Spin defects in solids are quantum objects — localized electronic states with spin degrees of freedom that can be initialized, manipulated, and read out. The nitrogen-vacancy center in diamond is the prototype: a nitrogen atom next to a missing carbon atom creates a spin-1 system that can be individually addressed with lasers and microwaves. The NV center works because diamond is a wide-bandgap insulator that isolates the defect state from the conduction electrons, and because the diamond lattice is rigid and chemically inert, protecting the spin coherence.
Au-Yeung, Huang, and collaborators (arXiv 2602.22301, February 2026) demonstrate single-spin quantum control of defects in monolayer molybdenum disulfide — a two-dimensional semiconductor one atomic layer thick.
The approach combines scanning tunneling microscopy with electron spin resonance. The STM tip serves three roles: it creates the defects by removing individual sulfur atoms or implanting carbon substituents; it reads the spin state through spin-polarized tunneling; and it drives coherent spin manipulation through microwave-frequency voltage applied to the tip. All three operations — creation, control, measurement — occur within one instrument, at the single-defect level, with atomic spatial resolution.
The two-dimensionality is the key advantage. In diamond, the NV center sits inside a three-dimensional lattice. Reaching it with an STM tip is impossible — the defect is buried beneath the surface. In monolayer MoS2, every atom is a surface atom. The defect is fully exposed to the tip. There is no bulk to shield the defect from the probe, and no bulk to protect the probe from the defect. The entire system is accessible.
The authors engineer spin-spin interactions between defect pairs by creating two vacancies at controlled separations and measuring the coupling between their spin states. The coupling depends on the inter-defect distance with atomic precision — moving the defects by one lattice site changes the interaction strength measurably. This controlled coupling is a prerequisite for multi-qubit operations: two spins that can be individually addressed and controllably coupled form the minimal unit for quantum information processing.
The spin coherence in monolayer MoS2 is shorter than in diamond — the two-dimensional semiconductor has more noise sources (substrate phonons, charge fluctuations, nuclear spins of neighboring molybdenum atoms) than the pristine diamond lattice. But the trade-off is access: every operation that requires reaching the defect — creation, coupling, local manipulation — is simpler when the defect is on the surface rather than inside a crystal. The NV center has better coherence but worse controllability. The MoS2 defect has worse coherence but complete accessibility.
The result establishes a new class of solid-state quantum system. Not engineered quantum dots, not implanted ions, not grown heterostructures — but missing atoms in a single-layer material, positioned with atomic precision by a scanning probe, manipulated by the same probe, and read out by the same probe. The vacancy is the qubit. The tip is everything else.