  Atom interferometry offers new perspectives for geophysics and inertial
sensing. We present the industrial prototype of a new type of quantum-based
instrument: a compact, transportable, differential quantum gravimeter capable
of measuring simultaneously the absolute values of both gravitational
acceleration, $g$, and its vertical gradient, $\Gamma_{zz}$. While the
sensitivity to g is competitive with the best industrial gravimeters, the
sensitivity on $\Gamma_{zz}$ reaches the limit set by quantum projection
noise-leading to an unprecedented long-term stability of 0.1 E ($1E=1\times
10^{-9}s^{-2}$). This unique, dual-purpose instrument, paves the way for new
applications in geophysics, civil engineering, and gravity-aided navigation,
where accurate mapping of the gravitational field plays an important role.
