229ThF4 thin films for solid-state nuclear clocks.

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After nearly 50 years of searching, the vacuum ultraviolet 229Th nuclear isomeric transition has recently been directly laser excited1,2 and measured with high spectroscopic precision3. Nuclear clocks based on this transition are expected to be more robust4,5 than and may outperform6,7 current optical atomic clocks. These clocks also promise sensitive tests for new physics beyond the standard model5,8-12. In light of these important advances and applications, a substantial increase in the need for 229Th spectroscopy targets in several platforms is anticipated. However, the growth and handling of high-concentration 229Th-doped crystals5 used in previous measurements1-3,13,14 are challenging because of the scarcity and radioactivity of the 229Th material. Here we demonstrate a potentially scalable solution to these problems by performing laser excitation of the nuclear transition in 229ThF4 thin films grown using a physical vapour deposition process, consuming only micrograms of 229Th material. The 229ThF4 thin films are intrinsically compatible with photonics platforms and nanofabrication tools for integration with laser sources and detectors, paving the way for an integrated and field-deployable solid-state nuclear clock with radioactivity up to three orders of magnitude smaller than typical 229Th-doped crystals1-3,13. The high nuclear emitter density in 229ThF4 also potentially enables quantum optics studies in a new regime. Finally, we present the estimation of the performance of a nuclear clock based on a defect-free ThF4 crystal.