English: An example of a simple system that would exhibit the bulk photovoltaic effect. There are two electronic levels per unit cell, separated by a large energy gap, say 3eV. The blue arrows indicate radiative transitions, i.e. an electron can absorb a UV photon to go from A to B, or it can emit a UV photon to go from B to A. The purple arrows indicate nonradiative transitions, i.e. an electron can go from B to C by emitting many phonons, or can go from C to B by absorbing many phonons. When light is shining, an electron will occasionally move right by absorbing a photon and going from A to B to C. However, it will almost never move in the reverse direction, C to B to A, because the transition from C to B requires an improbably large thermal fluctuation. Therefore, there is a net rightward photocurrent.