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Description
We have performed Doppler-free two-photon spectroscopy of cold, trapped HD$^+$ ions to measure a ro-vibrational transition frequency with a relative uncertainty of a few parts-per-trillion. Using highly precise ab-initio calculations [1], these measurements allow – for the first time – to determine the proton-electron mass ratio, $\mu$, from molecular spectroscopy with a precision competitive with that of state-of-the-art Penning trap mass measurements [2, 3]. Hence, our method provides both a new value of $\mu$ and a cross check of existing methods. In addition, our results provide an indication of the values of the proton radius and Rydberg constant, and may even serve as a probe of physics beyond the Standard Model.
[1] V.I. Korobov, J.-Ph. Karr, L. Hilico, Phys. Rev. Lett. 118, 233001 (2017).
[2] F. Heiße et al. Phys. Rev. Lett. 119, 033001 (2017).
[3] S. Sturm et al. Nature 506, 467 (2014).
