超高分辨率NV–色心金刚石弱磁场测量的组合解耦方法研究

Based on the electron spin theory, the negatively charged nitrogen-vacancy (NV–) center in diamond appears ultra-sensitive to the magnetic-field, which can facilitate the measurement of magnetic fields with ultra-high resolution to be realized. Hence, this technology has become a hot topic in high sensitive magnetic-field image research field and molecule-scale nuclear magnetic resonance research field. However, there are many coupling effects between the electron spin state of the NV– centers and the variety of impurities present. The coupling effects can lead to the phenomena of decoherence of the electron spin, which makes it experimentally difficult to improve the resolution of the magnetic-field measurement. Moreover, the magnetic-field resolution is degraded by the general measurement method, which has a low fluorescence intensity measurement efficiency for the electron spin state detection. In this project, in order to resolve the above problems, we propose the following works to be investigated. First of all, based on microscopic interactions of defects in diamond, the complex coupling mechanism of the NV– center electron spin ensemble will be analyzed. Then in order to better understand the NV– center electron spin ensemble system, a model will be constructed. Secondly, according to the spin decoherence and dynamical decoupling theory, the Carr-Purcell-Meiboom-Gill (nCPMG) decoupling sequence and the Mansfield-Rhim-Eillis-Vaughan (MREV) decoupling sequence which are previously used to resolve the nuclear spin coupling and dipole-dipole coupling resulting from the impurity, will be combined for decoupling the NV– center electron spin ensemble. Third and finally, considering the energy-level configuration of the NV– center, a multi-spectrum optical detection technology will be used to improve the optical detection efficiency, thereby improving the magnetic measurement resolution further. Additionally, a semi-physical simulation will be processed for experimental verification.

根据电子自旋理论，负价氮原子-空位（NV–）金刚石色心的电子自旋对磁场强度极其敏感，可用于实现超高分辨率的磁场测量，已成为高精度磁成像、分子级核磁共振等领域的新研究热点。但色心的电子自旋态与材料内杂质存在耦合现象，引起电子自旋的退相干，使实际磁场测量分辨率难以提高。传统的利用测量色心荧光强度实现自旋态检测的方法，因荧光的检测效率低，进一步降低了磁场测量分辨率。本项目针对上述问题，从色心的微观相互作用出发，分析高浓度色心金刚石材料中，NV–色心电子自旋系综的复杂耦合机制，并建立其合理模型；根据自旋退相干和动力学解耦理论，提出nCPMG方法与MREV方法相结合的组合解耦方法，通过理论推导与参数匹配，实现核自旋耦合与偶极耦合解耦方面的优势互补；进一步分析色心在磁场作用下的光学特性，优化多光谱成像光学谱段与测量系统，以提高光学检测效率，并通过半物理实验进行实验验证。

在国家对“无损、灵敏、高分辨率等特征的生命科学检测、成像”技术迫切需求的背景下，负价氮原子-空位（NV–）金刚石色心因可实现室温下磁场的高灵敏、高空间分辨率测量，已成为生物成像检测领域的新研究热点。其传感能力受到了内部杂质耦合效应而导致的退相干时间短以及光学检测效率低的严重制约。因此，本研究从模型建立、解耦方法、光学测量及实验验证四个方面进行了深入研究。通过第一性原理仿真软件VASP与计算软件Mathmatica对色心电子自旋进行了动力学建模；提出了新的色心解耦序列并根据其特点命名为双参数动态解耦脉冲序列（SDPDD）与非对称双参数动态解耦脉冲序列（ADPDD）；通过上述两种解耦序列对色心中影响其退相干的13C进行了定位，并在此基础上消除其弱相互作用，以达到延长退相干时间的目的，指标上，核自旋定位线宽精度达到1.7kHz，退相干时间延长最优达到37.6μs；设计并制作了用于解耦操控用的偏置磁场、微波天线，增强色心操控的同步性与效率，最终达到了0.95×1.2 mm2面积内操控均匀度95.6%；设计并加工了全反射透镜、高品质光波导镀膜，并结合CCD成像设备，使色心放出荧光导出金刚石体、金刚石体外放出荧光的收集效率较常用的共聚焦系统提高了64倍，荧光对比度提高了1.9倍；结合所搭建的专用的金刚石色心磁场测量平台及为其编制的操控、检测系统，进行了实验验证，最终效果达到了磁场测量灵敏度优于1nT。通过本课题的研究，更加明确了金刚石色心的动力学模型，创新的提出了较现有方法更好的色心解耦方法，设计并制作了更优的色心操控微波导线，以及更高效的荧光测量系统，上述方法不仅对基于金刚石色心的磁场测量技术，对基于金刚石色心的其他量子传感、量子计算、量子通信，乃至与金刚石色心相类似的诸如B-N材料中的B-V色心等，均有这重要的研究借鉴意义。如将该技术走向应用，则在浅层医疗磁成像、材料磁探测等诸多领域有着极好的应用前景。