  Today's best atomic clocks are limited by frequency noise on the lasers used
to interrogate the atoms. A proposed solution to this problem is to create a
superradiant laser using an optical clock transition as its gain medium. This
laser would act as an active atomic clock, and would be highly immune to the
fluctuations in reference cavity length that limit today's best lasers. Here,
we demonstrate and characterize superradiant emission from the mHz linewidth
clock transition in an ensemble of laser-cooled $^{87}$Sr atoms trapped within
a high-finesse optical cavity. We measure a collective enhancement of the
emission rate into the cavity mode by a factor of more than 10,000 compared to
independently radiating atoms. We also demonstrate a method for seeding
superradiant emission and observe interference between two independent
transitions lasing simultaneously. We use this interference to characterize the
relative spectral properties of the two lasing sub-ensembles.
