Molecules at ultralow temperatures represent an exciting new frontier for atomic, molecular, and optical physics; that frontier is endowed with a strong interdisciplinary character and connections to other scientific fields, including chemistry, quantum information, condensed-matter physics, and astrophysics.1 The connections, and many possibilities for technological advances, arise naturally, as molecules are the ubiquitous building blocks of materials. Control of molecular interactions has thus been an outstanding scientific quest for generations.
In the past three decades, researchers have been tremendously successful in creating ultracold atomic gases, in which atoms brought to a near standstill exhibit striking quantum behaviors. Bringing molecules into the ultracold regime, in which interaction dynamics are fully quantum mechanical, opens up exciting new opportunities. Reaching the ultracold regime with molecules has long been hindered by the many molecular internal degrees of freedom, such as vibrational and rotational levels, fine and hyperfine structure, and symmetry-breaking doublets, but the...