Figure 1: (a) Energy spectrum of a clean integer quantum Hall system as a function of guiding center position $X$. States near the edges disperse and carry current. By filling them to different Fermi energies $E_F^{(L)}$ and $E_F^{(R)}$, a net current flows down the sample which yields a quantized Hall resistance $R_H$. (b) Interpretation in terms of classical orbits: In bulk, electrons orbit around a fixed position and do not carry current. At the edges specular reflection induces current-carrying motion.