Figure 1: (a) Multiple scattering of a plane wave with a wave vector ${\mathbf{\text{k}}}_i$ at points $1$, $2$, …, $n-1$, $n$ changes its wave vector to ${\mathbf{\text{k}}}_f$. The angular distribution of multiply scattered intensity $I({\mathbf{\text{k}}}_f)$ results from the interference of waves scattered along all possible paths. It is a random function of ${\mathbf{\text{k}}}_f$ because the phases of different waves differ by more that $2\pi$ and hence can be considered random. This random intensity distribution is called a “speckle pattern.” (b) When the speckle pattern $I({\mathbf{\text{k}}}_f)$ is averaged over many realizations of the disordered potential, its random spatial structure washes out, whereas a peak builds up in the backscattering direction ${\mathbf{\text{k}}}_f=-{\mathbf{\text{k}}}_i$. This coherent backscattering (CBS) results from the constructive interference of time-reversed waves that follow the same scattering path in opposite directions.