Lighter colors represent higher energies in a contour map of the two highest valence bands in the moiré Brillouin zone of twisted bilayer graphene.

Nitrogen-vacancy (NV) centers in diamond carry spin degrees of freedom that enable their use for sensing of weak magnetic fields with high spatial resolution and under ambient conditions. This article reviews measurement technologies and identifies the physical mechanisms that limit the sensitivity with a focus on NV ensembles. Present-day NV sensors are far from theoretical limits imposed by quantum mechanics, and strategies are suggested for improvement of methods and materials.

COVID-19 has impacted many institutions and organizations around the world, disrupting the progress of research. Through this difficult time APS and the Physical Review editorial office are fully equipped and actively working to support researchers by continuing to carry out all editorial and peer-review functions and publish research in the journals as well as minimizing disruption to journal access.

We appreciate your continued effort and commitment to helping advance science, and allowing us to publish the best physics journals in the world. And we hope you, and your loved ones, are staying safe and healthy.

Many researchers now find themselves working away from their institutions and, thus, may have trouble accessing the Physical Review journals. To address this, we have been improving access via several different mechanisms. See Off-Campus Access to Physical Review for further instructions.

Opening for submissions mid-2020, PRX Quantum will be a highly selective, open access journal featuring quantum information science and technology research with an emphasis on lasting and profound impact. The journal expands on the excellence and innovation of Physical Review X (PRX).

Guest Editor David Tománek introduces a collection of papers in Physical Review Applied and Physical Review Materials on two-dimensional materials and devices, in a snapshot of the leading edge of this hot field.