Toward high-efficiency repumping for ultra-stable Yb inner-shell optical lattice clocks

Ytterbium inner-shell optical clocks, based on transitions to the long-lived 4f135d6s2 (J = 2) state, are a promising platform for metrology and fundamental physics studies but have been hindered by inefficient state detection. We overcome this critical bottleneck by identifying the 593 nm transition as the optimal single-laser repumping pathway and developing a corresponding high-performance laser. A Pound–Drever–Hall lock to a 10-cm Fabry–Pérot cavity, with active drift cancellation via an ultra-stable 578 nm reference, achieves an instability <2.0 × 10−15 at 1000 s. Spectroscopy on the 431 nm inner-shell clock transitions using this laser yields a repumping efficiency of 93(5)%, confirming our theoretical prediction of ∼94% and demonstrating a decisive advance over prior low-efficiency or complex multi-laser approaches. This achievement resolves a key obstacle, enabling the realization of functional, ultra-stable Yb inner-shell optical lattice clocks.