(a) Schematic of the conversion from a charge current to a spin current. (b) The electron-electron repulsion changes the flow of the charge current. The areas shaded in red in (a) and yellow in (b) indicate the change of electron density by the correlation effect. Although the electron-electron interaction depends on the material, a similar change of the electron density could be induced by an electric field and/or light. Therefore, the charge current converted from the spin current will be controlled by an electric field and/or light. The blue circles with arrows denote electrons, wherein circles and arrows indicate charge and spin, respectively. A magnet with N and S poles is a suitable analogy for spin.