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.
