Development of a Frequency-Locking Optical System for Metastable-State Control in Trapped Barium Ions*

Trapped atomic ions are one of the most promising approaches to realizing quantum computers. One active topic in trapped-ion quantum information research focuses on the so-called “omg” architecture which utilizes a combination of optical, metastable, and ground-state qubits, where qubit states are encoded in the internal electronic states of each ion. Among the ion species available, barium ions offer several advantages for implementing omg schemes, such as long-lived metastable states and isotopes with favorable values of nuclear spin. The barium ion has a key transition of relevance to the omg architecture, between the S_1/2 ground state and D_5/2 state, that occurs at a wavelength of 1762 nm. Due to the long lifetime of the D_5/2 metastable state, a laser with very narrow linewidth is needed to drive the transition with precise control and achieve long coherence times. We demonstrate an optical system to lock a 1762 nm laser to an ultra-high finesse cavity by utilizing the Pound-Drever-Hall (PDH) technique. Experimental results from single-ion spectroscopy validate the achievement of the desired narrow linewidth, thus paving the way for quasi-dual-species operation within a chain of identical ions and potentially eliminating challenges in the manipulation of dual-species chains of ions.