Frequency-comb spectroscopy on pure quantum states of a single molecular ion

Abstract

Spectroscopy is a powerful tool for studying molecular properties and is commonly performed on large thermal ensembles of molecules that are perturbed by motional shifts and interactions with the environment and one another, resulting in convoluted spectra and limited resolution. Here, we use generally applicable quantum-logic techniques to prepare a single trapped molecular ion in a pure quantum state, drive terahertz rotational transitions with an optical frequency comb, and read out the final state non-destructively, leaving the molecule ready for further manipulation. We resolve rotational transitions to 11 significant digits and derive the rotational constant of 40CaH+ to be BR= 142 501 777.9(1.7) kHz. Our approach suits a wide range of molecular ions, including polyatomics and species relevant for tests of fundamental physics, chemistry, and astrophysics.