  Terahertz (THz) technologies, generally defined as operating in the 0.1-10THz
range, bridge the gap between electronic and photonic devices. Because THz
radiation passes readily through materials such as plastics, paper and cloth it
can be employed in non-destructive testing, and as it is non-ionising it is
considered safe for security and biomedical applications. There is significant
demand for high speed THz imaging across a range of applications but, despite
ongoing efforts, fast full-field imaging remains an unfulfilled goal. Here we
demonstrate a THz imaging system based upon efficient THz-to-optical conversion
in atomic vapour, where full-field images can be collected at ultra-high speeds
using conventional optical camera technology. For a 0.55 THz field we show an
effective 1 cm $^{2}$ sensor with near diffraction-limited spatial resolution
and a minimum detectable power of 190 $\pm$ 30 fW s$^{-1/2}$ per 40x40${\mu}$m
pixel capable of video capture at 3000 frames per second. This combination of
speed and sensitivity represents a step change in the state of the art of THz
imaging, and will likely lead to its uptake in wider industrial settings. With
further improvements we expect that even higher frame rates of up to 1 MHz
would be possible.
