  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 relaxation rates. 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 3.8(+0.9/-0.3) K, with the ground state population increasing
from ~3% to 95.4(+1.3/-2.0) %. 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.
