A theoretical study led by SKKU researchers identifies a molybdenum-oxygen-vacancy complex in zinc oxide as a candidate for high-fidelity spin qubits operating at room temperature, potentially addressing fabrication challenges faced by diamond NV centers
An international team led by Professor Hosung Seo at Sungkyunkwan University (SKKU), working with collaborators from the University of Wisconsin-Madison and the University of Washington, has published a theoretical analysis identifying a new atomic defect in zinc oxide (ZnO) that could serve as a high-fidelity spin qubit at room temperature. The research, appearing in PRX Quantum, focuses on a molybdenum-oxygen-vacancy complex, denoted (MoZnvO)2+, which is predicted to combine optical addressability with robust spin coherence in a wide-bandgap semiconductor already used in commercial manufacturing.
Diamond nitrogen-vacancy (NV) centers have long set the standard for solid-state spin qubits, offering room-temperature operation and optical readout. However, diamond's incompatibility with standard semiconductor processing and the difficulty of growing large, high-purity crystals have limited its scalability. In contrast, ZnO can be fabricated as large-area, ultra-high-purity wafers using molecular beam epitaxy, and its dominant oxygen isotope is nearly free of nuclear spin, reducing magnetic noise that can degrade quantum coherence. Previous attempts to realize spin qubits in ZnO using shallow donor defects such as indium or gallium have been constrained by low binding energies, restricting operation to cryogenic temperatures and ultraviolet emission.
Defect Properties and Theoretical Predictions
The SKKU-led team used density functional theory (DFT) and GW calculations to systematically screen substitutional defects in ZnO, ultimately identifying the (MoZnvO)2+ complex as a promising candidate. The defect is predicted to host a localized spin-triplet ground state, with three key properties relevant for quantum information applications: a low Huang-Rhys factor (∼5), indicating minimal phonon coupling and a sharp zero-phonon line in the visible spectrum; a spin coherence time (T2) of approximately 4 milliseconds, limited mainly by paramagnetic impurities rather than intrinsic lattice noise; and a defect geometry that supports high-fidelity, single-shot optical readout via strong spin-orbit coupling and suppressed Jahn-Teller distortion.
These theoretical results suggest that (MoZnvO)2+ could enable optically addressable spin qubits that operate at room temperature, with performance metrics comparable to or exceeding those of diamond NV centers. The predicted millisecond-scale coherence time is particularly notable, as it would allow for quantum error correction protocols and practical quantum networking if realized in hardware. However, the findings are based on first-principles calculations, and experimental confirmation will be required to validate the defect's formation, stability, and quantum properties under realistic fabrication and measurement conditions.
Manufacturing and Integration Challenges
One of the central motivations for exploring ZnO-based spin qubits is the potential for integration with established semiconductor manufacturing processes. Unlike diamond, ZnO is compatible with standard CMOS workflows and can be grown as large, uniform wafers. The material's low abundance of nuclear spins further reduces decoherence, a persistent challenge in many solid-state qubit platforms. If the (MoZnvO)2+ defect can be reliably engineered and controlled, it could provide a scalable route to quantum devices that leverage existing semiconductor infrastructure.
Despite these advantages, significant engineering challenges remain. Theoretical identification of a suitable defect does not guarantee that it can be created with high yield, controlled placement, or reproducible properties at scale. Device-to-device variability, defect clustering, and unintentional doping could all impact performance. Furthermore, the optical and spin properties predicted by simulation must be confirmed in fabricated devices, and the impact of real-world noise sources-including charge fluctuations, strain, and interface effects-must be quantified. These issues have also been encountered in other solid-state qubit platforms, as seen in efforts to integrate trapped-ion and superconducting qubits with classical hardware, such as the hybrid quantum-classical architectures described in recent work on quantum-classical supercomputing integration.
Scientific and Technological Implications
If experimentally realized, a room-temperature spin qubit in ZnO could accelerate the development of integrated quantum networks and solid-state quantum sensors. The ability to fabricate large numbers of identical, optically addressable qubits on a semiconductor wafer would address a major bottleneck in scaling quantum hardware beyond laboratory prototypes. However, the transition from theoretical proposal to practical device will require advances in defect engineering, materials characterization, and quantum control. Independent replication and benchmarking against established platforms such as diamond NV centers and silicon spin qubits will be essential to establish the true potential of ZnO-based quantum devices.
The research highlights the importance of combining computational materials discovery with experimental validation in the search for scalable quantum technologies. As the field moves beyond proof-of-principle demonstrations, the ability to integrate quantum functionality into standard semiconductor processes will be a decisive factor in determining which platforms achieve practical utility.
Spin qubits are quantum two-level systems based on the spin state of an electron or nucleus, which can be manipulated and measured to encode quantum information. The performance of a spin qubit depends on its coherence time-the duration over which quantum superpositions are preserved-and the fidelity of state preparation and measurement. In solid-state systems, decoherence arises from interactions with the surrounding lattice, nuclear spins, and environmental noise. Materials with low nuclear-spin abundance, such as isotopically purified diamond or ZnO, can suppress one major source of decoherence. However, engineering defects with the right combination of optical addressability, spin coherence, and integration compatibility remains a central challenge for scalable quantum hardware.
