Infrared absorption spectroscopy of a single trapped polyatomic molecular ion

Quantum logic spectroscopy has proven to be a powerful tool for high-precision spectroscopic studies of diatomic molecular ions co-trapped with atomic ions in an ion trap, contributing to fundamental physics research and quantum information applications. However, extending this protocol to larger polyatomic molecular species is challenging due to their more complex internal structure and has not yet been demonstrated experimentally. Using an adapted version of the quantum logic methods, we investigate polyatomic molecular ions by probing their photon-absorption signal. This is achieved by preparing the shared motional mode of the trapped molecular and atomic ions in a non-classical state that is sensitive to the momentum recoil associated with the absorption of a single photon. We demonstrate this scheme by measuring the infrared absorption spectrum corresponding to the O–H stretching vibration of a trapped CaOH⁺ ion co-trapped with a Ca⁺ ion. The measurements are performed with broadband femtosecond laser pulses on molecular ions that are internally thermalized to room temperature. The measured spectrum shows good agreement with ab initio theoretical predictions for the transition frequency and transition dipole moment. The experiment thus showcases the potential of this method for the initial identification of transitions in complex molecular ions, and for the development of ultrafast pump–probe experiments at the single-molecule and single-photon level that facilitates the investigations of ultrafast intramolecular dynamics. With further improvements in the signal-to-noise ratio, the technique may enable quantum non-demolition measurements of molecular states and thus measurement-based state preparation of a wide range of molecular species.