All-optical preparation of molecular ions in the rovibrational ground state

In the field of cold quantum matter, control of the motional degrees of freedom of both neutral and charged gas-phase molecules has been achieved for a wide range of species1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11. However, cooling of the internal degrees of freedom remains challenging. Recently, transfer to the internal ground state by sophisticated optical techniques has been demonstrated for neutral alkali dimers created in single quantum states from ultracold atoms12, 13, 14, 15. Here we demonstrate cooling of the rotational degree of freedom of heteronuclear diatomic molecules with a thermal distribution of internal states, using a simple, robust and general optical-pumping scheme with two low-power continuous-wave lasers. With trapped and translationally cooled hydrogen deuteride (HD+) molecular ions as a model system, we achieve 78(4)% rovibrational ground-state population. The rotationally, vibrationally and translationally cold molecular ion ensemble is suitable for a number of applications, such as generation of long-lived coherences or frequency metrology of fundamental constants16, 17.