  We present multi-wavelength fast timing observations of the black hole X-ray
binary MAXI J1820+070 (ASASSN-18ey), taken with the Karl G. Jansky Very Large
Array (VLA), Atacama Large Millimeter/Sub-Millimeter Array (ALMA), Very Large
Telescope (VLT), New Technology Telescope (NTT), Neutron Star Interior
Composition Explorer (NICER), and XMM-Newton. Our data set simultaneously
samples ten different electromagnetic bands (radio - X-ray) over a 7-hour
period during the hard state of the 2018-2019 outburst. The emission we observe
is highly variable, displaying multiple rapid flaring episodes. To characterize
the variability properties in our data, we implemented a combination of
cross-correlation and Fourier analyses. We find that the emission is highly
correlated between different bands, measuring time-lags ranging from hundreds
of milliseconds between the X-ray/optical bands to minutes between the
radio/sub-mm bands. Our Fourier analysis also revealed, for the first time in a
black hole X-ray binary, an evolving power spectral shape with electromagnetic
frequency. Through modelling these variability properties, we find that MAXI
J1820+070 launches a highly relativistic ($\Gamma=6.81^{+1.06}_{-1.15}$) and
confined ($\phi=0.45^{+0.13}_{-0.11}$ deg) jet, which is carrying a significant
amount of power away from the system (equivalent to $\sim0.6 \, L_{1-100{\rm
keV}}$). We additionally place constraints on the jet composition and magnetic
field strength in the innermost jet base region. Overall, this work
demonstrates that time-domain analysis is a powerful diagnostic tool for
probing jet physics, where we can accurately measure jet properties with
time-domain measurements alone.
