Decoherence of V-B(-) spin defects in monoisotopic hexagonal boron nitride

Spin defects in hexagonal boron nitride (hBN) are promising quantum systems for the design of flexible two-dimensional quantum sensing platforms. Here we rely on hBN crystals isotopically enriched with either B-10 or B-11 to investigate the isotope-dependent properties of a spin defect featuring a broadband photoluminescence signal in the near infrared. By analyzing the hyperfine structure of the spin defect while changing the boron isotope, we first confirm that it corresponds to the negatively charged boron-vacancy center (V-B(-)). We then show that its spin coherence properties are slightly improved in B-10-enriched samples. This is supported by numerical simulations employing cluster correlation expansion methods, which reveal the importance of the hyperfine Fermi contact term for calculating the coherence time of point defects in hBN. Using cross-relaxation spectroscopy, we finally identify dark electron spin impurities as an additional source of decoherence. This work provides new insights into the properties of V-B(-) spin defects, which are valuable for the future development of hBN-based quantum sensing foils. ; Funding Agencies|French Agence Nationale de la Recherche under the program ESR/EquipEx+ [ANR-21-ESRE-0025]; Institute for Quantum Technologies in Occitanie through the project BONIQs; European Union H2020 Quantum Flagship Program [820394]; U.S. Department of Energy, Office of Nuclear Energy under DOE Idaho Operations Office, Nuclear Science User Facilities experiment [DE-AC07-051D14517]; Materials Engineering and Processing program of the National Science Foundation [CMMI 1538127]; Ohio State University Nuclear Reactor Laboratory; MTA Premium Postdoctoral Research Program; Knut and Alice Wallenberg Foundation through WBSQD2 project [2018.0071]; National Research, Development, and Innovation Office of Hungary of the National Excellence Program of Quantum-Coherent Materials Project [KKP129866]; Quantum Information National Laboratory - Ministry of Innovation and Technology of Hungary