We demonstrate diamond-based quantum imaging of the current flow in graphene structures with submicrometer resolution.

Stray magnetic fields contain significant information about the electronic and magnetic properties of condensed-matter systems. For two-dimensional (2D) systems, stray field measurements can even allow full determination of the source quantity. For instance, a 2D map of the stray magnetic field can be uniquely transformed into the 2D current density that gives rise to the field and, under some conditions, into the equivalent 2D magnetization. However, implementing these transformations typically requires truncation of the initial data and involves singularities that may introduce errors, artefacts, and amplify noise. Here we investigate the possibility of mitigating these issues through vector measurements. For each scenario (current reconstruction and magnetization reconstruction) the different possible reconstruction pathways are analyzed and their performances compared. In particular, we find that the simultaneous measurement of both in-plane components ($B_x$ and $B_y$) enables near-ideal reconstruction of the current density, without singularity or truncation artefacts, which constitutes a significant improvement over reconstruction based on a single component (e.g., $B_z$). On the other hand, for magnetization reconstruction, a single measurement of the out-of-plane field ($B_z$) is generally the best choice, regardless of the magnetization direction. We verify these findings experimentally using nitrogen-vacancy-center magnetometry in the case of a 2D current density and a 2D magnet with perpendicular magnetization.

Controlling and manipulating individual quantum systems in solids underpins the growing interest in the development of scalable quantum technologies. Recently, hexagonal boron nitride (hBN) has garnered significant attention in quantum photonic applications due to its ability to host optically stable quantum emitters. However, the large bandgap of hBN and the lack of efficient doping inhibits electrical triggering and limits opportunities to study the electrical control of emitters. Here, we show an approach to electrically modulate quantum emitters in an hBN-graphene van der Waals heterostructure. We show that quantum emitters in hBN can be reversibly activated and modulated by applying a bias across the device. Notably, a significant number of quantum emitters are intrinsically dark and become optically active at non-zero voltages. To explain the results, we provide a heuristic electrostatic model of this unique behavior. Finally, employing these devices we demonstrate a nearly-coherent source with linewidths of ~160 MHz. Our results enhance the potential of hBN for tunable solid-state quantum emitters for the growing field of quantum information science.

The application of imaging techniques based on ensembles of nitrogen-vacancy ($\mathrm{N}$-$V$) sensors in diamond to characterize electrical devices has been proposed, but the compatibility of $\mathrm{N}$-$V$ sensing with operational gated devices remains largely unexplored. Here we report the fabrication of graphene field-effect transistors directly on the diamond surface and their characterization by $\mathrm{N}$-$V$ microscopy. The current density within the gated graphene is reconstructed from $\mathrm{N}$-$V$-magnetometry measurements under both mostly $p$- and $n$-type doping, but the exact doping level is found to be affected by the measurements. Additionally, we observe a surprisingly large modulation of the electric field at the diamond surface under an applied gate potential, seen in $\mathrm{N}$-$V$-photoluminescence and $\mathrm{N}$-$V$-electrometry measurements, suggesting a complex electrostatic response of the oxide-graphene-diamond structure. Possible solutions to mitigate these effects are discussed.

Magnetic imaging with ensembles of nitrogen-vacancy (NV) centres in diamond is a recently developed technique that allows for quantitative vector field mapping. Here we uncover a source of artefacts in the measured magnetic field in situations where the magnetic sample is placed in close proximity (a few tens of nm) to the NV sensing layer. Using magnetic nanoparticles as a test sample, we find that the measured field deviates significantly from the calculated field, in shape, amplitude and even in sign. By modelling the full measurement process, we show that these discrepancies are caused by the limited measurement range of NV sensors combined with the finite spatial resolution of the optical readout. We numerically investigate the role of the stand-off distance to identify an artefact-free regime, and discuss an application to ultrathin materials. This work provides a guide to predict and mitigate proximity-induced artefacts that can arise in NV-based wide-field magnetic imaging, and also demonstrates that the sensitivity of these artefacts to the sample can make them a useful tool for magnetic characterisation.