  The search for extraterrestrial intelligence at radio frequencies has largely
been focused on continuous-wave narrowband signals. We demonstrate that
broadband pulsed beacons are energetically efficient compared to narrowband
beacons over longer operational timescales. Here, we report the first extensive
survey searching for such broadband pulsed beacons towards 1883 stars as a part
of the Breakthrough Listen's search for advanced intelligent life. We conducted
233 hours of deep observations across 4 to 8 GHz using the Robert C. Byrd Green
Bank Telescope and searched for three different classes of signals with
artificial (or negative) dispersion. We report a detailed search -- leveraging
a convolutional neural network classifier on high-performance GPUs -- deployed
for the very first time in a large-scale search for signals from
extraterrestrial intelligence. Due to the absence of any signal-of-interest
from our survey, we place a constraint on the existence of broadband pulsed
beacons in our solar neighborhood: $\lesssim$1 in 1000 stars have transmitter
power-densities $\gtrsim$10$^5$ W/Hz repeating $\leq$500 seconds at these
frequencies.
