The boron-vacancy spin defect (V − B) in hexagonal boron nitride (hBN) has a great potential as a quantum sensor in a two-dimensional material that can directly probe various external perturbations in atomic-scale proximity to the quantum sensing layer. Here, we apply first principles calculations to determine the coupling of the V − B electronic spin to strain and electric fields. Our work unravels the interplay between local piezoelectric and elastic effects contributing to the final response to the electric fields. The theoretical predictions are then used to analyse optically detected magnetic resonance (ODMR) spectra recorded on hBN crystals containing different densities of V − B centres. We prove that the orthorhombic zero-field splitting parameter results from local electric fields produced by surrounding charge defects. By providing calculations of the spin-strain and spin-electric field couplings, this work paves the way towards applications of V − B centres for quantitative electric field imaging and quantum sensing under pressure.

Despite the considerable interest for antiferromagnets that appeared with the perspective of using them for spintronics, their experimental study, including the imaging of antiferromagnetic textures, remains a challenge. To address this issue, quantum sensors, and, in particular, the nitrogen-vacancy (NV) defects in diamond have become a widespread technical solution. We review here the recent applications of single NV centers to study a large variety of antiferromagnetic materials, from quantitative imaging of antiferromagnetic domains and non-collinear states, to the detection of spin waves confined in antiferromagnetic textures and the non-perturbative measurement of spin transport properties. We conclude with recent developments improving further the magnetic sensitivity of scanning NV microscopy, opening the way to detailed investigations of the internal texture of antiferromagnetic objects.

In the room-temperature magnetoelectric multiferroic, BiFeO3, the non-collinear antiferromagnetic state is coupled to the ferroelectric order, opening applications for low-power electric-field-controlled magnetic devices. While several strategies have been explored to simplify the ferroelectric landscape, here we directly stabilize a single domain ferroelectric and spin cycloid state in epitaxial BiFeO3(111) thin films grown on orthorhombic DyScO3(011). Comparing with films grown on SrTiO3(111), we identify anisotropic in-plane strain as a powerful handle to tailor the single antiferromagnetic state. In this single domain multiferroic state, we establish the thickness limit of the coexisting electric and magnetic orders and directly visualize the suppression of the spin cycloid induced by the magnetoelectric interaction below the ultrathin limit of 1.4 nanometers. This as-grown single domain multiferroic configuration in BiFeO3 thin films opens an avenue both for fundamental investigations and for electrically-controlled non-collinear antiferromagnetic spintronics.

We employ the electronic spin of a single nitrogen-vacancy (NV) defect in diamond to detect and control the quantum state of remote nuclear spins coupled by hyperfine interaction. More precisely, our work focuses on individual 13C nuclei featuring a moderate hyperfine coupling strength (∼ 1 MHz) with the NV’s electron spin. Two different methods providing an efficient room-temperature polarization of these peculiar 13C nuclear spins are described. The first one is based on a polarization transfer from the NV electron spin to the 13C nucleus, which is mediated by the anisotropic component of the hyperfine interaction. The second one relies on coherent population trapping (CPT) within a Λ-type energy-level configuration in the microwave domain, which enables to initialize the 13C nuclear spin in any quantum state superposition on the Bloch sphere. This CPT protocol is performed in an unusual regime for which relaxation from the excited level of the Λ-scheme is externally triggered by optical pumping and separated in time from coherent microwave excitations. For these two polarization techniques, we investigate the impact of optical illumination on the nuclear spin polarization efficiency. This work adds new methods to the quantum toolbox used for coherent control of individual nuclear spins in diamond, which might find applications in quantum metrology.