Confined free-electrons in an applied electric field: Discontinuous   electron density
  We consider free electrons in rectangular quantum dots, with either hard wall boundary conditions or anharmonic confinement. In both cases, due to finite size effects, a homogeneous electric field applied along one of the rectangular axis is shown to induce abrupt changes in the electron density, parallel and perpendicularly to the field direction: the electron density jumps from one configuration to another. Making use of this property, we propose a purely electrical mechanism to control the magnitude of the effective exchange coupling between two quantum dots. This system has been proposed recently as a quantum gate for quantum computation. 
Stephane Pleutin