Fig. 3: PL enhancement of V_B^- defects coupled to qBIC resonances and optical spin read-out.

a Experimental transmission spectra of the hBN qBIC metasurfaces as a function of the scaling factor S. b V_B^- PL intensity when coupled to metasurfaces with varying scaling factors. The PL emission of the coupled defects is maximized for resonant coupling with the qBIC spectral position. c PL spectra of coupled defects for the full set of metasurfaces. In gray: reference V_B^- PL spectra, normalized over the unit cell filling factor and multiplied by a factor of 75. The resonant enhancement of the qBIC metasurface shows excellent overlap with the V_B^- PL emission spectrum. d Integrated PL intensity of the coupled system for different scaling factors. The PL is integrated over a 20 nm spectral window positioned around the PL maximum of the coupled defects. In black, the reference hBN integrated PL intensity values, normalized over the unit cell filling factor, obtained by integrating over the same 20 nm spectral window. e PL emission from V_B^- defects coupled to an hBN metasurface (in red) with a scaling factor of S = 1.39. The spectrum is fitted with a single Lorentzian peak (black dashed curve) showing a full width at half maximum (FWHM) below 4 nm. f Optically detected magnetic resonance (ODMR) contrast of the V_B^- defects for the reference hBN crystal. The solid line represents the averaged values. g ODMR spectrum of V_B^- defects coupled to a qBIC metasurface. The black dashed line is a fit to a two Lorentzian function, centered at 3.48 GHz. Inset: level scheme of the triplet ground state of the V_B^- defect. Both spectra are integrated for 150 sweeps with an integration time of 1 ms at each microwave (MW) frequency. The PL is filtered with a 50 nm bandpass filter centered around 775 nm.