Genome editing with a Cas9 scalpel: The Cas9 nuclease forms the heart of the CRISPR-Cas genome editing system. Cas9 binds small guide RNAs that direct it to its target sites, where the nuclease either cleaves or binds to genomic DNA. Knight et al. used single-molecule imaging to track Cas9 in living cells. Cas9 searches the genome for its target sites using rapid threedimensional diffusion. It spends very little time binding to off-target sites, which explains the high accuracy of the CRISPRCas9 editing machine. Science , this issue p. 823

Deviations from Brownian motion leading to anomalous diffusion are found in transport dynamics from quantum physics to life sciences. The characterization of anomalous diffusion from the measurement of an individual trajectory is a challenging task, which traditionally relies on calculating the trajectory mean squared displacement. However, this approach breaks down for cases of practical interest, e.g., short or noisy trajectories, heterogeneous behaviour, or non-ergodic processes. Recently, several new approaches have been proposed, mostly building on the ongoing machine-learning revolution. To perform an objective comparison of methods, we gathered the community and organized an open competition, the Anomalous Diffusion challenge (AnDi). Participating teams applied their algorithms to a commonly-defined dataset including diverse conditions. Although no single method performed best across all scenarios, machine-learning-based approaches achieved superior performance for all tasks. The discussion of the challenge results provides practical advice for users and a benchmark for developers.

Phase retarders usually present a strong frequency dependence. We discuss the design and characterization of a terahertz achromatic quarter-wave plate. This wave plate is made from six birefringent quartz plates precisely designed and stacked together. Phase retardation has been measured over the whole terahertz range by terahertz polarimetry. This achromatic wave plate demonstrates a huge frequency bandwidth $({\nu }_{\max }∕{\nu }_{\min }\approx 7)$, and therefore can be applied to terahertz time domain spectroscopy and polarimetry.