Nitrogen-vacancy (NV) defect centers in diamond are promising solid-state magnetometers. Single centers allow for high-spatial-resolution field imaging but are limited in their magnetic field sensitivity. Using defect-center ensembles, sensitivity can be scaled with $\sqrt{N}$ when $N$ is the number of defects. In the present work, we use an ensemble of $N\sim {10}^{11}$ defect centers within an effective sensor volume of $8.5\times {10}^{-4}{\mathrm{mm}}^3$ for sensing at room temperature. By carefully eliminating noise sources and using high-quality diamonds with large NV concentrations, we demonstrate, for such sensors, a sensitivity scaling as $1/\sqrt{t}$, where $t$ is the total measurement time. The associated photon-shot-noise-limited magnetic-field sensitivity for ac signals of $f=20\mathrm{kHz}$ is $0.9\mathrm{pT}/\sqrt{\mathrm{Hz}}$. For a total measurement time of 100 s, we reach a standard deviation of about 100 fT. Further improvements using decoupling sequences and material optimization could lead to $\mathrm{fT}/\sqrt{\mathrm{Hz}}$ sensitivity.

We report the experimental realization of a hybrid quantum circuit combining a superconducting qubit and an ensemble of electronic spins. The qubit, of the transmon type, is coherently coupled to the spin ensemble consisting of nitrogen-vacancy centers in a diamond crystal via a frequency-tunable superconducting resonator acting as a quantum bus. Using this circuit, we prepare a superposition of the qubit states that we store into collective excitations of the spin ensemble and retrieve back into the qubit later on. These results constitute a proof of concept of spin-ensemble based quantum memory for superconducting qubits.

NMR on diamonds gets down to chemistry: Nuclear magnetic resonance (NMR) spectroscopy is immensely useful for chemical characterization, but it requires relatively large amounts of sample. Recent studies have leveraged nitrogen vacancy centers in diamond to detect NMR signals from samples of just a few cubic nanometers, but with low resolution. Aslam et al. optimized this technique to achieve a resolution of 1 part per million—sufficient to distinguish among alkyl, vinyl, and aryl protons in solution (see the Perspective by Bar-Gill and Retzker). They also demonstrated solid-state implementation and fluorine detection. Science , this issue p. 67 ; see also p. 38

The silicon-vacancy (${\mathrm{SiV}}^{-}$) color center in diamond has attracted attention because of its unique optical properties. It exhibits spectral stability and indistinguishability that facilitate efficient generation of photons capable of demonstrating quantum interference. Here we show optical initialization and readout of electronic spin in a single ${\mathrm{SiV}}^{-}$ center with a spin relaxation time of $T_1=2.4\pm 0.2\mathrm{ms}$. Coherent population trapping (CPT) is used to demonstrate coherent preparation of dark superposition states with a spin coherence time of $T_2^{\star }=35\pm 3\mathrm{ns}$. This is fundamentally limited by orbital relaxation, and an understanding of this process opens the way to extend coherence by engineering interactions with phonons. Hyperfine structure is observed in CPT measurements with the ${}^{29}\mathrm{Si}$ isotope which allows access to nuclear spin. These results establish the ${\mathrm{SiV}}^{-}$ center as a solid-state spin-photon interface.