<b>SYNERGIZING SPIN DEFECTS IN WIDE-BANDGAP MATERIALS WITH MAGNETS FOR QUANTUM SENSING</b>

<p dir="ltr">Solid-state spin-defects in wide-bandgap materials have emerged as crucial building blocks for quantum technologies. Among these defects, the negatively charged Nitrogen-Vacancy (NV-) center in diamond and the negatively charged Boron-Vacancy center (Vb-) in hBN have become leading platforms for quantum sensing technologies. These defects can sensitively probe magnetic noise across a wide frequency range, spanning from DC to GHz, and detect signals from small sample volumes such as a few-layered van der Waals (vdW) flakes. Our work focuses on understanding the intrinsic properties of these defects and designing heterostructures to enhance their sensitivity to external fields for quantum sensing applications. </p><p dir="ltr">To this end, we propose a hybrid quantum system that couples spin defects with magnons —collective excitations in magnetically ordered systems. By integrating NV- centers into a multiferroic heterostructure combining ferroelectric and ferromagnetic order, we demonstrate electric-field control of the NV-magnon coupling. This design enables enhanced sensitivity of the NV- centers to DC electric fields. For quantum sensing applications, the Vb- center in hBN offers an additional advantage due to its two-dimensional nature. We demonstrate that coupling these defects with carefully designed nano-plasmonic cavities can significantly enhance their PL signal and spatial resolution. </p><p dir="ltr">For practical applications, understanding the intrinsic properties of spin-defects and the mechanisms governing their population relaxation rate (1/T1) and decoherence rate (1/T2) is critical. We investigate the broadband spin-lattice relaxation dynamics of Vb- centers over a wide temperature (3-250K) and magnetic field up to 7 Tesla. At low fields (<2 Tesla), we observe that interactions with a fast-fluctuating paramagnetic spin bath dominate the relaxation process. At higher fields (2-7 Tesla), the dynamics are primarily governed by first-order spin-phonon coupling. Overall, our results lay the essential groundwork for designing quantum sensors capable of operating at high magnetic fields. </p>