  We report observation of the electroweak production of single top quarks in
ppbar collisions at sqrt(s) = 1.96 TeV based on 2.3 fb^-1 of data collected by
the D0 detector at the Fermilab Tevatron Collider. Using events containing an
isolated electron or muon and missing transverse energy, together with jets
originating from the fragmentation of b quarks, we measure a cross section of
sigma(ppbar -> tb + X, tqb + X) = 3.94 +- 0.88 pb. The probability to measure a
cross section at this value or higher in the absence of signal is 2.5 x 10^-7,
corresponding to a 5.0 standard deviation significance for the observation.
