  We present the detection of a bright radio burst at radio frequencies between
2.2--2.3 GHz with the NASA Deep Space Network (DSN) 70 m dish (DSS-63) in
Madrid, Spain from FRB~20200120E. This repeating fast radio burst (FRB) was
recently discovered by the Canadian Hydrogen Intensity Mapping Experiment Fast
Radio Burst (CHIME/FRB) instrument and reported to be associated with the M81
spiral galaxy at a distance of 3.6 Mpc. The high time resolution capabilities
of the recording system used in this observation, together with the small
amount of scattering and intrinsic brightness of the burst, allow us to explore
the burst structure in unprecedented detail. We find that the burst has a
duration of roughly 30 $\mu$s and is comprised of several narrow components
with typical separations of 2--3 $\mu$s. The narrowest component has a width of
$\lesssim$ 100 ns, which corresponds to a light travel time size as small as 30
m, the smallest associated with an FRB to date. The peak flux density of the
narrowest burst component is 270 Jy. We estimate the total spectral luminosity
of the narrowest component of the burst to be 4 $\times$ 10$^{\text{30}}$ erg
s$^{\text{-1}}$ Hz$^{\text{-1}}$, which is a factor of $\sim$500 above the
luminosities of the so-called "nanoshots" associated with giant pulses from the
Crab pulsar. This spectral luminosity is also higher than that of the radio
bursts detected from the Galactic magnetar SGR 1935+2154 during its outburst in
April 2020, but it falls on the low-end of the currently measured luminosity
distribution of extragalatic FRBs. These results provide further support for
the presence of a continuum of FRB burst luminosities.
