  The electric field of light induces--in a non centrosymmetric insulator--a dc
current, quadratic in the field magnitude, and called "shift current". When
addressed from a many-electron viewpoint, the shift current has a simple
explanation and a simple formulation as well, deeply rooted in quantum
geometry. The basic formula is then specialized to the independent-electron
case, first for a disordered system in a supercell formulation, and then for a
crystalline system. In the latter case the known shift-current formula is
retrieved in a very transparent way.
