Path distinguishability in double scattering of light by atoms

Wave-particle duality finds a natural application for electrons or light propagating in disordered media where coherent corrections to transport are given by two-wave interference. For scatterers with internal degrees of freedom, these corrections are observed to be much smaller than would be expected for structureless scatterers. By examining the basic example of the scattering of one photon by two spin-1/2 atoms—a case study for coherent backscattering—we demonstrate that the loss of interference strength is associated with which-path information stored by the scattering atoms.

Figure 1

(a) The two paths in coherent backscattering. Along path $A$ the photon is first scattered by atom 1, then by atom 2; along path $B$ the order is reversed. (b) Level scheme of the $1∕2\leftrightarrow 1∕2$ transition. Both the ground state and the excited state are doublets with total angular momentum $j=1∕2$, and the magnetic quantum numbers $m=\pm 1∕2$ label the sublevels.