State control and vibrational spectroscopy of a single molecular ion

In recent years, quantum-logic spectroscopy of single molecular ions (QLS-SMI) emerged as a novel platform for high-resolution spectroscopy, offering advantages including precise initial state preparation [1], minimal line broadening from collisional or motional effects, and long interrogation times [1,2,3]. The efficient control of the quantum state of a single molecule, however, remains difficult due to environmental perturbations, notably from thermal radiation (TR) emitted by surrounding surfaces. To mitigate this effect, we developed a QLS-based protocol to track and control the evolution of molecular state. By monitoring and reversing TR-induced quantum jumps from a rotational state with quantum number J = 1 to those with J = 0 and 2, we can confine the population in J = 1 to, on average, ~20 times its natural lifetime of 1.7 s, resulting in an improvement of our experimental duty cycle from 7% to 64%. Leveraging this improvement in state control, we performed vibrational overtone spectroscopy on CaH+ at the 10^-13 fractional uncertainty level. Using a single frequency comb source, we locate and probe two rotational lines within the v = 0 -> 5 vibrational series. We discuss the generalizability of these state control and spectroscopy techniques to other molecular species which are of astrophysical interest or suitable for various precision measurement tasks.

[1] Chou et al., Nature 545, 203 (2017) [2] Chou et al., Science 367, 6485 (2020) [3] Collopy et al., PRL 130, 223201 (2023)