The other response functions have a similar expression as Eq. 22, with μ showing up in different places with ρ(j) . From Eq. 20, one can easily observe that, for each consecutive laser operation, one needs to spawn more trajectories due to the sum, nj ñj , and each dτ j contains additional Gaussian sampling for the mapping variables. To reduce this scaling in the number of trajectories, Provazza et al. suggested an efficient importance sampling strategy with focusing and important sampling techniques to stochastically choose one pair of labels {n j , ñj }, with details outlined below.