  An ensemble of excited atoms can synchronize emission of light collectively
in a process known as superradiance when its characteristic size is smaller
than the wavelength of emitted photons. The underlying superradiance depends
strongly on electromagnetic (photon) fields surrounding the atomic ensemble.
High mode densities of microwave photons from $300\,$K blackbody radiation
(BBR) significantly enhance decay rates of Rydberg states to neighbouring
states, enabling superradiance that is not possible with bare vacuum induced
spontaneous decay. Here we report observations of the superradiance of
ultracold Rydberg atoms embedded in a bath of room-temperature photons. The
temporal evolution of the Rydberg $|nD\rangle$ to $|(n+1)P\rangle$ superradiant
decay of Cs atoms ($n$ the principal quantum number) is measured directly in
free space. Theoretical simulations confirm the BBR enhanced superradiance in
large Rydberg ensembles. We demonstrate that the van der Waals interactions
between Rydberg atoms change the superradiant dynamics and modify the scaling
of the superradiance. In the presence of static electric fields, we find that
the superradiance becomes slow, potentially due to many-body interaction
induced dephasing. Our study provides insights into many-body dynamics of
interacting atoms coupled to thermal BBR, and might open a route to the design
of blackbody thermometry at microwave frequencies via collective, dissipative
photon-atom interactions.
