ChatAtomic clocks measure electron orbital energy jumps that are vulnerable to external electromagnetic interference; directly exciting the ultralow-energy nuclear isomer of Thorium-229 with vacuum ultraviolet lasers paves the way for nuclear clocks of unprecedented stability.
Modern atomic clocks keep time with breathtaking precision by locking laser frequencies to electron transitions in strontium or cesium atoms, powering global GPS navigation and telecommunications.
However, electrons orbit on the fragile exterior of atoms, where stray magnetic and electric fields constantly perturb energy levels, placing a hard physical ceiling on long-term timekeeping stability.
In a historic Physical Review Letters breakthrough, physicists achieved direct resonant laser excitation of the Thorium-229 nuclear isomer using vacuum ultraviolet laser pulses in a calcium fluoride crystal. By driving a transition inside the tightly shielded atomic nucleus rather than the outer electron cloud, the nuclear transition proves immune to external environmental noise.
The realization of the world's first solid-state nuclear clock promises to surpass today's best atomic clocks by orders of magnitude, opening revolutionary probes into fundamental constant drift, dark matter waves, and gravitational redshift on millimeter scales.
Laser Excitation of the Th-229 Nucleus
The 8.4 eV nuclear isomer state in Th-229 is resonantly excited in Th-doped CaF_{2} crystals using a tabletop tunable laser system. A resonance fluorescence signal is observed in two crystals with different Th-229 dopant concentrations, while it is absent in a control experiment using Th-232. The nuclear resonance for the Th^{4+} ions in Th:CaF_{2} is measured at the wavelength 148.3821(5) nm, frequency 2020.409(7) THz, and the fluorescence lifetime in the crystal is 630(15) s, corresponding to an isomer half-life of 1740(50) s for a nucleus isolated in vacuum. These results pave the way toward Th-229 nuclear laser spectroscopy and realizing optical nuclear clocks.
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