  Singly ionized nitrogen molecules in ambient air pumped by near-infrared
femtosecond laser give rise to superradiant emission. Here we demonstrate
coherent control of this superradiance by injecting a pair of resonant seeding
pulses inside the nitrogen gas plasma. Strong modulation of the 391.4 nm
superradiance with a period of 1.3 fs is observed when the delay between the
two seeding pulses are finely tuned, pinpointing the essential role of
macroscopic coherence in this lasing process. Based on this time-resolved
method, the complex temporal evolution of the macroscopic coherence between two
involved energy levels has been experimentally revealed, which is found to last
for around 10 picoseconds in the low gas pressure range. These observations
provide a new level of control on the "air lasing" based on nitrogen ions,
which can find potential applications in optical remote sensing.
