Infrared absorption spectroscopy of a single polyatomic molecular ion

TL;DR

This work addresses the challenge of performing infrared absorption spectroscopy on a single polyatomic molecular ion by implementing recoil spectroscopy with cat-state amplification in a co-trapped two-ion crystal. A single-photon absorption event on CaOH+ imprints momentum on the shared motion, which is amplified by preparing a non-classical cat state and read out via the co-trapped Ca+ ion, enabling nondestructive detection. Applying the method to the CaOH+ O–H stretching vibration, the authors obtain a single-photon absorption spectrum whose center aligns with a high-level ab initio value around $ν_0 \approx 3783$ cm$^{-1}$ and whose width agrees with simulations, though the absolute magnitude requires improved dynamical modeling and higher pulse intensities. This work demonstrates a pathway toward quantum-nondemolition measurements of complex molecular ions and lays groundwork for high-fidelity quantum-state preparation and readout across a broad class of molecular species.

Abstract

Absorption spectroscopy is a fundamental tool for probing molecular structure. However, performing absorption spectroscopy on individual molecules is challenging due to the low signal-to-noise ratio. Here, we report on a nondestructive absorption spectroscopy on a mid-infrared vibrational transition in a single molecular ion that is co-trapped with an atomic ion. The absorption of a single photon is detected via the momentum transfer from the absorbed photon onto the molecule. This recoil signal is amplified using a non-classical state of motion of the two-ion crystal and subsequently read out via the atomic ion. We characterize the recoil detection method and use it to investigate the interaction between femtosecond laser pulses and the O-H stretching vibration in individual CaOH+ molecular ions. Furthermore, we present the single-photon absorption spectrum obtained for the vibrational transition. This method represents a milestone towards quantum non-demolition measurements of complex polyatomic molecules, providing high-fidelity methods for preparation and measurement of the quantum state of a wide range of molecular species.

Figures (5)

• Figure 1: Schematic of the system of a trapped two-ion mixed-species crystal for performing molecular spectroscopy. The system is considered here to possess three degrees of freedom: a harmonic oscillator describing the in-phase motion of the crystal, a two-level system in the atomic ion for quantum logic operations, and the intramolecular vibration.
• Figure 2: Sequence diagram of the cat state spectroscopy for detecting the photon absorption recoil on the molecule (left panels) and the evolution of the motional wavepacket of the ion crystal in phase space (right panels). The procedure consists of the following steps: initial ground state cooling of the ion motion, generation of the recoil sensitive cat state, excitation of the molecular transition, reversal of the cat state generation operation, and the detection of the photon absorption recoil on the atomic ion. This process generates entanglement between the atomic qubit and the motion of the ion crystal, such that signal of single photon absorption recoil is amplified and transformed into a geometric phase illustrated by the shaded area. This is then mapped to the state of the atomic ion which can be detected via fluorescence.
• Figure 3: Characterization of the cat state spectroscopy on an electric kick-induced displacement. (a) Measured atomic excitation signal with $|\alpha|=6.5(7)$ on the displacement produced by a single electric kick pulse of size $\eta_k = 0.0193(2)$ (blue) and the background signal with no electric pulse applied (orange) as a function of the bichromatic light phase $\phi_-$. (b) Variation of the signal with respect to cat state amplitude $|\alpha|$ for a fixed displacement. (c) The signal measured with $|\alpha|=6.5(7)$ on electric displacements of various magnitudes $\eta$. This is compared to the signal expected from a perfect direct measurement of the displacement on the motional ground state (dashed gray). The measured signal can be explained with the model described in Equation \ref{['eq:exp signal model']} indicated by the blue curve.
• Figure 4: (a) The effective single-photon absorption probability from the cat state spectroscopy measured at $\nu=3703$ with different laser pulse number $n_\text{pulse}$ (red) and the results from a Monte-Carlo simulation (purple). (b) Schematic of the pulse sequence triggered on the femtosecond laser pulse train. (c) An example of the detected atomic excitation variation in the measurement from which the peak-to-peak amplitude $\mathcal{S}$ is obtained.
• Figure 5: Absorption spectrum of the O--H stretching mode of CaOH$^+$ measured with cat state spectroscopy (red dots), along with the expected spectrum from simulation (purple dots). The value for the transition frequency from ab initio theory is indicated (purple line).