  We present Neutron Star Interior Composition Explorer X-ray and Arcminute
Microkelvin Imager Large Array radio observations of a rapid hard-to-soft state
transition in the black hole X-ray transient MAXI J1820+070. During the
transition from the hard state to the soft state a switch between two
particular types of quasiperiodic oscillations (QPOs) was seen in the X-ray
power density spectra, from type-C to type-B, along with a drop in the strength
of the broadband X-ray variability and a brief flare in the 7-12 keV band. Soon
after this switch (~1.5-2.5 hr) a strong radio flare was observed that
corresponded to the launch of superluminal ejecta. Although hints of a
connection between QPO transitions and radio flares have been seen in other
black hole X-ray transients, our observations constitute the strongest
observational evidence to date for a link between the appearance of type-B QPOs
and the launch of discrete jet ejections.
