![]() With high-precision laser spectroscopy ruled out, such searches relied on observables that are indirectly related to the transition frequency. The precise determination of the 2 2 9 m Th energy remained an elusive experimental objective for decades. And finding the precise frequency with a narrow-band laser would have required prohibitively long scans and multiple types of lasers. However, since the transition frequency wasn’t known with sufficient precision, it wasn’t clear which laser technology would be most appropriate. These energies correspond to wavelengths where lasers are, in principle, available. The energy difference between its ground state and its first, metastable excited state, denoted 2 2 9 m Th, is exceptionally low-with previously reported values between 3.5 and 8.3 eV. The reason is simple: Transitions involving nuclear excited states have typical energies in the keV to MeV range, which is inaccessible by today’s laser technology.Ī transition of the thorium-229 nucleus is the only known exception. Conversely, laser spectroscopy of a nuclear transition remains elusive. Since the laser was invented in 1960, laser spectroscopy of the electronic shells of atoms has become an established technique and has enabled spectacular applications, including optical atomic clocks. Knowledge of the exact transition frequency would immediately make a nuclear optical clock possible. The results pave the way for even more precise measurements based on laser spectroscopy. Tomas Sikorsky of Heidelberg University, Germany, and colleagues have now reported a high-precision measurement of the thorium-229 transition that significantly narrows the spectral range on which future searches should focus. However, until recently, the transition frequency was not determined with sufficient precision to allow its direct excitation with narrow-band lasers-a prerequisite for the operation of an optical clock. Long ago, researchers identified a nuclear transition suitable for a nuclear clock in the thorium-229 isotope. 1) is expected to be less sensitive to external perturbations. Since the atomic nucleus is much smaller than the atomic electron shell, such a “nuclear optical clock” (Fig. However, an even more accurate clock could, in principle, be built by using a nuclear transition instead of an electronic transition. ![]() These optical atomic clocks are accurate to within one part in 1 0 1 8, meaning that they’d slip by less than one second over the age of the Universe. Today’s most accurate clocks tick at frequencies defined by ultranarrow electronic transitions of atoms at optical wavelengths. Figure 1: Artist’s rendition of a nuclear optical clock. ![]()
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