The nitrogen-vacancy colour centre in diamond

Abstract: The nitrogen-vacancy (NV) colour centre in diamond is an important physical system for emergent quantum technologies, including quantum metrology, information processing and communications, as well as for various nanotechnologies, such as biological and sub-diffraction limit imaging, and for tests of entanglement in quantum mechanics. Given this array of existing and potential applications and the almost 50 years of NV research, one would expect that the physics of the centre is well understood, however, the study of the NV centre has proved challenging, with many early assertions now believed false and many remaining issues yet to be resolved. This review represents the first time that the key empirical and ab initio results have been extracted from the extensive NV literature and assembled into one consistent picture of the current understanding of the centre. As a result, the key unresolved issues concerning the NV centre are identified and the possible avenues for their resolution are examined.

• The paper clarifies the NV center's electronic structure, highlighting its ground state triplet and hyperfine interactions.
• The paper demonstrates experimental breakthroughs such as room-temperature quantum registers, spin-photon entanglement, and nanoscale magnetometry.
• The paper identifies unresolved challenges like charge state control and photoconversion dynamics, setting a roadmap for future quantum research.

Overview of "The Nitrogen-Vacancy Colour Centre in Diamond"

This comprehensive review paper authored by Doherty et al. explores the complex physics of the nitrogen-vacancy (NV) color center in diamond, a system that is pivotal to the development of several quantum technologies. The NV center is extensively researched not only for its utility in quantum metrology, information processing, and communication but also in nanoscale imaging and entanglement tests in quantum mechanics. Despite nearly five decades of investigation, there remain significant unresolved issues surrounding the NV center's properties and interactions.

Key Findings and Contributions

The review meticulously assembles empirical and ab initio results into a coherent picture to detail current understanding and unknowns in NV center physics. The authors emphasize that the NV$^-$ center has already witnessed numerous critical experimental demonstrations, exemplifying its versatility. Among these are room-temperature quantum registers using electronic and nuclear spins, spin-photon entanglement, and nanoscale magnetometry. They recognize that while the NV$^-$ center has been extensively used in various experiments, the NV$^0$ remains underexplored.

Empirical and Theoretical Insights

A significant contribution of the paper is the clarification of the NV center's electronic structure. Notably, the NV$^-$ center's ground state is a triplet ($^3A_2$), and its fine structure is characterized by hyperfine interactions attributable to the nitrogen nucleus. The empirical data, supported by theoretical models, suggest these interactions arise mainly due to the effect of unpaired electron spin densities. The facile manipulation and readout of these states underscore the importance of NV centers in quantum applications.

In addressing the low temperature properties of the $^3E$ electronic state of the NV$^-$ center, the paper identifies the state’s fine structure resulting from spin-spin interactions and axial spin-orbit coupling. A significant breakthrough discussed is the low temperature spectroscopic resolution of NV$^-$ optical ZPL fine structure, a feat that was previously limited by inhomogeneously broadened ensemble measurements.

Another strong focus is the review of challenges in the NV system, such as its charge state control and the impact of varying electromagnetic interactions. The authors assert that the relative concentrations of NV$^-$ and NV$^0$ charge states are highly sensitive to microscopic donor distributions and subsequent photoconversion equilibria, necessitating further detailed research.

Future Directions

The paper identifies several outstanding challenges for future inquiry:

1. Photoconversion Mechanisms: Understanding charge state dynamics remains critical. The review suggests that resolving this will enhance the NV system’s utility in practical applications.
2. Temperature Dependence: The NV center’s properties, including its fine structure, show variation with temperature. Further empirical work is needed to thoroughly understand these dependencies and their implications for NV center utility under realistic operating conditions.
3. Jahn-Teller Effects: These play a subtle role in altering the NV center's interactions with various fields. Detailed models are needed to predict the influence of vibronic interactions on the NV center's electronic transitions.
4. Spin Dynamics: The comprehensive understanding of spin-polarization mechanisms is essential for advances in quantum information science. Future work should explore this through both theoretical models and experimental studies.

Conclusion

This review serves as an invaluable reference point, consolidating decades of research and clarifying current knowledge boundaries in NV center science. By identifying key unresolved issues, the authors provide a strategic roadmap that can guide both current and future researchers in the field. Such an understanding is crucial, not only for optimizing the NV center's capabilities in emerging quantum technologies but for laying the groundwork for the discovery and utilization of new defect systems with potentially superior functionalities.