Advanced quantum systems are emerging as powerful drivers of scientific research and technologies, enabling a wide range of applications. Color centers hosting an accessible spin in solid-state materials are naturally relevant in this context due to their transformative potential, especially for quantum sensing and quantum communication technologies, with impact for quantum computing being an area of active consideration. Many of the excellent features of color centers derive from their quantum-mechanical nature, including quantum coherence, a controllable entanglement, and functionality over a broad temperature range. There is intense activity to develop and understand color center physics and to develop new applications. The success of these efforts relies on advances in theory, materials, device engineering, and the development of sensitive metrology tools. To summarize the recent progress in this field, the “Materials, Methods, and Applications of Color Centers with Accessible Spin” Special Topic in the Journal of Applied Physics covers materials, methods, and applications of color centers with accessible spin in a broad range of quantum science and technologies.
Figure 1 summarizes the major research areas based on color centers with accessible spin hosted by a variety of wide bandgap substrate materials. The research areas include applications—quantum networking and quantum sensing—as well as basic research into the materials of the color centers themselves. These application areas take advantage of the unique properties of defect color centers. For example, the term color center derives from the fact that a high concentration of optically active defect centers residing in an otherwise transparent material can give that material color, which has long been an important concern of the gem industry. Today, however, the optical transition of a single color center can be optically isolated1 and detected as a source of single photons.2 This is an important ingredient for communicating quantum information and quantum entanglement over a network.3 When the color center also has a spin degree of freedom, it is sometimes accessible through spin-dependent optical transitions.4 This makes the color center even more useful because it enables the measurement of an atom-scale spin through emitted photons1 or entanglement between the spin and the emitted photon.5,6 The former enables optically detected quantum sensors based on spin,7,8 while the latter is a second crucial ingredient for quantum networks.9 It is not surprising that the performance of color centers depends on their atomic composition, their local environment, and their material host,4 motivating research into the nature of color centers, a search for new color centers, especially within new materials. Thus, the “Materials, Methods, and Applications of Color Centers with Accessible Spin” Special Topic represents the recent work and reviews from all three areas presented in Fig. 1.
First, we highlight works focused on quantum networks. Ruf et al., a joint team of authors from MIT and Delft, have written an invited review on the progress toward establishing a quantum network based on color centers in diamond.10 They describe the essential physics behind color center-based quantum networking, the merits of different color center choices and their respective challenges, strategies for enhancing the spin–photon interface, the progress in device integration, and the opportunities that lay ahead. This issue also includes the analysis by Inam and Castelletto that contributes to understanding the detailed mechanisms of enhanced photon extraction when a single color center is placed in a dielectric nanopillar.11
Next, we consider diamond nitrogen-vacancy (NV) centers in sensing applications and development. NV centers are optically active spin defects in diamond that have emerged as a promising multimodal sensing platform to investigate condensed matter systems in a broad range of experimental conditions. Trimble et al. have investigated soliton-like gyrotropic modes generated from a magnetic vortex in a permalloy disk using NV-based optically detected magnetic resonance technique,12 bringing new opportunities for developing hybrid NV-based quantum devices. Nava Antonio et al. demonstrated stray field imaging of FeRh with an ensemble of NV centers and observed a reorientation of the domain walls across the metamagnetic phase transition.13 Moreover, diamond NV-based quantum sensing techniques have been employed to study materials under extreme conditions, such as at high frequency, in a large magnetic field, and at high pressure. Fortman et al. performed nuclear magnetic resonance measurements using NV centers with the application of an external magnetic field up to 8.3 T and a microwave frequency of 230 GHz,14 enabling a successful detection of 13C nuclear bath spins in diamond. In addition, NV centers play an increasingly important role in high-pressure research to study the pressure-induced exotic quantum phases of condensed matter systems. Ho et al. have presented a detailed review of the recent progress along this research direction.15 The development of quantum sensing techniques also relies on advances in theory and measurement protocols. McMichael et al. reported that a sequential Bayesian experimental design protocol could effectively improve the data acquisition efficiency of Ramsey sequence measurements.16 More recently, color centers beyond NVs and their application in quantum sensing are under intensive study. For instance, Bates et al. demonstrated measurements of the full strain tensor in diamond using silicon vacancy centers.17
Finally, we draw your attention to several works advancing the knowledge of defect and materials physics. Khan and Leuenberger18 and Turiansky and Van de Walle19 apply ab initio methods to identify color centers in 2D materials: erbium in tungsten disulfide and boron dangling bonds in hexagonal boron nitride, respectively, and assess their potential for applications in quantum technology. Such ab initio simulations not only are difficult but are also highly interesting and rewarding with new physics, due to layer dependencies, extreme anisotropies, and long-range interactions. Switching from theory to experiment, the work by Son and Ivanov20 is a comprehensive empirical study of the influence of doping and intrinsic defect concentration on the charge states of the silicon vacancy and divacancy color centers in silicon carbide. This work seeks to resolve the charge instability of the centers, which is one of the key limitations of their performance in quantum technology. The final two works by Cao et al.21 and Yang et al.22 focus on the properties and applications of low-dimensional materials rather than color centers. They are, respectively, motivated to apply these materials for the creation of ultra-thin photodetectors and spintronic devices. Both fit within the future vision of integrated 2D quantum devices that employ color centers alongside photodetectors and spintronics.
This Special Topic, “Materials, Methods, and Applications of Color Centers with Accessible Spin,” touches on several key areas of development that are driving color centers with accessible spin in relevant application areas. These new developments spanning quantum networking, quantum sensing, and defect materials are exciting, and we expect a bright future for applications and fundamental science using color centers with accessible spin.
G.D.F. acknowledges support from the ONR (No. N00014-21-1-2614) and the National Science Foundation (NSF) (No. ECCS-1839196). C.R.D. acknowledges support from the Air Force Office of Scientific Research under Award No. FA9550-20-1-0319 and its Young Investigator Program under Award No. FA9550-21-1-0125.