  Fast radio bursts (FRBs) are extragalactic transients with typical durations
of milliseconds. FRBs have been shown, however, to fluctuate on a wide range of
timescales: some show sub-microsecond sub-bursts while others last up to a few
seconds in total. Probing FRBs on a range of timescales is crucial for
understanding their emission physics, how to detect them effectively, and how
to maximize their utility as astrophysical probes. FRB 20121102A is the
first-known repeating FRB source. Here we show that FRB 20121102A is able to
produce isolated microsecond-duration bursts whose total durations are more
than ten times shorter than all other known FRBs to date. The polarimetric
properties of these micro-bursts resemble those of the longer-lasting bursts,
suggesting a common emission mechanism producing FRBs spanning a factor of
1,000 in duration. Furthermore, this work shows that there exists a population
of ultra-fast radio bursts that current wide-field FRB searches are missing due
to insufficient time-resolution. These results indicate that FRBs occur more
frequently and with greater diversity than initially thought. This could also
influence our understanding of energy, wait time, and burst rate distributions.
