(a) The ln–ln plots of the standard deviation of the phase distribution σ versus step number N with the initial walker state |α = 3〉, coin state and different field strengths with ɛ = (0.01,0.012,0.015,0.018) GHz, the step size in PS Δθ = 0.3. The dots represent the standard deviation of even steps up to 25 steps. (b) The ln–ln plots with various ɛ values showing the slopes of the plots which represent the values of the walker spreading speed ς.