Nuclear clock

A nuclear clock or nuclear optical clock is an atomic clock which uses photons from a nuclear isomeric transition as its reference frequency, instead of the atomic electron transition energy used by conventional atomic clocks. Such a clock is expected to be more precise than the best current atomic clocks by a factor of about 10, with an achievable precision approaching the 10 −19 level. The only nuclear state suitable for the development of a nuclear clock using existing technology is thorium-229m, an isomer of thorium-229 and the lowest-energy nuclear isomer known. With an energy of , this corresponds to a frequency of , or wavelength of , in the vacuum ultraviolet region, making it accessible to laser excitation.

A nuclear clock or nuclear optical clock is an atomic clock which uses photons from a nuclear isomeric transition as its reference frequency, instead of the atomic electron transition energy used by conventional atomic clocks. Such a clock is expected to be more precise than the best current atomic clocks by a factor of about 10, with an achievable precision approaching the 10 −19 level. The only nuclear state suitable for the development of a nuclear clock using existing technology is thorium-229m, an isomer of thorium-229 and the lowest-energy nuclear isomer known. With an energy of , this corresponds to a frequency of , or wavelength of , in the vacuum ultraviolet region, making it accessible to laser excitation. ===Ionization=== An excited atomic nucleus can shed its excess energy by two alternative paths: radiatively, by direct photon (gamma ray) emission, or by internal conversion, transferring the energy to a shell electron which is ejected from the atom. For most nuclear isomers, the available energy is sufficient to eject any electron, and the inner-shell electrons are the most frequently ejected. In the special case of , the energy is sufficient only to eject an outer electron (thorium's first ionization energy is ), and if the atom is already ionized, there is not enough energy to eject a second (thorium's second ionization energy is ). The two decay paths have different half-lives. Neutral decays almost exclusively by internal conversion, with a half-life of . In thorium cations, internal conversion is energetically prohibited, and is forced to take the slower path, decaying radiatively with a half-life of around half an hour. Thus, in the typical case that the clock is designed to measure radiated photons, it is necessary to hold the thorium in an ionized state. This can be done in an ion trap, or by embedding it in an ionic crystal with a band gap greater than the transition energy. In this case, the atoms are not 100% ionized, and a small amount of internal conversion is possible (reducing the half-life to approximately 10 minutes ), but the loss is tolerable. Alternatively, the thorium can be embedded in a material with a bandgap less than the transition energy, and the resonance can be found by measuring the count of ejected electrons as a function of UV excitation wavelength. This method of interrogation has both benefits and disadvantages. The main drawback is that since the decay is much faster, the linewidth is greater, and the stability is limited to "only" about 1 part in . An advantage is that the readout is obtained by measuring ejected electrons, a common lab technique and one that is easy to miniaturize. Readout is also much faster due to the faster decay. Finally, only a very thin surface layer ( ) of thorium is required, potentially important as Th-229 is exceedingly rare the world supply is estimated at 40 grams. The first two prototype nuclear clocks were announced in June 2026. Both use solid-state crystals of CaF 2 doped with . Their performance is so far similar to more mature atomic clock technologies (stability for intervals \tau up to 10,000 seconds about 3\cdot 10^{-12} \sqrt{\text{s}/\tau} , better than 4\cdot 10^{-12} \sqrt{\text{s}/\tau} for an HP 5071A/001 caesium atomic clock, but far worse than 1.55\cdot 10^{-13} \sqrt{\text{s}/\tau} for a caesium fountain ), and development is expected to be rapid. ==External links== EU thorium nuclear clock (nuClock) project

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