We have directly detected millimeter-wave (mm-wave) free-space superradiant emission from Rydberg states (principal quantum number $n\sim 30$) of barium atoms in single-shot experiments. We trigger the cooperative effects with a weak initial pulse and detect the time-dependent amplitude and phase of the emitted radiation with single-shot sensitivity and 20 ps time resolution. This allows measurement and shot-by-shot analysis of the statistical distribution of decay rates, time delays, and time-dependent frequency shifts. Cooperative line shifts and decay rates are observed that exceed the 250 kHz Doppler width by a factor of 20 and the 50 Hz spontaneous emission rate by a factor of ${10}^5$.

• Received 13 April 2016
• Revised 1 March 2017

DOI:https://doi.org/10.1103/PhysRevA.95.043818