You shall not pass! Shielding and control of ultracold collisions

By virtue of moving slowly, molecules in microkelvin gases are extraordinarily sensitive to their long-range interactions. This sensitivity can be exploited to control the way in which the molecules interact and collide, by subjecting them to electric or microwave fields. This control includes shielding that can prevent the molecules from reacting; and “field-linked” resonances that can bind pairs of molecules non-chemically. I discuss some of the background concepts that make this manipulation possible, with reference to its recent realization in several experiments. The ultracold community is tired of hearing me talk about this, but if it’s new to you, you might learn something interesting!