Precision vibrational spectroscopy of a single ⁴⁰CaH⁺ molecular ion

The precise study of molecules by direct manipulation of their quantum state is of interest in multiple fields, providing stringent tests for fundamental physics and chemistry [1]. In particular, the characterization of vibrational transitions in molecules could find applications in quantum information processing due to their ability to convert quantum information across a wide range of frequencies [2]. Quantum-logic spectroscopy (QLS), used here in a ⁴⁰CaH⁺ – ⁴⁰Ca⁺ crystal, provides a general method for molecular state control, since it demands few requirements on the molecular level structure and allows for sympathetic cooling of translational motion, pure quantum-state preparation, and non-destructive state readout of the molecule [1, 3, 4]. In this contribution, we will discuss the progress made towards a refined investigation and control of the vibrational states of ⁴⁰CaH⁺. Firstly, we have performed direct-comb precision spectroscopy on the v (vibrational quantum number) = 0 → 5 overtone transition at ~1450 nm within the electronic ground state of ⁴⁰CaH⁺ and are working on evaluating the accuracy of the results. Accurately determining the frequencies of additional overtone transitions will further improve the characterization of the vibrational structure of ⁴⁰CaH⁺. Besides our ongoing efforts on the v = 0 → 7 transition at ~1066 nm, we are developing a more general method to coherently excite vibrational transitions with │Δv│ = 1 . We will use a dual-branch frequency comb to connect adjacent vibrational levels via a Raman process. The dispersion and coherence between the two branches need to be precisely controlled to achieve constructive interference among all participating comb teeth.

[1] Y. Liu et al., Science 385, 790 (2024) and references therein.

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

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

[4] C. -W. Chou et al., Science 367, 1458 (2020).