Broadband optical cooling of molecular rotors from room temperature to the ground state

Laser cycling of resonances can remove entropy from a system via spontaneously emitted photons, with electronic resonances providing the fastest cooling timescales because of their rapid spontaneous relaxation. Although atoms are routinely laser-cooled, even simple molecules pose two interrelated challenges for cooling: every populated rotational–vibrational state requires a different laser frequency, and electronic relaxation generally excites vibrations. Here we cool trapped AlH+ molecules to their ground rotational–vibrational quantum state using an electronically exciting broadband laser to simultaneously drive cooling resonances from many different rotational levels. Undesired vibrational excitation is avoided because of vibrational–electronic decoupling in AlH+. We demonstrate rotational cooling on the 140(20) ms timescale from room temperature to , with the ground-state population increasing from ~3 to . This cooling technique could be applied to several other neutral and charged molecular species useful for quantum information processing, ultracold chemistry applications and precision tests of fundamental symmetries.