Towards full quantum state control for a molecular ion in ground electronic state

Molecules possess rotational and vibrational degrees of freedom absent in atoms. The additional complexities are challenging for quantum state control but present unique opportunities in quantum science. To exploit the diversity in molecules, we are developing general capabilities for manipulating molecular ions with single quantum-state resolution (SQSR). In quantum-logic spectroscopy [1], a molecular ion and an atomic ion are held in a trap. The coupled ion motion enables cooling for the translational motion and state measurement of the molecular ion via the atomic ion [2-5]. This leads to SQSR in tracking of state dynamics in a proof-of-principle species, CaH⁺ [6]. In a cryogenic setup, we demonstrate high-fidelity state preparation and measurement (SPAM) of rotational sublevels [7]. Beyond controlling molecular rotation with an optical frequency comb (OFC) [8], we are developing a toolbox for controlling molecular vibration. With multiple teeth of an OFC simultaneously probing a CaH⁺ prepared in a pure state, we efficiently identify its vibrational transitions near 1070 and 1450 nm. The spectroscopy of the latter reaches a relative uncertainty below one part per trillion, a prelude for precise vibrational control. To access more vibrational manifolds in the electronic ground state, we are developing a dual-branch OFC. It is expected to enable excitation of numerous Raman transitions near 40 THz between adjacent vibrational levels in CaH⁺. The SPAM protocols and frequency comb-based molecular state manipulation are applicable to a broad range of molecular ion species with applications in quantum science. We are working toward applying the approach to more molecular ion species, including polyatomic ones.

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[2] F. Wolf et. al., Nature 530, 457 (2016).

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

[4] M. Sinhal et. al., Science 367, 6483 (2020).

[5] D. Holzapfel et. al., Phys. Rev. X 15, 031009 (2025).

[6] Y. Liu et. al., Science 385, 790 (2024).

[7] D. Chaffee et. al., Phys. Rev. Lett. 135, 240801 (2025).

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