Protons and neutrons are held together through interactions associated with the strong nuclear force, producing a system whose energy levels depend upon nuclear structure, angular momentum, and the interactions between its constituent particles. Nuclear excited states are familiar in radioactive decay and nuclear spectroscopy, but their energy differences generally require gamma rays or other highly energetic processes to access them. This creates a major obstacle for attempts to drive a specific nuclear transition coherently using the stable, precisely controlled electromagnetic sources employed in modern optical measurements.

Thorium-229 is exceptional because its nucleus possesses a low-lying excited state known as a nuclear isomer. An isomer is a nuclear configuration that differs from another state of the same nucleus in its internal energy and quantum properties. The metastable thorium-229 isomer, conventionally designated thorium-229m, has an excitation energy near 8.4 electronvolts. This places its transition within the vacuum-ultraviolet region of the electromagnetic spectrum, where specialized laser sources can produce radiation suitable for precision spectroscopy. The small energy separation does not mean the nucleus has lost the physical properties associated with nuclear matter. Instead, its internal structure permits two distinct configurations to exist at an unusually small energy difference.

Evidence for this nuclear state emerged from investigations conducted during the 1970s. Studies examining the structure of thorium-229 identified indications of an unusually low-lying isomer, although determining its precise energy proved difficult. For decades, the transition energy was inferred through indirect nuclear spectroscopy and successive experimental refinements. The challenge was substantial because the nucleus could not simply be interrogated using ordinary visible-light spectroscopy, while the exceptionally small energy difference complicated methods designed for much higher-energy nuclear radiation. The situation required new approaches capable of linking nuclear measurements with precise optical frequency control.

The importance of the isomer also lies in the possibility that a nuclear reference may respond differently to external disturbances than an electronic atomic transition. Electrons occupy comparatively extended quantum states and can be sensitive to surrounding electric fields, magnetic interactions, and thermal radiation. The nucleus is spatially much more compact, potentially reducing its response to certain environmental effects. This property makes thorium-229 an attractive candidate for precision timekeeping, although the actual performance of a nuclear clock still depends upon the physical environment, the properties of the host material, and the design of the measurement apparatus.

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The Long Road From Nuclear Spectroscopy to Direct Laser Excitation

The development of the thorium nuclear clock concept required a sequence of advances extending across nuclear physics, atomic spectroscopy, and optical engineering. An early experimental challenge involved establishing that the isomeric state could produce a measurable transition suitable for comparison with a stable electromagnetic source. Another was identifying the energy accurately enough to construct a laser system capable of reaching the resonance. Because the transition lies in the vacuum-ultraviolet region, conventional optical components and ordinary laser systems cannot simply be used without substantial adaptation. The theoretical proposal to use thorium-229 as a nuclear frequency standard gained attention during the early 2000s. Its unusually low nuclear excitation suggested the possibility of a clock in which the controlling transition originated inside the nucleus rather than in the surrounding electron structure. The proposed advantage extended beyond potentially improved stability, offering an experimental probe of interactions influencing nuclear energy and opportunities to investigate whether certain fundamental physical quantities remain constant over time.

A crucial experimental milestone came in 2016, when the decay of the thorium-229 isomer was directly detected through internal conversion electrons. Internal conversion is a nuclear de-excitation process in which energy is transferred to an atomic electron, causing that electron to be emitted rather than producing a photon through ordinary radiative decay. The observation provided direct evidence of the isomeric transition and strengthened the experimental foundation for future investigations. It also revealed that the electronic environment surrounding the nucleus could influence how the excited state released its energy, establishing an important relationship between nuclear behavior and the atomic or solid-state system containing it.

By 2024, multiple laboratories had demonstrated direct excitation of the thorium nuclear transition using specially generated ultraviolet radiation. These experiments marked a major achievement because they established that the transition could be driven through controlled electromagnetic interactions rather than inferred exclusively through radioactive decay products or indirect nuclear measurements. Direct excitation supplied the necessary foundation for frequency stabilization, allowing the nuclear transition to be treated as a reproducible reference whose response could be measured through precisely tuned radiation. Generating suitable light remained technically demanding. Radiation near 148 nanometers is strongly absorbed by ordinary air and many materials used in conventional optical systems, requiring specialized sources and experimental arrangements. Techniques involving nonlinear optical interactions, harmonic generation, and carefully selected materials capable of transmitting or producing the required wavelengths made it possible to interrogate the nuclear state with sufficient spectral precision to distinguish its response from surrounding background signals.

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The 2024 JILA Experiment Connected a Nuclear Transition to an Atomic Clock

On September 4, 2024, an international scientific collaboration led by JILA published a landmark investigation in Nature titled Frequency Ratio of the ²²⁹ᵐTh Nuclear Isomeric Transition and the ⁸⁷Sr Atomic Clock. The participating scientists included Chuankun Zhang, Tian Ooi, Jacob S. Higgins, Jack F. Doyle, Lars von der Wense, Kjeld Beeks, Adrian Leitner, Georgy Kazakov, Peng Li, Peter G. Thirolf, Thorsten Schumm, and Jun Ye. Their work established a direct relationship between the frequency associated with the thorium-229 nuclear transition and the optical reference provided by a strontium-87 atomic clock.

The experimental arrangement used thorium nuclei embedded within a calcium fluoride crystal. Vacuum-ultraviolet radiation was generated to excite the nuclear resonance, while a frequency comb established its frequency relative to the strontium optical clock. An optical frequency comb provides a precisely defined set of electromagnetic frequencies that can connect widely separated regions of the optical spectrum. Through this method, the nuclear transition frequency could be determined with exceptional precision while maintaining a direct relationship to an established atomic reference. The experiment also resolved features associated with the interaction between the thorium nucleus and its surroundings. Nuclear quadrupole interactions can produce distinct spectral components when the electric-field environment of a nucleus interacts with the distribution of its nuclear charge. Examining these features supplied information about the nuclear state’s properties and the influence of the surrounding crystal, demonstrating that precision nuclear spectroscopy could reveal structural details inaccessible through less refined methods.

The significance of the JILA achievement was substantial, but its status required scientific precision. The 2024 apparatus demonstrated essential components for future nuclear timekeeping, including optical access to the nuclear resonance and a direct frequency comparison with an atomic clock. It was not yet an operating nuclear clock whose laser frequency was continuously stabilized through feedback from the nuclear reference. That additional step required a sufficiently rapid and sensitive method for obtaining nuclear resonance information and using it to control an oscillator during sustained operation. The experiment consequently established a crucial foundation for the operating nuclear clocks that followed.

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The Challenge of Building a Nuclear Clock Inside a Crystal

A particularly promising approach to thorium nuclear timekeeping involves embedding thorium-229 nuclei within a solid material. This method provides access to an enormous number of nuclear references in a comparatively small physical volume, allowing a collective response to be obtained from many nuclei without individually trapping each atom or ion. Calcium fluoride has become an important host material because of its optical properties and compatibility with experimental arrangements designed to investigate the vacuum-ultraviolet transition.

Embedding nuclei within a crystal creates advantages and complications. The host material provides mechanical stability and permits many nuclei to participate in a spectroscopic measurement, potentially improving signal strength. At the same time, the local environment influences the nuclear resonance through electric-field gradients, temperature-dependent changes, and interactions associated with the material’s structure. Thorium ions may occupy different sites or configurations within the crystal, producing distinct spectral features that must be characterized before a particular transition can be selected for accurate timekeeping. The electric quadrupole moment of a nucleus can interact with the electric-field gradient created by neighboring ions, shifting or splitting nuclear energy levels. Such interactions provide valuable information about nuclear structure and the local environment, but they also introduce frequency shifts that may limit the reproducibility of a clock if they are not properly understood.

The development of suitable host crystals therefore requires more than introducing thorium into a transparent material. Dopant concentrations, local lattice arrangements, temperature sensitivity, and the mechanisms responsible for observed frequency differences must all be characterized. These requirements connect nuclear spectroscopy with crystal growth, materials engineering, and precise optical measurement. The successful operation of a solid-state nuclear clock depends upon maintaining a sufficiently stable nuclear resonance while obtaining a signal strong enough to guide the laser frequency.

A separate research direction has examined thorium-containing thin films. In December 2024, work involving thorium tetrafluoride films was reported as a potential platform for solid-state nuclear clocks. Such materials offer opportunities for integrating nuclear references with smaller optical structures and future photonic components. Their usefulness depends upon preserving the nuclear transition’s measurable properties while controlling the interactions introduced by the surrounding material.

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The October 2026 Breakthrough: Nuclear Clocks Become Operational

A major advance occurred on October 7, 2026, when Nature published research titled A Nuclear Clock Synchronized to ²²⁹Th. The study demonstrated an operating nuclear clock by stabilizing a continuous-wave vacuum-ultraviolet laser to a resolved thorium-229 nuclear transition within calcium fluoride crystals. The experiment established the complete feedback process required for a functioning clock, connecting the nuclear resonance to an oscillator whose frequency could be repeatedly corrected through measurements of the nucleus itself.

The apparatus used a continuous-wave light source operating near 148.4 nanometers, with radiation generated through four-wave mixing in cadmium vapor. This system produced approximately 10 microwatts of vacuum-ultraviolet radiation while maintaining a narrow spectral linewidth suitable for interrogating the nuclear transition. The availability of continuous-wave radiation was particularly important because earlier experiments had relied heavily upon pulsed light sources, complicating sustained frequency stabilization and rapid acquisition of resonance information. A calcium fluoride crystal containing thorium-229 nuclei served as the nuclear reference, with spectroscopy used to identify suitable resonance components. The measurements distinguished a broader spectral structure from a set of narrower quadrupole-resolved transitions. A particular component exhibited sufficiently favorable linewidth and temperature sensitivity to serve as the selected clock reference.

Changes in the nuclear absorption signal were measured as the laser frequency was adjusted around the selected resonance. The resulting information was converted into a frequency correction applied to the optical system through an acousto-optic modulator. This completed the feedback loop, allowing the nuclear transition to influence the operating frequency of the interrogating laser rather than functioning solely as a signal observed during spectroscopy.

The team reported a fractional frequency instability of approximately 5 × 10⁻¹³ divided by the square root of the averaging time in seconds. Nuclear transition frequencies measured in two independently fabricated crystals also agreed at approximately the 10⁻¹³ fractional level, providing evidence concerning the reproducibility of the selected solid-state reference. These results established that a nuclear transition could be incorporated into an operating frequency standard. The achievement was not simply the observation of another unusual atomic phenomenon. It demonstrated that the internal quantum structure of a nucleus could directly regulate a precision optical oscillator through a measurable feedback process.

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Independent Experiments Confirm the Practical Development of Nuclear Timekeeping

The October 2026 progress was reinforced by additional independent work demonstrating similar operating principles through different experimental arrangements. Another investigation published in Nature, titled A Thorium-229 Optical Nuclear Clock with Feedback Loop, reported a nuclear clock based upon continuous-wave laser interrogation and rapid absorption measurements of thorium nuclei embedded within a millimeter-scale calcium fluoride crystal at room temperature.

This experimental system used a laser whose wavelength was converted through nonlinear optical interactions to produce the required vacuum-ultraviolet radiation. Absorption associated with the nuclear resonance provided the information necessary to stabilize the laser frequency. A continuing comparison with a ytterbium-ion optical clock allowed the nuclear instrument’s performance to be evaluated against an independently established atomic reference. The reported shot-noise-limited fractional frequency instability was approximately 3 × 10⁻¹² divided by the square root of the averaging time in seconds. Over extended operation, the system approached fractional instabilities near 10⁻¹⁵ on a timescale of one day, demonstrating sustained nuclear-reference operation and establishing performance characteristics suitable for comparison with conventional optical frequency standards.

The study also used the nuclear clock to investigate possible variations in the thorium transition energy associated with theoretical models of ultralight dark matter. Frequency behavior was examined over timescales extending from approximately 20 seconds to one day, seeking signatures consistent with periodic fluctuations or slow changes in the nuclear reference. The investigation produced constraints on proposed dark-matter interactions rather than a confirmed detection.

Another important report was accepted by Physical Review Letters on October 26, 2026. The study, titled Solid-State Nuclear Clock Based on VUV Absorption Spectroscopy of ²²⁹Th, involved Pengfei Wang, Xu-Fei Yin, Hanlin Wang, Jinming Liu, Qichen Qin, Zhouzhi Tan, and an extensive international collaboration that included Thorsten Schumm, Jian-Wei Pan, and Yu-Ao Chen. The experiment demonstrated sustained operation of a solid-state nuclear clock through continuous-wave absorption measurements at 148.4 nanometers.

The apparatus incorporated a calcium fluoride crystal containing distinct thorium centers, with multiple quadrupole-related components resolved through spectroscopy. Locking the laser to one of the stronger narrow transitions maintained an active feedback process throughout a 30-hour measurement record, with control updates occurring approximately every 10 seconds. The output frequency was measured using a frequency comb referenced to a hydrogen maser, providing an established timing reference against which the nuclear clock’s behavior could be evaluated.

The reported fractional frequency instability reached approximately 1.24 × 10⁻¹³ at an averaging time of 10,000 seconds. A separate experiment maintained feedback on another nuclear resonance component for approximately 6.6 hours. These results demonstrated that multiple nuclear spectral features could support sustained stabilization while revealing differences requiring further characterization when comparing transitions and crystal environments.

These independent achievements are important because operating thorium nuclear clocks are no longer confined to a single experimental implementation. Different laboratories have developed systems capable of generating the necessary radiation, detecting nuclear resonance signals, and maintaining feedback control through a nuclear reference. Their performance remains below the best established optical atomic clocks, but the essential principle has been demonstrated experimentally.

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Why Nuclear Timekeeping Could Become Important for Fundamental Physics

The development of operating nuclear clocks provides science with a new frequency reference governed by interactions within the atomic nucleus. This has consequences beyond the immediate objective of measuring time. Different physical systems respond differently to hypothetical changes in fundamental constants, making comparisons between nuclear and electronic transitions valuable for testing predictions that would otherwise be difficult to investigate.

A significant area of investigation concerns the fine-structure constant, which characterizes the strength of electromagnetic interactions. Atomic transition frequencies depend upon this constant through the electronic structure and electromagnetic interactions of the atom. Nuclear transition energies also depend upon electromagnetic contributions, but they are governed by additional interactions associated with internal nuclear structure. Comparing the behavior of nuclear and atomic frequency references can therefore provide sensitivity to different combinations of physical effects.

Thorium-229 is particularly interesting because its unusually low transition energy emerges from a near-degeneracy between two nuclear configurations. The individual contributions determining nuclear energy can be substantially larger than the small difference separating the isomer and ground state. The resulting transition may be especially sensitive to changes in certain underlying physical parameters, although the size and interpretation of that sensitivity depend upon nuclear-structure calculations.

Another potential application involves searches for ultralight dark matter. Certain theoretical models predict that a dark-matter field could produce extremely small oscillations in physical constants or interaction strengths. If such effects exist, a nuclear transition and an electronic atomic transition may respond differently, allowing precision frequency comparisons to search for correlated variations. The nuclear-clock measurements reported in October 2026 provide an important experimental development for these investigations because they establish an operating reference that can be observed continuously over extended periods. Rather than relying solely upon isolated nuclear spectroscopy measurements, the behavior of a stabilized oscillator can be analyzed while comparing it with established atomic frequency standards.

The implications also extend to the study of nuclear structure. Precision measurements can reveal small energy shifts associated with electric quadrupole interactions, local crystal environments, and other physical effects influencing nuclear resonance. Such information provides experimental constraints for theoretical calculations describing the internal configurations of protons and neutrons. These opportunities are scientifically significant because the nuclear clock combines exceptionally precise electromagnetic measurement with a system governed by nuclear interactions, creating a new experimental connection between atomic physics, nuclear physics, and tests of fundamental physical laws.

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The Scientific Limits That Still Separate Nuclear Clocks From Established Optical Standards

Although the October 2026 experiments established functioning nuclear clocks, their achievements should not be confused with demonstrating a replacement for the world’s most accurate optical atomic clocks. The newly reported instruments remain experimental systems whose frequency stability and systematic uncertainties require further improvement. Their successful operation establishes an important technical foundation, but the precision of an instrument depends upon more than the existence of a narrow physical resonance.

Establishing reproducibility between independently manufactured nuclear clocks requires a comprehensive evaluation of systematic frequency uncertainties. Calibration procedures must determine the contributions of temperature measurement, magnetic shielding, reference-laser stability, and differences between crystal specimens. These effects must be quantified individually and incorporated into an uncertainty budget before measurements from separate instruments can establish whether they reproduce the same nuclear frequency within their stated precision.

Future improvements in nuclear-clock performance will also depend upon maintaining the spectral purity and long-term frequency stability of the interrogation laser. Frequency-conversion stages must preserve a sufficiently narrow optical linewidth while minimizing fluctuations introduced by nonlinear interactions, thermal variations, and mechanical instability. Efficiently transferring a stable frequency reference through the optical system will be essential for reducing technical noise and improving the reliability of sustained nuclear-clock operation.

Another obstacle concerns the method used to detect nuclear excitation. Fluorescence measurements can provide direct evidence of radiative decay, but the long lifetime of the excited nuclear state can limit the speed at which information is acquired. Absorption spectroscopy provides an alternative by detecting changes in transmitted light, potentially supporting faster feedback. Conversion-electron detection offers another approach, particularly in host materials where the nuclear excitation can transfer energy into electronic degrees of freedom.

Each measurement technique introduces different physical considerations. A material suitable for efficient fluorescence may not provide the most favorable conditions for electron-based detection, while the local electronic environment can influence the nuclear decay mechanism. The combined behavior of the nucleus, host material, optical system, and detector must therefore be evaluated when determining which architecture offers the strongest performance.

Radioactive material handling also remains a practical consideration. Thorium-229 is radioactive, requiring appropriate containment, preparation, and laboratory controls. The small quantities used in precision experiments do not remove the need for specialized procedures, particularly when developing reproducible materials and considering possible future instrument deployment.

The progress reported in October demonstrates that these difficulties can be addressed sufficiently to operate nuclear clocks under controlled conditions. The next scientific challenge is improving their performance, establishing reproducible uncertainty budgets, and determining whether the nuclear reference can ultimately offer practical advantages over the advanced electronic atomic standards already in use.

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TRJ VERDICT

The creation of operating thorium-229 nuclear clocks represents an important achievement in the development of quantum measurement technology. For decades, the scientific community understood that transitions inside an atomic nucleus could theoretically provide a frequency reference distinct from those based upon electronic atomic states. The challenge was reaching a suitable nuclear transition with sufficiently precise electromagnetic radiation, detecting its response, and incorporating that signal into an instrument capable of sustained frequency stabilization. The independent experiments reported in October 2026 demonstrate that these requirements can now be met under controlled laboratory conditions.

Scientific progression leading to this achievement deserves recognition. Early nuclear spectroscopy established the unusual energy structure of thorium-229, while the direct detection of its isomeric decay provided evidence necessary for future experimental work. The laser-excitation breakthroughs of 2024 demonstrated that the nuclear state could be addressed electromagnetically, and JILA’s frequency comparison connected the transition directly with an established strontium atomic clock. These accomplishments created the experimental foundation upon which the 2026 nuclear-clock demonstrations were constructed.

The most consequential development is the successful integration of the thorium nuclear resonance into operating feedback systems. Nuclear absorption signals can now guide a continuously operating optical oscillator, with separate laboratories establishing sustained measurement records and quantifiable frequency stability. The 30-hour operation reported in the Physical Review Letters study further demonstrates that nuclear-reference stabilization can be maintained over extended intervals, providing valuable evidence concerning the engineering requirements of future instruments.

The broader significance extends into fundamental physics. A nuclear clock offers access to energy differences governed by the internal structure of the nucleus, providing a reference whose dependence upon physical interactions differs from that of conventional electronic atomic clocks. This opens opportunities for investigating nuclear properties, comparing fundamental frequency standards, and testing selected theoretical models involving possible variations in physical constants or interactions associated with dark matter. Such investigations remain subject to experimental limitations and theoretical interpretation, but the establishment of operating nuclear clocks provides a new instrument for pursuing them.

TRJ recognizes that the achievement is not the replacement of existing atomic time standards, nor proof that nuclear clocks have already surpassed the most accurate instruments available. The accomplishment is the successful demonstration of a new category of quantum timekeeping that previously existed primarily as a scientific objective. Future progress will depend upon improved frequency stability, reproducibility between independently manufactured references, systematic uncertainty control, and the continued refinement of vacuum-ultraviolet instrumentation.

For more than a century, quantum physics has revealed how matter changes between discrete energy states. Scientists have now brought one of those transitions into the heart of the atomic nucleus and used it to regulate an operating clock. The October 2026 breakthroughs demonstrate that nuclear quantum states can serve as measurable, reproducible frequency references, establishing an important new foundation for precision timekeeping and the experimental investigation of fundamental physics.

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