McCallum works on the materials and detector physics of donor qubits in silicon and colour centres in diamond and silicon carbide: defect engineering by ion implantation and annealing, characterisation of the resulting spin coherence, and — most relevant to a sensing postdoc — the development of superconducting and semiconductor detectors capable of registering single implanted ions with near-unit efficiency, which is what turns implantation from a statistical process into a deterministic one. He also works on near-surface colour centres, where surface termination and Fermi-level control set the achievable coherence. Positioned against the established body of NV-ensemble quantum sensing work — DEER, nanoscale NMR and T1 relaxometry protocols operating at pT/sqrt(Hz) field sensitivity — his group supplies the near-surface, coherence-optimised spin ensembles that DEER, nanoscale NMR and T1-relaxometry protocols at pT/sqrt(Hz) sensitivity actually depend on.