image: Schematic of graphene-diamond heterostructure hybridization
Credit: ©Science China Press
Recently, researchers from the CAS Key Laboratory of Microscale Magnetic Resonance at the University of Science and Technology of China (USTC), in collaboration with Nanjing University and other institutions, have successfully achieved a 2-fold enhancement in the coherence time of shallow nitrogen-vacancy (NV) centers by graphene-diamond heterostructure hybridization. This study provides the first experimental demonstration of graphene's capability to enhance the quantum coherence time of shallow NV center spins, and clarifies the underlying physical mechanism. This work will benefit to this kind of solid-state spin systems, and provides a novel approach to improve the performance of nanoscale quantum sensors. These results were published in National Science Review (2025, Issue X) with the title "Coherence Enhancement via Diamond-Graphene Hybrid for Nanoscale Quantum Sensing". PhD student Yucheng Hao, Associate Researcher Zhiping Yang, and Zeyu Li are listed as co-first authors and Associate Professor Xi Kong, Professor Fazhan Shi, and Professor Jiangfeng Du are listed as corresponding authors.
Quantum sensing is a fast-developing research field, serves as a critical tool for research at microscopic scale. High sensitivity for weak magnetic signals sensing benefits various fields, such as physics, chemistry, and life science. Due to its long coherence time at room temperature, NV centers have rapidly emerged as a novel quantum sensor in recent years, enabling single-molecule magnetic resonance and nanoscale magnetic imaging technology. Creating shallow NV centers close to the diamond surface can greatly enhance the spatial resolution, however, the electric or magnetic noise from unpaired electron spins at the surface significantly reduces the coherence time of NV centers, which in turn degrades the sensitivity. Suppressing near-surface noise and further improving the coherence properties of shallow NV centers are essential issues that need to be addressed for the development of high-sensitivity nanomagnetic sensing.
Conventional methods to enhance the quantum coherence time such as dynamic decoupling and driving surface spin bath can suppress noise on NV centers, while they require additional microwave manipulation. The research team took a new approach to enhance the coherence time of NV centers by covering monolayer graphene on the diamond surface and using the hybridization of the heterogeneous interface to modulate density and relaxation time of surface electron spins. The experimental results show a substantial enhancement of about 2 times for shallow NV centers (depth < 20 nm) on average, with some instances showing improvements of over 2.5 times. Using double electron-electron resonance method, the researchers quantitatively measured the density and relaxation time of surface electron spins before and after the deposition of the graphene, revealing a remarkable reduction in the surface spin density. First-principles calculations also indicate that electrons transfer from graphene to the diamond surface, forming an orbital hybridization that effectively depletes unpaired electrons, which explains the physical mechanism of coherence enhancement by graphene-diamond heterointerface hybridization.
This technique is simple for application, leveraging mature graphene transfer processes to reduce the electric and magnetic noise from diamond surface. Thus, the coherence times of NV centers can be directly enhanced without complex fabrication. The interface modulation strategy is applicable to similar solid-state quantum systems, providing a new approach for the enhancement of nanoscale quantum sensing technology. Meanwhile, the phenomenon that shallow NV centers are influenced by the interface can in turn allows NV centers to be used as quantum sensors for in situ detection of the properties of such heterogeneous interfaces, presenting a new method for the study of two-dimensional material and interfaces.