Room Temperature Initialisation and Readout of Intrinsic Spin Defects in a Van der Waals Crystal

Optically addressable spins in wide-bandgap semiconductors have become one of the most prominent platforms for exploring fundamental quantum phenomena. While several candidates in 3D crystals including diamond and silicon carbide have been extensively studied, the identification of spindependent processes in atomically-thin 2D materials has remained elusive. Although optically accessible spin states in hBN are theoretically predicted, they have not yet been observed experimentally. Here, employing rigorous electron paramagnetic resonance techniques and photoluminescence spectroscopy, we identify fluorescence lines in hexagonal boron nitride associated with a particular defect—the negatively charged boron vacancy ( )—and determine the parameters of its spin Hamiltonian. We show that the defect has a triplet (S = 1) ground state with a zero-field splitting of ≈3.5 GHz and establish that the centre exhibits optically detected magnetic resonance (ODMR) at room temperature. We also demonstrate the spin polarization of this centre under optical pumping, which leads to optically induced population inversion of the spin ground state—a prerequisite for coherent spin-manipulation schemes. Our results constitute a leap forward in establishing twodimensional hBN as a prime platform for scalable quantum technologies, with extended potential for spin-based quantum information and sensing applications, as our ODMR studies on hBN NV diamonds hybrid structures show. The emergence of 2D materials and van der Waals (vdW) crystals has enabled the observation and realisation of unique optoelectronic and nanophotonic effects such as unconventional superconductivity, Moire excitons and quantum spin Hall effects at elevated temperatures, to name a few.1-3 Amidst the large variety of studied vdW crystals, hexagonal Boron Nitride (hBN) offers a combination of unique physical, chemical and optical properties4. Most relevant to this work, is the ability of hBN to host atomic impurities (or point defects), that give rise to quantized optical transitions, well below its bandgap.5,6 hBN colour centres are ultrabright with narrow and tuneable linewidth,7-9 and photostability up to 800 K.10 Whilst the nature of many of the defects is still uncertain,11-15 they are being extensively studied as promising candidates for quantum photonic applications requiring on-demand, ultrabright single-photon emission. A step forward, which will significantly extend the functionality of hBN emitters for quantum applications, is to interface their optical properties with spin transitions, and realise spin-polarization and optical spinreadout schemes.16, 17 The concept of spin-photon interface has been extensively studied in quantum dots18 and the nitrogen-vacancy (NV) centre in diamond.19, 20 The latter has been harnessed to realise basic two-node quantum networks19 and a plethora of advanced quantum sensing schemes.21-23 The basic principle is that the triplet spin ground state of the defect can be polarised, manipulated and read out optically owing to the spin-dependent excitation, decay and intersystem crossing pathways available to the system during the optical excitation-recombination cycle.24 Yet, extending the optical control of single-spin states beyond defects in 3D crystals, to 2D systems, has remained elusive. If achieved, it will open up a stretch of novel possibilities both fundamental and technological. The two-dimensional nature of these materials inherently allows for seamless integration with heterogeneous, opto-electronic devices where the hosted solid-state qubits can be readily interfaced with cavities, resonators and nanophotonic components from foreign materials. Further, it naturally grants nanoscale proximity of the spin probe to target samples for high-resolution quantum sensing realizations. Reliable and deterministic transfer of hBN layers on stacks of other 2D materials is well-established and is part of one of the currently most relevant endeavours of condensed matter physics—engineering heterostructures made by purposefully-chosen sequences of atomically-thin 2Dmaterials.25 Here we report on the optical initialisation and readout of an ensemble of spins in hBN. We perform rigorous electron paramagnetic resonance (EPR) spectroscopy and optically detected magnetic resonance (ODMR) measurements to establish that the defect has a triplet ground state with zero field splitting (ZFS) of D/h ≈ –3.5 GHz and almost isotropic Landé factor g = 2.000. From the analysis of the angular dependence and nitrogen hyperfine structure, we confirm the intrinsic nature of the defect and assign it to the negatively-charged boron vacancy ( ). The alternative nitrogen vacancy ( ) structure was also considered, but discarded upon analysis of the experimental data (see discussion below). Figure 1a is a schematic illustration of the proposed defect. The defect is a negatively charged boron vacancy ( ) centre consisting of a missing boron atom surrounded by three equivalent nitrogen atoms in the hBN lattice. The defect has D3h point-group symmetry, typical for a substitutional defect in hBN, and exhibits a strong room temperature photoluminescence (PL) emission at λmax ≈ 850 nm, under λexc = 532 nm laser excitation (Figure 1b). Figure 1. ODMR of an hBN single crystal at room temperature, T = 300 K. a) Schematic of an hBN monolayer and its crystalline hexagonal structure with alternating boron (red) and nitrogen (blue) atoms. The green arrows indicate the spins of the negatively-charged boron-vacancy defects, . b) Photoluminescence spectrum of the sample at room temperature displaying a pronounced emission at 850 nm. c) ODMR spectra measured with zero magnetic field (bottom) and with magnetic field B = 10 mT (top); d) Dependence of ODMR frequencies and on the magnetic field (BIIc). Experimental (red) and fit (blue line) obtained using Equation 2 with parameters D/h = 3.48 GHz, E/h = 50 MHz and g=2.000. Most interestingly, we find that the PL from this hBN colour centre is spin-dependent. Figure 1c shows the ODMR spectrum recorded for an hBN single crystal at T = 300 K. In ODMR experiments, microwaveinduced magnetic dipole transitions between spin-sublevels manifest as changes in PL intensity (ΔPL). The prerequisite for optical (PL) detection of EPR is thus the existence of a dependence between the optical excitation-recombination cycle and the defect’s spin orientation. Figure 1c shows the spectrum of the investigated sample as normalized change of PL intensity (ΔPL/PL)—i.e. ODMR contrast—as a function of the applied microwave frequency for two static magnetic fields = 0 and = 10 . Even without external magnetic field, the ODMR spectrum shows two distinct resonances and , located symmetrically around the frequency . We tentatively assign them to the ΔmS = ±1 spin transitions between triplet energy sublevels with completely lifted three-fold degeneracy, due to a splitting induced by dipolar interaction between the unpaired electron spins, forming the triplet. This so-called zero field splitting is described by parameters and which can be derived from the spectrum as /h = and , = ( ± )/h. In order to verify this assignment, we have studied the dependence of the and resonant microwave frequencies on the magnitude of the external static magnetic field. The evolution of the ODMR spectrum with the field applied parallel to the hexagonal c-axis (B II c) of hBN is presented in Figure 1d. To explain the observed transitions and their variation with magnetic field, we use the standard spin Hamiltonian given by Equation 1 with z as the principle symmetry axis oriented perpendicular to the plane (collinear with the c-axis of the hBN crystal). = ( − ( + 1)/3) + − ! + "# $ ⋅ & (1) where D and E are the ZFS parameters, S = 1 is the total spin, g is the Landé factor, # is the Bohr magneton, $ is the static magnetic field and , , are the spin-1 operators. According to Equation 1 and for $ applied parallel to the c-axis the resonant microwave frequencies at which the transitions occur