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Optically addressable solid-state spins are an important platform for practical quantum technologies. Van der Waals material hexagonal boron nitride (hBN) is a promising host as it contains a wide variety of optical emitters, but thus far observations of addressable spins have been sparse, and most of them lacked a demonstration of coherent spin control. Here we demonstrate robust optical readout of spin pairs in hBN with emission wavelengths spanning from violet to the near-infrared. We find these broadband spin pairs exist naturally in a variety of hBN samples from bulk crystals to powders to epitaxial films, and can be coherently controlled across the entire wavelength range. Furthermore, we identify the optimal wavelengths for independent readout of spin pairs and boron vacancy spin defects co-existing in the same sample. Our results establish the ubiquity of the optically addressable spin pair system in hBN across a broad parameter space, making it a versatile playground for spin-based quantum technologies.

Widefield quantum diamond microscopy is a powerful technique for imaging magnetic fields with high sensitivity and spatial resolution. However, current methods to approach the ultimate spatial resolution ($<500\,$nm) are impractical for routine use as they require time-consuming fabrication or transfer techniques to precisely interface the diamond sensor with the sample to be imaged. To address this challenge, we have designed a co-axial sensor holder that enables simple, repeatable sensor-sample interfacing while being compatible with high numerical aperture (NA) optics. With our new design we demonstrate low standoffs $<500\,$nm with a millimetre sized sensor. We also explore the relationship between spatial resolution and NA spanning from 0.13 to 1.3. The spatial resolution shows good agreement with the optical diffraction limit at low NA but deviates at high NA, which is shown to be due to optical aberrations. Future improvements to our design are discussed, which should enable magnetic imaging with $<500\,$nm resolution in an accessible, easy-to-use instrument.

Optically detected magnetic resonance (ODMR) has been observed from emitters in hexagonal boron nitride across a broad range of wavelengths, but so far an understanding of their microscopic structure and the physical origin of ODMR has been lacking. Here we perform comprehensive measurements and modelling of the spin-resolved photodynamics of ensembles and single emitters, and uncover a universal model that accounts, and provides an intuitive physical explanation, for all key experimental features. The model assumes a pair of nearby point defects -- a primary optically active defect and a secondary defect. Charge transfer between the two defects creates a metastable weakly coupled spin pair with ODMR naturally arising from selection rules. Our optical-spin defect pair (OSDP) model resolves several previously open questions including the asymmetric envelope of the Rabi oscillations, the large variability in ODMR contrast amplitude and sign, and the wide spread in emission wavelength. It may also explain similar phenomena observed in other wide bandgap semiconductors such as GaN. The presented framework will be instrumental in guiding future theoretical and experimental efforts to study and engineer solid-state spin pairs and unlock their potential for quantum technology applications.

Boron vacancy centre ($V_{\rm B}^-$) ensembles in hexagonal boron nitride (hBN) have attracted recent interest for their potential as two-dimensional solid-state quantum sensors. Irradiation is necessary for $V_{\rm B}^-$ creation, however, to date only limited attention has been given to optimising the defect production process, especially in the case of bulk irradiation with high-energy particles, which offers scalability through the potential for creating ensembles in large volumes of material. Here we systematically investigate the effect of electron irradiation by varying the dose delivered to a range of hBN samples, which differ in their purity, and search for an optimum in measurement sensitivity. We find that moderate electron irradiation doses ($\approx 5\times 10^{18}$~cm$^{-2}$) appear to offer the best sensitivity, and also observe a dependence on the initial crystal purity. These results pave the way for the scalable and cost-effective production of hBN quantum sensors, and provide insight into the mechanisms limiting $V_{\rm B}^-$ spin properties., Comment: 8pages, 5 figures

Optically addressable spin defects hosted in two-dimensional van der Waals materials represent a new frontier for quantum technologies, promising to lead to a new class of ultrathin quantum sensors and simulators. Recently, hexagonal boron nitride (hBN) has been shown to host several types of optically addressable spin defects, thus offering a unique opportunity to simultaneously address and utilise various spin species in a single material. Here we demonstrate an interplay between two separate spin species within a single hBN crystal, namely $S=1$ boron vacancy defects and visible emitter spins. We unambiguously prove that the visible emitters are $S=\frac{1}{2}$ spins and further demonstrate room temperature coherent control and optical readout of both spin species. Importantly, by tuning the two spin species into resonance with each other, we observe cross-relaxation indicating strong inter-species dipolar coupling. We then demonstrate magnetic imaging using the $S=\frac{1}{2}$ defects, both under ambient and cryogenic conditions, and leverage their lack of intrinsic quantization axis to determine the anisotropic magnetic susceptibility of a test sample. Our results establish hBN as a versatile platform for quantum technologies in a van der Waals host at room temperature.

Detecting magnetic noise from small quantities of paramagnetic spins is a powerful capability for chemical, biochemical, and medical analysis. Quantum sensors based on optically addressable spin defects in bulk semiconductors are typically employed for such purposes, but the 3D crystal structure of the sensor inhibits the sensitivity by limiting the proximity of the defects to the target spins. Here we demonstrate the detection of paramagnetic spins using spin defects hosted in hexagonal boron nitride (hBN), a van der Waals material which can be exfoliated into the 2D regime. We first create negatively charged boron vacancy (V$_{\rm B}^-$) defects in a powder of ultrathin hBN nanoflakes ($<10$~atomic monolayers thick on average) and measure the longitudinal spin relaxation time ($T_1$) of this system. We then decorate the dry hBN nanopowder with paramagnetic Gd$^{3+}$ ions and observe a clear $T_1$ quenching, under ambient conditions, consistent with the added magnetic noise. Finally, we demonstrate the possibility of performing spin measurements including $T_1$ relaxometry using solution-suspended hBN nanopowder. Our results highlight the potential and versatility of the hBN quantum sensor for a range of sensing applications, and pave the way towards the realisation of a truly 2D, ultrasensitive quantum sensor., Comment: 19 pages, 11 figures