Figure 1. Overview of single-molecule MRI using a quantum spin probe.

(a) The set-up consists of a controllable electronic probe situated 2–4 nm below the surface of the substrate, above which a molecule is positioned. The probe acts as both sensor and gradient field source for the spatial-frequency encoding of the nuclear spins in the target molecule. The equipotential slices (frequency, ω_S) of the probe's coupling field gradient have a characteristic dipole–dipole lobe shape, and can be spatially rotated by varying the direction (θ_B0, ϕ_B0) of the background magnetic field B₀ (magnitude of order 1-2 T). (b) Initially, the NMR spectrum of the molecule is broadened by the numerous nuclear dipole–dipole interactions (blue lines). The protocol decouples nuclei from each other in the presence of the coupling gradient field of the probe, and resonant excitation of target spins at ω_S provides a spatial MRI signal encoded on the probe state. In the spectrum obtained by sweeping the excitation frequency, the peak amplitudes characterize the spin density over the corresponding probe coupling slice. (c) High-level schematic of the interleaved control protocol structure consisting of a spin-echo sequence on the probe spin, and the slice-selective controlled nuclear spin rotation (excitation) embedded into a nuclear decoupling sequence. (d)The target molecule's nuclear density is sampled for multiple orientations of the interaction slices, followed by transformation from dipolar-slice space to cartesian space, to produce a 3D nuclear spin density image of ¹H atoms, or other non-zero nuclear spin species such as ¹³C. Atomic positions are directly extracted from the density image data.