Long-Term Spin State Storage Using Ancilla Charge Memories

Advances in Stabilization and Enrichment of Shallow Nitrogen-Vacancy Centers in Diamond for Biosensing and Spin-Polarization Transfer

Negatively charged nitrogen-vacancy (NV^-) centers in diamond have unique magneto-optical properties, such as high fluorescence, single-photon generation, millisecond-long coherence times, and the ability to initialize and read the spin state using purely optical means. This makes NV^- centers a powerful sensing tool for a range of applications, including magnetometry, electrometry, and thermometry. Biocompatible NV-rich nanodiamonds find application in cellular microscopy, nanoscopy, and in vivo imaging. NV^- centers can also detect electron spins, paramagnetic agents, and nuclear spins. Techniques have been developed to hyperpolarize ¹⁴N, ¹⁵N, and ¹³C nuclear spins, which could open up new perspectives in NMR and MRI. However, defects on the diamond surface, such as hydrogen, vacancies, and trapping states, can reduce the stability of NV^- in favor of the neutral form (NV⁰), which lacks the same properties. Laser irradiation can also lead to charge-state switching and a reduction in the number of NV^- centers. Efforts have been made to improve stability through diamond substrate doping, proper annealing and surface termination, laser irradiation, and electric or electrochemical tuning of the surface potential. This article discusses advances in the stabilization and enrichment of shallow NV^- ensembles, describing strategies for improving the quality of diamond devices for sensing and spin-polarization transfer applications. Selected applications in the field of biosensing are discussed in more depth.

Robustness improvement of a nitrogen-vacancy magnetometer by a double driving method

The nitrogen vacancy (NV) color center in diamonds is an electron spin that can measure magnetic fields with high sensitivity and resolution. Furthermore, the robustness of an NV-based quantum system should be improved for further application in other sensing methods and in the exploration of basic physics. In this work, the robustness of an NV magnetometer is improved by the double driving method. The sensitivity of the NV magnetometer was improved 2.1 times by strengthening the pumping power from 100 to 600 mW. In this process, thermal drift was introduced, which affects the measurement accuracy. The temperature drift of a diamond matrix was measured using an infrared camera, and the temperature change of a diamond host drifted to ∼80 K under high laser and microwave power. To address the drift of temperature owing to sensitivity improvement by pumping enhancement, the double driving method was introduced, to suppress the drift of the resonance frequency, to improve the robustness of a continuous-wave NV magnetometer. The magnetic noise density was improved from 10 to 1.2 nT/Hz1/2. This study checked the source of temperature noise in the process of measuring with the NV color centers and proposes a double driving measurement method to track the resonant frequency change due to environmental temperature drift and improve sensitivity. The findings of this study are useful in applying complex pulse protocols in high-level sensing applications based on solid-state spin.

Detection and Modeling of Hole Capture by Single Point Defects under Variable Electric Fields

Understanding carrier trapping in solids has proven key to semiconductor technologies, but observations thus far have relied on ensembles of point defects, where the impact of neighboring traps or carrier screening is often important. Here, we investigate the capture of photogenerated holes by an individual negatively charged nitrogen-vacancy (NV) center in diamond at room temperature. Using an externally gated potential to minimize space-charge effects, we find the capture probability under electric fields of variable sign and amplitude shows an asymmetric-bell-shaped response with maximum at zero voltage. To interpret these observations, we run semiclassical Monte Carlo simulations modeling carrier trapping through a cascade process of phonon emission and obtain electric-field-dependent capture probabilities in good agreement with experiment. Because the mechanisms at play are insensitive to the characteristics of the trap, we anticipate the capture cross sections we observe─largely exceeding those derived from ensemble measurements─may also be present in materials platforms other than diamond.

Room-Temperature Photochromism of Silicon Vacancy Centers in CVD Diamond

The silicon vacancy (SiV) center in diamond is typically found in three stable charge states, SiV⁰, SiV^-, and SiV^2-, but studying the processes leading to their formation is challenging, especially at room temperature, due to their starkly different photoluminescence rates. Here, we use confocal fluorescence microscopy to activate and probe charge interconversion between all three charge states under ambient conditions. In particular, we witness the formation of SiV⁰ via the two-step capture of diffusing, photogenerated holes, a process we expose both through direct SiV⁰ fluorescence measurements at low temperatures and confocal microscopy observations in the presence of externally applied electric fields. In addition, we show that continuous red illumination induces the converse process, first transforming SiV⁰ into SiV^- and then into SiV^2-. Our results shed light on the charge dynamics of SiV and promise opportunities for nanoscale sensing and quantum information processing.

Imaging dark charge emitters in diamond via carrier-to-photon conversion

The application of color centers in wide-bandgap semiconductors to nanoscale sensing and quantum information processing largely rests on our knowledge of the surrounding crystalline lattice, often obscured by the countless classes of point defects the material can host. Here, we monitor the fluorescence from a negatively charged nitrogen-vacancy (NV⁻) center in diamond as we illuminate its vicinity. Cyclic charge state conversion of neighboring point defects sensitive to the excitation beam leads to a position-dependent stream of photo-generated carriers whose capture by the probe NV⁻ leads to a fluorescence change. This “charge-to-photon” conversion scheme allows us to image other individual point defects surrounding the probe NV, including nonfluorescent “single-charge emitters” that would otherwise remain unnoticed. Given the ubiquity of color center photochromism, this strategy may likely find extensions to material systems other than diamond.

Probing Charge Dynamics in Diamond with an Individual Color Center

Control over the charge states of color centers in solids is necessary to fully utilize them in quantum technologies. However, the microscopic charge dynamics of deep defects in wide-band-gap semiconductors are complex, and much remains unknown. We utilize a single-shot charge-state readout of an individual nitrogen-vacancy (NV) center to probe the charge dynamics of the surrounding defects in diamond. We show that the NV center charge state can be converted through the capture of holes produced by optical illumination of defects many micrometers away. With this method, we study the optical charge conversion of silicon-vacancy (SiV) centers and provide evidence that the dark state of the SiV center under optical illumination is SiV^2-. These measurements illustrate that charge carrier generation, transport, and capture are important considerations in the design and implementation of quantum devices with color centers and provide a novel way to probe and control charge dynamics in diamond.