Two-step spin decoherence in transition metal dichalcogenide defect qubits

Two-dimensional transition metal dichalcogenides (TMDCs) have recently emerged as promising defect qubit hosts due to their low nuclear spin density. Millisecond-scale spin coherence times were predicted for ideal model qubit systems under magnetic field strengths of a few teslas, where the magnetic noise due to the dilute nuclear spin bath was significantly suppressed. However, the effects of the specific defect structure and magnetic fields remain unexplored. Combining hybrid density functional theory and cluster correlation expansion, we investigate the spin decoherence of specific defect qubit candidates, which are a carbon radical ion (S = 1/2) and an antisite defect (S = 1), under varying magnetic field strengths. A key finding is that a spin in a TMDC undergoes early collapse of coherence (ECC) within a sub-microsecond timescale depending on the defect type, host material, and magnetic field. In particular, defect spins in a Mo-based TMDC show pronounced ECC over a broad range of magnetic field strengths, leading to a clear two-step decoherence process: (1) an initial sub-microsecond (T_ECC) decay driven by single nuclear spin dynamics, followed by (2) a slower millisecond-scale decay (T₂) arising from nuclear spin flip-flop interactions. Notably, at specific magnetic field strengths, coherence associated with the slower T₂ decay is completely suppressed due to enhanced nuclear spin modulation effects, leaving ECC as the dominant decoherence mechanism. These results identify ECC as a central feature of spin decoherence in TMDCs and highlight the interplay between defect characteristics and the nuclear spin environment in determining qubit coherence.