High-resolution direct optical frequency comb Raman spectroscopy of single ions: from atomic fine structures to rotational spectra of molecular ions

Abstract

Optical frequency combs have in the recent past revolutionized the field of high-resolution spectroscopy by being applied both as frequency references and light sources for direct comb spectroscopy. With respect to the latter application, we have demonstrated the use of an optical frequency comb to coherently drive stimulated Raman transitions between terahertz-spaced atomic energy levels. Specifically, we have measured the 3d 2D3/2 - 3d 2D5/2 fine structure splitting of a single trapped 40Ca+ ion to be 1,819,599,021,534±8Hz, which is five times more accurate than previous measurements, and currently only limited by the stability of our atomic clock reference. Furthermore, Rabi oscillations with a contrast of 99.3(6)% and millisecond coherence time have been realized experimentally, indicating great potentials for future qubit applications. Importantly, the technique should generally be applicable to drive Raman transitions spanning the level spacings ranging from sub-kHz to tens of THz range, including hyperfine transitions in highly charged ions and spin-resolved rovibrational transitions in molecular ions. High-resolution spectroscopy of such systems may find applications in the search for new physics beyond the Standard Model.