DC Magnetic Field Sensitivity Optimization of Spin Defects in Hexagonal Boron Nitride.

Spin-active defects in layered hexagonal boron nitride (hBN) crystals have attracted increased attention in quantum sensing. Notably, the recently discovered negatively charged boron vacancy (V B −) center stands out due to its optical addressability and coherent controllability. Among the various methods reported for generating such defects, ion implantation is notable as a readily accessible technique. In this paper, the properties of V B − defects in hBN generated via ion implantation are extensively studied. We achieve a ubiquitous distribution of highly stable defects across the crystal sample, and find that the ion beam current density, rather than fluence, plays a critical role in determining the uniformity and density of defects. The generated defects display bright and stable photoluminescence, and we explicitly investigate the dependence of spin properties on factors such as laser, microwave power, and duration. An intriguing phenomenon is observed wherein the peak contrast exceeds 20% without any enhancing techniques in the optically detected magnetic resonance spectrum for some special defects. Our results provide valuable insights and suggestions for the controlled generation of V B − defects in hBN through ion implantation.

Spin defects in hexagonal boron nitride, and specifically the negatively charged boron vacancy (VB - ) centers, are emerging candidates for quantum sensing. However, the VB - defects suffer from low quantum efficiency and, as a result, exhibit weak photoluminescence. In this work, a scalable approach is demonstrated to dramatically enhance the VB - emission by coupling to a plasmonic gap cavity. The plasmonic cavity is composed of a flat gold surface and a silver cube, with few-layer hBN flakes positioned in between. Employing these plasmonic cavities, two orders of magnitude are extracted in photoluminescence enhancement associated with a corresponding twofold enhancement in optically detected magnetic resonance contrast. The work will be pivotal to progress in quantum sensing employing 2D materials, and in realization of nanophotonic devices with spin defects in hexagonal boron nitride.

Optically active spin defects in wide-bandgap materials have many potential applications in quantum information and quantum sensing. Spin defects in two-dimensional layered van der Waals materials are just emerging to be investigated. Here we demonstrate that optically-addressable spin ensembles in hexagonal boron nitride (hBN) can be generated by femtosecond laser irradiation. We observe optically detected magnetic resonance (ODMR) of hBN spin defects created by laser irradiation. We show that the creation of spin defects in hBN is strongly affected by the pulse energy of the femtosecond laser. When the laser pulse number is less than a few thousand, the pulse number only affects the density of defects but not the type of defects. With proper laser parameters, spin defects can be generated with a high probability of success. Our work provides a convenient way to create spin defects in hBN by femtosecond laser writing, which shows promising prospects for quantum technologies.

We report optically detected magnetic resonance (ODMR) measurements of an ensemble of spin-1 negatively charged boron vacancies in hexagonal boron nitride. The photoluminescence decay rates are spin-dependent, with intersystem crossing rates of 1.02 ns-1 and 2.03 ns-1 for the mS = 0 and mS = ±1 states, respectively. Time gating the photoluminescence enhances the ODMR contrast by discriminating between different decay rates. This is particularly effective for detecting the spin of the optically excited state, where a zero-field splitting of |DES| = 2.09 GHz is measured. The magnetic field dependence of the photoluminescence exhibits dips corresponding to the ground (GSLAC) and excited-state (ESLAC) anticrossings and additional anticrossings due to coupling with nearby spin-1/2 parasitic impurities. Comparison to a model suggests that the anticrossings are mediated by the interaction with nuclear spins and allows an estimate of the ratio of the singlet to triplet spin-dependent relaxation rates of κ0/κ1 = 0.34.

The past decade has witnessed a growing research interest in quantum sensing using spin-active boron vacancy (VB-) defects in hexagonal boron nitride (hBN). While hBN enables easy chip integration, current quantum sensing devices suffer from low optical readout and noisy signals due to inefficient waveguide designs that limit microwave absorption, resulting in reduced optically detected magnetic resonance (ODMR) contrast. This study advances hBN-integrated quantum sensors through three generations, culminating in a compact single-port coplanar waveguide (CPW). Unlike conventional two-port waveguides, our design allows on-chip optical and microwave excitation, ensuring improved impedance stability and high radio frequency (RF) magnetic field concentration without altering spin properties. As a result, we achieve a high ODMR contrast of ∼28% at low microwave power (MW) of 400 mW. This miniaturized sensor enhances efficiency three times, reduces the use of RF power to five times, and supports robust performance at lower MW power, making it ideal for scalable quantum sensing applications such as magnetic field detection.

Quantum technology grown out of quantum information theory, including quantum communication, quantum computation and quantum sensing, not only provides powerful research tools for numerous fields, but also is expected to go to civilian use in the future. Solid-state spin-active defects are one of promising platforms for quantum technology, and the host materials include three-dimensional diamond and silicon carbide, and the emerging two-dimensional hexagonal boron nitride (hBN) and transition-metal dichalcogenides. In this review, we will focus on the spin defects in hBN, and summarize theoretical and experimental progresses made in understanding properties of these spin defects. In particular, the combination of theoretical prediction and experimental verification is highlighted. We also discuss the future advantages and challenges of solid-state spins in hBN on the path towards quantum information applications.

Optically addressable solid-state color center spin qubits have become important platforms for quantum information processing, quantum networks and quantum sensing. The readout of color center spin states with optically detected magnetic resonance (ODMR) technology is traditionally based on Stokes excitation, where the energy of the exciting laser is higher than that of the emission photons. Here, we investigate an unconventional approach using anti-Stokes excitation to detect the ODMR signal of silicon vacancy defect spin in silicon carbide, where the exciting laser has lower energy than the emitted photons. Laser power, microwave power and temperature dependence of the anti-Stokes excited ODMR are systematically studied, in which the behavior of ODMR contrast and linewidth is shown to be similar to that of Stokes excitation. However, the ODMR contrast is several times that of the Stokes excitation. Coherent control of silicon vacancy spin under anti-Stokes excitation is then realized at room temperature. The spin coherence properties are the same as those of Stokes excitation, but with a signal contrast that is around three times greater. To illustrate the enhanced spin readout contrast under anti-Stokes excitation, we also provide a theoretical model. The experiments demonstrate that the current anti-Stokes excitation ODMR approach has promising applications in quantum information processing and quantum sensing.

Recently, Stone–Wales (SW) defects gradually attracted people’s research interest because of their unique properties. The theoretical research indicated that the SW defect in hexagonal boron nitride (h-BN) can lead to new defect levels in bandgap, making h-BN apply in ultraviolet emitters. However, the SW defect is always observed in graphene and rarely observed in h-BN in the experiments. Here, we confirmed the SW defects are not easily formed in h-BN under thermodynamic conditions by first-principles calculations. Specifically, the monolayer h-BN with SW defect (h-BN-SW) has the weak bond strength, dynamic stability and high-temperature thermal stability, facilitating the healing of SW defects under high-temperature conditions and the role of hydrogen. Additionally, we found the SW defect in AB stacked h-BN (AB-h-BN) have good mechanical stability, dynamic stability and thermodynamic stability than h-BN-SW, especially for AB-h-BN-2SW (2SW defects formed in upper and lower layer of AB-h-BN, respectively), which can meet the requirements for its application in electronic devices. Even under thermodynamic conditions, the formation of SW defects is extremely challenging. Electron beam irradiation technology provides a window for the generation of SW defects in h-BN. This offers opportunities for the introduction and control of SW defects, while also creating potential for their application in electronic devices. Moreover, we found that the absorption peak broadens, and a new absorption peak appears with the generation of SW defects, which is mainly induced by the decrease of bandgap and the generation of defect levels. Our research can provide theoretical guidance at atomic scale for designing and applying h-BN with SW defect in the experiments.

Optically active spin defects in van der Waals materials are promising platforms for modern quantum technologies. Here we investigate the coherent dynamics of strongly interacting ensembles of negatively charged boron-vacancy ({{{{{{{{rm{V}}}}}}}}}_{{{{{{{{rm{B}}}}}}}}}^{-}) centers in hexagonal boron nitride (hBN) with varying defect density. By employing advanced dynamical decoupling sequences to selectively isolate different dephasing sources, we observe more than 5-fold improvement in the measured coherence times across all hBN samples. Crucially, we identify that the many-body interaction within the {{{{{{{{rm{V}}}}}}}}}_{{{{{{{{rm{B}}}}}}}}}^{-} ensemble plays a substantial role in the coherent dynamics, which is then used to directly estimate the concentration of {{{{{{{{rm{V}}}}}}}}}_{{{{{{{{rm{B}}}}}}}}}^{-}. We find that at high ion implantation dosage, only a small portion of the created boron vacancy defects are in the desired negatively charged state. Finally, we investigate the spin response of {{{{{{{{rm{V}}}}}}}}}_{{{{{{{{rm{B}}}}}}}}}^{-} to the local charged defects induced electric field signals, and estimate its ground state transverse electric field susceptibility. Our results provide new insights on the spin and charge properties of {{{{{{{{rm{V}}}}}}}}}_{{{{{{{{rm{B}}}}}}}}}^{-}, which are important for future use of defects in hBN as quantum sensors and simulators.

Understanding and engineering atomic defects in hexagonal boron nitride (hBN) provides a powerful platform for realizing solid-state quantum emitters and spin qubits, advancing the field of quantum information science and technologies. However, the full potential of such quantum defects remains locked by the critical lack of a deterministic structure-property relationship at the atomic scale. Here, we demonstrate a strategy to atomically engineer and decipher quantum defects in hBN by integrating scanning tunneling microscopy/spectroscopy (STM/STS) and noncontact atomic force-microscopy with a CO-functionalized tip. We implemented controllable argon ion bombardment to create both boron vacancies (VB) and nitrogen vacancies (VN) in submonolayer hBN grown on Cu(111). Simultaneously, encapsulated Ar species trapped between hBN and Cu(111) locally lift the hBN to form nanobubbles, thereby decoupling atomic vacancies from the metal substrate and enabling direct probing of their electronic states. For the on-bubble VN, STS measurement reveals a prominent in-gap state with a phonon replica. Furthermore, with aid of STM tip-assisted manipulation, we demonstrate that the tuning of nanobubble sizes modulates their strain profile, thereby modulating the energetic positions of electronic states in on-bubble defects, corroborated by density functional calculations. Our studies offer insight into the intrinsic defect structures in hBN and quantum defect engineering via local strain engineering.

We have used optically detected magnetic resonance (ODMR) to characterize the degree of solvent availability of the tryptophan residues in lysozyme that are likely to be responsible for the observed phosphorescence. From the phosphorescence spectra, ODMR zero-field splittings (zfs), and ODMR line widths, we concur with the X-ray structure [Blake, C. C., Mair, G. A., North, A. C. T., Phillips, D. C., & Sarma, V. R. (1967) Proc. R. Soc. London, ser. B 167, 365-377] that Trp-62 behaves as an exposed residue and Trp-108 is buried. In addition, we present evidence that ODMR can be used in conjunction with conventional phosphorescence to evaluate the degree of order in the microenvironments of tryptophan in a protein containing several tryptophans. By the specific modification of residues Trp-62 and Trp-108, we have identified those portions of the ODMR lines in the native enzyme that are due to those specific residues. Barring major enzyme conformational changes in the vicinity of unmodified tryptophan residues when Trp-62 or Trp-108 are selectively modified, we find that Trp-108 dominates both the phosphorescence and the ODMR signals in native lysozyme. The results are discussed in view of previous fluorescence findings.

Single-photon emitters play an essential role in quantum technologies, including quantum computing and quantum communications. Atomic defects in hexagonal boron nitride ( h-BN) have recently emerged as new room-temperature single-photon emitters in solid-state systems, but the development of scalable and tunable h-BN single-photon emitters requires external methods that can control the emission energy of individual defects. Here, by fabricating van der Waals heterostructures of h-BN and graphene, we demonstrate the electrical control of single-photon emission from atomic defects in h-BN via the Stark effect. By applying an out-of-plane electric field through graphene gates, we observed Stark shifts as large as 5.4 nm per GV/m. The Stark shift generated upon a vertical electric field suggests the existence of out-of-plane dipole moments associated with atomic defect emitters, which is supported by first-principles theoretical calculations. Furthermore, we found field-induced discrete modification and stabilization of emission intensity, which were reversibly controllable with an external electric field.

We present temperature-dependent micro-PL studies on a single defect in hexagonal boron nitride. A zero-phonon line emission accompanied by Stokes and anti-Stokes phonon sidebands (≈ 6.5 meV) with a Debye-Waller factor of 0.59 is observed.

The atomic structure, stability, and dynamics of defects in hexagonal boron nitride (h-BN) are investigated using an aberration-corrected transmission electron microscope operated at 80 kV between room temperature and 1000 °C. At temperatures above 700 °C, parallelogram- and hexagon-shaped defects with zigzag edges become prominent, in contrast to the triangular defects typically observed at lower temperatures. The appearance of 120° corners at defect vertices indicates the coexistence of both N- and B-terminated zigzag edges in the same defect. In situ dynamics studies show that the hexagonal holes grow by electron-induced sputtering of B-N chains, and that at high temperatures these chains can migrate from one defect corner to another. We complement the experiments with first-principles calculation which consider the thermal equilibrium formation energy of different defect configurations. It is shown that, below a critical defect size, hexagonal defects have the lowest formation energy and therefore are the more-stable configuration, and triangular defects are energetically metastable but can be "frozen in" under experimental conditions. We also discuss the possible contributions of several dynamic processes to the temperature-dependent defect formation.

Hexagonal boron nitride (hBN) is an emerging two-dimensional material for quantum photonics owing to its large bandgap and hyperbolic properties. Here we report two approaches for engineering quantum emitters in hBN multilayers using either electron beam irradiation or annealing and characterize their photophysical properties. The defects exhibit a broad range of multicolor room-temperature single photon emissions across the visible and the near-infrared spectral ranges, narrow line widths of sub-10 nm at room temperature, and a short excited-state lifetime, and high brightness. We show that the emitters can be categorized into two general groups, but most likely possess similar crystallographic structure. Remarkably, the emitters are extremely robust and withstand aggressive annealing treatments in oxidizing and reducing environments. Our results constitute a step toward deterministic engineering of single emitters in 2D materials and hold great promise for the use of defects in boron nitride as sources for quantum information processing and nanophotonics.