Two-level atom

The basic unit of information in computer science is the binary digit or bit, whose physical realization is a component capable of two stable states, say, 0) and 1). In a conventional computer, these usually correspond to charged and uncharged states of tiny capacitors within a silicon microchip. On an atomic level, bits might in concept be represented by the two orientations of an electron spin or the two polarization states of a photon. Apart from economy of scale, an atomic two-level system has a capability beyond that of a classical component, namely, the possibility of being in a coherent superposition such as... 

We now turn to the question of line shapes. Absorption bands in proteins are determined by both homogeneous and inhomogeneous effects. First consider homogeneous effects. Simple atomic two-level transitions usually are approximated by a Lorentzian line shape of the form... 

---