Fig. 1.

Quantum probe hyperpolarisation of nuclear spin ensembles. a Schematic of the system showing a near-surface nitrogen vacancy (NV) spin probe in diamond and a hydrogen nuclear spin target ensemble in molecular Poly(methyl methacrylate) (PMMA) on the surface. The NV probe is initialised by a green laser (532 nm), and read out via its photoluminescence (PL) signal. The shaded blue surfaces denote different regimes of polarisation capabilities arising from the spatial dependence of the nuclear spin coupling to the NV qubit. b Schematic of cross relaxation induced polarisation (CRIP) implemented on a spin system illustrating the build up of polarisation from repeated application of the CRIP sequence. Diffusion effects act in competition with the CRIP entropy pumping mechanism, but also allow for polarisation at distances beyond that reachable via the hyperfine interaction. c Energy-level diagram of the NV (left) showing the relative positions of various target nuclear spins (right) resonance conditions. d, e The control sequences (laser pulses in green, RF pulses in red) used for polarising a target spin ensemble using CRIP (d) and for controlled depolarisation using the combined CRIP⁻¹ × CRIP protocol (e). f Schematic showing the cross-relaxation spectrum obtained by measuring the PL during the CRIP (blue) or depolarisation (orange) sequence with a constant interaction time τ, while scanning the NV frequency ω_NV. g Similarly, the cross-relaxation curve is obtained by scanning τ with ω_NV set at the resonance