{
  "id": "nuclear-clock",
  "title": "Nuclear clock",
  "lang": "en",
  "source_lang": "en",
  "summary": "A proposed timekeeping device that would use a nuclear transition, most prominently in thorium-229, as its frequency reference instead of an electronic transition.",
  "category": "science",
  "subcategory": "physics",
  "type": "concept",
  "aliases": [
    "Nuclear optical clock",
    "thorium-229 nuclear clock",
    "nuclear clock transition"
  ],
  "generated_at": "2026-10-16",
  "model": "pro/claude-haiku-5-5",
  "url": "https://aiwiki.page/en/nuclear-clock/",
  "translations": {
    "zh-Hans": {
      "url": "https://aiwiki.page/zh/nuclear-clock/",
      "json": "https://aiwiki.page/zh/nuclear-clock.json"
    }
  },
  "sections": [
    {
      "heading": "",
      "text": "A nuclear clock is a proposed frequency standard whose oscillator is a transition between two energy levels of an atomic nucleus, rather than the electron-shell transitions used by conventional atomic clocks. The only known candidate that is realistically accessible to laser excitation is the first excited state of the thorium-229 nucleus, a nuclear isomer with unusually low energy. Because the nuclear frequency is far less affected by the surrounding environment than electron-shell frequencies, researchers expect a nuclear clock to be more robust than current optical clocks and to serve as a sensitive probe of fundamental physics. As of 2026 the concept has been demonstrated in laboratory prototypes but has not become a practical timekeeping instrument."
    },
    {
      "heading": "Physical basis",
      "text": "Ordinary atomic clocks count oscillations of light or microwaves emitted when electrons change orbitals in an atom or ion. Atomic nuclei have their own quantized energy levels, typically spaced by hundreds of keV to MeV, far above the eV scale of electronic transitions. Most nuclear transitions therefore lie in the X-ray or gamma-ray range and cannot serve as clock references. Nuclear transition frequencies are much higher than shell transitions, and for this reason they have been unusable for atomic clocks, which have been based exclusively on microwaves or laser light. The thorium-229 case is the notable exception, because its isomeric state lies at an exceptionally low energy.\n\nThe unusually low energy has a structural explanation. It results from a near-cancellation of the Coulomb and nuclear contributions to the binding energies of the ground and excited states, which are each on the MeV scale. That same sensitivity makes the transition frequency highly responsive to small changes in the coupling constants of the fundamental interactions, which is why the system is of interest in tests of physics beyond the Standard Model.\n\nThe current best value places the transition at about 8.36 eV. This corresponds to a frequency of roughly 2.02 PHz, or a wavelength of about 148.38 nm in the vacuum ultraviolet region, which makes it accessible to laser excitation. Earlier estimates, such as an electron-spectrometry result of 8.28 ± 0.17 eV, had much larger uncertainties, and the precise value was only fixed after laser-based measurements became possible."
    },
    {
      "heading": "Thorium-229 isomer",
      "text": "The isomer's behaviour depends strongly on its chemical and charge environment. The ground state of thorium-229 is long-lived, with a half-life of about 7,932 years. The isomeric state has a half-life of only about 7 µs in the neutral atom, but more than 60 s for the ion, because the ion cannot emit a loosely bound electron. In a bare nucleus in vacuum the isomer's lifetime is expected to approach an hour, which would give a very narrow resonance. This transition offers a potential resonance quality factor on the order of 10^19.\n\nIn solid crystals the lifetime is shorter but still useful. Measurements in thorium-doped calcium fluoride (CaF₂) gave a radiative decay half-life of about 447 s, and researchers have also shown that X-ray irradiation can depopulate the isomer on demand, a so-called X-ray quenching effect."
    },
    {
      "heading": "Experimental history",
      "text": "Interest in the nuclear clock dates to a 2003 proposal by Peik and Tamm, who suggested that the thorium-229 isomer could form the basis of a nuclear frequency standard. The proposal built on the extraordinarily low-energy isomeric state of the thorium-229 isotope at about 8.4 eV. For many years, researchers could not excite the isomer directly by laser, mainly because the state was difficult to populate. Despite many years of research, there was no successful laser excitation for a long time, largely because of the difficulty in freely accessing the isomeric state.\n\nSeveral methods were developed to overcome this. X-ray pumping through a resonant channel was used to populate the isomer in a transparent crystal, and electron spectroscopy and laser-driven Mössbauer-type measurements have since provided further information on the transition. In 2025, a team at UCLA achieved laser-driven measurement of internal-conversion electron Mössbauer spectroscopy in thorium dioxide, directly observing nuclear transition signals within an opaque medium. Theoretical work from Fudan University has also calculated the transition frequency for isolated thorium ions, which provides a target for ion-trap experiments.\n\nThe three main experimental routes are:\n\n- Ion-based clocks, in which a single trapped thorium ion is probed with lasers. This offers the cleanest environment but a very small number of emitters.\n- Atom- and surface-based approaches, which exploit the isomer's behaviour in neutral atoms or thin films.\n- Solid-state clocks, in which many thorium nuclei are embedded in a crystal. This is the most practical route."
    },
    {
      "heading": "Solid-state nuclear clocks",
      "text": "Solid-state designs have attracted the most attention because they can host large numbers of emitters. Because the nuclear transition is weakly coupled to its surroundings, orders of magnitude more emitters can be hosted in a solid-state crystal compared with current optical lattice atomic clocks. Thorium is doped at very low concentrations into a crystal host, and the nuclei are held in the Lamb–Dicke regime, which gives a simple and robust device at a moderate cost in quality factor.\n\nSeveral studies have examined the properties of such crystals. A 2025 study found that one of the quadrupole-split transition lines has only weak temperature dependence, which means temperature fluctuations need not limit stability if the crystal is controlled adequately. A 2026 study characterized the nuclear clock transition as a function of doping concentration, temperature and time, and found high reproducibility along with identified operating conditions for these crystals.\n\nA key milestone came in 2026, when a team demonstrated a nuclear clock driven by a continuous-wave laser. The team stabilized a continuous-wave laser to the 148 nm nuclear transition using rapid feedback based on absorption spectroscopy. The thorium nuclei sit in a millimetre-sized, room-temperature calcium fluoride crystal, and the clock was compared against a ytterbium ion single-ion clock. Its shot-noise-limited fractional frequency instability is about 3×10⁻¹² divided by the square root of the averaging time in seconds, approaching 10⁻¹⁵ after one day of operation. The stated instability is written as \\[\\sigma_y(\\tau) \\approx \\frac{3\\times10^{-12}}{\\sqrt{\\tau/\\text{s}}}\\] where \\(\\tau\\) is the averaging time. The authors project that the instability could improve by several orders of magnitude in future devices."
    },
    {
      "heading": "Applications",
      "text": "Timekeeping and metrology. A nuclear clock could in principle match or exceed the accuracy of optical atomic clocks. For comparison, the best optical clock reported in 2025, based on a single aluminium ion at NIST, reached a short-term stability of 3.5×10⁻¹⁶ per square root of the averaging time and an uncertainty of 5.5×10⁻¹⁹. Solid-state designs may also allow compact, field-deployable devices because they need only modest thermal control.\n\nFundamental physics. The clock is especially sensitive to hypothetical changes in the fine-structure constant and other fundamental parameters. A clock based on the thorium-229 nuclear transition is of significant interest for testing theories beyond the Standard Model that suggest that fundamental constants vary over time. The 2026 feedback-loop study used the clock to search for ultralight dark-matter through periodic fluctuations and slow drifts in the transition energy. Its constraints compete with the best atomic clocks for dark-matter coupling to photons and go beyond previous measurements for coupling to the strong force."
    },
    {
      "heading": "Challenges and open questions",
      "text": "Several technical problems remain. Early solid-state measurements showed spectral line widths of roughly 300 kHz, which is far wider than the natural linewidth set by the isomer's lifetime. Later samples reduced this to about 20 kHz, but the gap still limits the achievable precision. Vibrations of the crystal lattice may also affect the transition, although they are expected to average out.\n\nMaterial choice is another issue. Calcium fluoride is transparent at vacuum-ultraviolet wavelengths, but other host materials are under study, and the isomer's behaviour may vary with the environment in ways that remain incompletely understood. The experimental difficulty is also considerable. More than ten research groups around the world are working on the feasibility of a thorium-229 nuclear clock, and the issue has proven extremely difficult experimentally.\n\nTheoretical questions are open as well. Calculations of how nuclear deformation contributes to the sensitivity have suggested that some standard assumptions, such as a constant nuclear volume during excitation, may need revision. Such results affect how strongly the clock can constrain new physics, and they remain under active study.\n\nTaken together, the 2025–2026 results show that a laser-driven, solid-state nuclear clock is feasible, while precision and long-term stability still need substantial improvement before the device can match the best existing atomic clocks."
    }
  ],
  "links": [
    "standard-model",
    "dark-matter"
  ],
  "sources": [
    {
      "title": "En route to the optical nuclear clock - PTB.de",
      "url": "https://www.ptb.de/cms/en/presseaktuelles/si-materialien-fuer-schulen/ptb-experiment/the-second-optical-atomic-clocks/news-for-time.html?contrast=1&tx_news_pi1%5Bnews%5D=8856&tx_news_pi1%5Bcontroller%5D=News&tx_news_pi1%5Baction%5D=detail&tx_news_pi1%5Bday%5D=17&tx_news_pi1%5Bmonth%5D=4&tx_news_pi1%5Byear%5D=2018&cHash=ccb796b268a53cf2ddca671506784253"
    },
    {
      "title": "A thorium-229 optical nuclear clock with feedback loop",
      "url": "https://arxiv.org/pdf/2606.04997"
    },
    {
      "title": "Nuclear clock",
      "url": "https://en.wikipedia.org/wiki/Nuclear_clock"
    },
    {
      "title": "Active production of the isomeric state of thorium-229 nucleus with the lowest excitation energy. Toward the nuclear clock",
      "url": "https://inis.iaea.org/search/search.aspx?orig_q=RN%3A53064400"
    },
    {
      "title": "Current Progress on 229 Th Nuclear Clock",
      "url": "https://www.mdpi.com/2304-6732/13/2/141"
    },
    {
      "title": "Frequency reproducibility of solid-state thorium-229 nuclear clocks",
      "url": "https://www.nature.com/articles/s41586-025-09999-5"
    },
    {
      "title": "A thorium-229 optical nuclear clock with feedback loop",
      "url": "https://www.nature.com/articles/s41586-026-11084-4"
    },
    {
      "title": "Controlling 229Th isomeric state population in a VUV transparent crystal",
      "url": "https://www.ncbi.nlm.nih.gov/pmc/articles/PMC11252313/"
    },
    {
      "title": "Temperature sensitivity of a Thorium-229 solid-state nuclear clock",
      "url": "https://arxiv.org/pdf/2409.11590"
    },
    {
      "title": "Redefining Timekeeping Accuracy with Thorium-229 Nuclear Clock",
      "url": "https://www.azom.com/article.aspx?ArticleID=24834"
    },
    {
      "title": "Prospects for a Solid-State Nuclear Clock",
      "url": "https://arxiv.org/html/2511.13017v1"
    },
    {
      "title": "A thorium-229 optical nuclear clock with feedback loop",
      "url": "https://arxiv.org/html/2606.04997v2"
    }
  ],
  "license": "https://creativecommons.org/licenses/by/4.0/"
}