Precision frequency-comb vibrational spectroscopy of a single molecular ion*

The ability to coherently control molecular degrees of freedom can open many new opportunities for applications, including investigating elementary chemical reaction mechanisms [1], testing fundamental physics [2], and transducing quantum information between qubits on different platforms [3]. Quantum-logic spectroscopy (QLS) offers a method to study and manipulate many species of molecular ions with high precision [4]. Here, we report our ongoing effort toward precision spectroscopy of vibrational transitions in a single CaH⁺ ion by using QLS. Starting with a CaH⁺ efficiently prepared in a single quantum state [5], we use infrared frequency combs to coherently drive, efficiently search for, and characterize narrow vibrational transitions in CaH⁺. We have characterized v = 0 -> 5 overtone transitions with single quantum-state resolution at ~1450 nm, reaching below one part in 10¹³ statistical uncertainty in their frequencies. We have also driven v = 0 -> 7 overtone transitions near 1070 nm and are currently in the process of accurately determining their unperturbed frequencies. In addition to reaching vibrational excited states via one-photon transitions, we are developing two branches of broadband frequency combs to drive numerous two-photon Raman transitions between adjacent vibrational states in CaH⁺. With sufficient bandwidth in each branch, many comb teeth pairs with correct frequency differences can contribute constructively to collectively drive the transitions [6]. This promises versatile control over molecular vibrations, and can serve as a proof of principle for other molecular ion species.

[1] S. Haze et al., Nat. Phys. 21.228 (2025)

[2] T. Roussy et al., Science 381, 46-50 (2023)

[3] Y. Lin et al., Nature 581, 273 (2020)

[4] C. -W. Chou et al., Nature 545, 203 (2017)

[5] Y. Liu et al., Science 385, 790-795 (2024)

[6] C. -W. Chou et al., Science 367, 6485 (2020)