Proton-electron mass ratio from laser spectroscopy of HD+ at the part-per-trillion level

The value of the ratio of the masses of the proton and the electron has a bearing on the values of other physical constants. This ratio is known to a very high precision. Patra et al. improved this precision even further by measuring particular frequencies in the rovibrational spectrum of the hydrogen deuteride molecular ion (HD+) (see the Perspective by Hori). To reach this high precision, the researchers placed the HD+ molecules in an ion trap and surrounded them by beryllium ions. The cold beryllium ions then helped cool the HD+ molecules, making the HD+ spectral lines narrow enough that the proton-electron mass ratio could be extracted by comparison with theoretical predictions.