Decoherence of V$_{\rm B}^{-}$ spin defects in monoisotopic hexagonal boron nitride

TL;DR

This study investigates the isotope-dependent decoherence of V$_{ m B}^{-}$ spin defects in monoisotopic hexagonal boron nitride, revealing slight coherence improvements in $^{10}$B-enriched samples and identifying dark impurities as decoherence sources.

Contribution

It confirms the V$_{ m B}^{-}$ defect as the studied spin system and demonstrates isotope-dependent coherence variations supported by numerical simulations and spectroscopy.

Findings

Slightly improved spin coherence in $^{10}$B-enriched hBN.

Hyperfine structure confirms V$_{ m B}^{-}$ as the defect.

Dark electron spin impurities contribute to decoherence.

Abstract

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 $^{10}$B or $^{11}$B 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 unambiguously confirm that it corresponds to the negatively-charged boron-vacancy center (${\rm V}_{\rm B}^-$). We then show that its spin coherence properties are slightly improved in $^{10}$B-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 ${\rm V}_{\rm B}^-$ spin defects, which are valuable for the future development of hBN-based quantum sensing foils.