²²⁹ThF₄ thin films for solid-state nuclear clocks.

Abstract

After nearly 50 years of searching, the vacuum ultraviolet ²²⁹Th nuclear isomeric transition has recently been directly laser excited^1,2 and measured with high spectroscopic precision³. Nuclear clocks based on this transition are expected to be more robust^4,5 than and may outperform^6,7 current optical atomic clocks. These clocks also promise sensitive tests for new physics beyond the standard model^5,8-12. 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 measurements^1-3,13,14 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 crystals^1-3,13. 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.