Precision vibrational overtone spectroscopy of a single molecular ion

Precision spectroscopy of molecules unravels their complex level structures and is foundational for many relevant fields, including quantum science and cold chemistry. It also offers new opportunities in precision measurements, such as measuring the electron electric dipole moment, observing parity violation in chiral systems, and exploring possible variation of fundamental constants. Using quantum-logic spectroscopy (QLS), many species of single molecular ions can be studied with high precision free of adverse effects due to ensemble averaging. Here, we report a precision spectroscopy of vibrational transitions in a single CaH⁺ ion using QLS. Co-trapped with a ⁴⁰Ca⁺, the CaH⁺ is efficiently prepared in a single initial quantum state in low-lying rotation manifolds [1, 2]. Using a spectrally broadened infrared frequency comb, we searched for and characterized the v = 0 -> 5 overtone transitions at ~1450 nm, reaching below part per 10¹³ statistical uncertainty in their frequencies . This work presents an efficient method for searching and probing narrow molecular transitions, and thus obtaining control of various molecular degrees of freedom suitable for diverse applications. Our demonstration of the coherent addressing of vibrational transitions in the telecom frequency regime paves the way to using a single molecule as a transducer in quantum networking applications [3].

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

[2] Liu et al., arXiv: 2312.17104 (2023)

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