  Quantum sensors exploiting matter waves interferometry promise to realize a
new generation of Gravitational Wave detectors. The intrinsic stability of
specific atomic energy levels makes atom interferometers and clocks ideal
candidates to extend the frequency window for the observation of Gravitational
Waves in the mid-frequency band, ranging from 10 mHz to 10 Hz. We present the
geometry and functioning of this new class of ground and space detectors and
detail their main noise sources. We describe the different projects undertaken
worldwide to realize large scale demonstrators and push further the current
limitations. We finally give the roadmap for achieving the instrumental
sensitivity required to seize the scientific opportunities offered by this new
research domain.
