Solid-state electronic spins are extensively studied in quantum information science, as their large magnetic moments offer fast operations for computing¹ and communication^2–4, and high sensitivity for sensing⁵. However, electronic spins are more sensitive to magnetic noise, but engineering of their spectroscopic properties, for example, using clock transitions and isotopic engineering, can yield remarkable spin coherence times, as for electronic spins in GaAs⁶, donors in silicon^7–11 and vacancy centres in diamond^12,13. Here we demonstrate simultaneously induced clock transitions for both microwave and optical domains in an isotopically purified ¹⁷¹Yb³⁺:Y₂SiO₅ crystal, reaching coherence times of greater than 100 μs and 1 ms in the optical and microwave domains, respectively. This effect is due to the highly anisotropic hyperfine interaction, which makes each electronic–nuclear state an entangled Bell state. Our results underline the potential of ¹⁷¹Yb³⁺:Y₂SiO₅ for quantum processing applications relying on both optical and spin manipulation, such as optical quantum memories^4,14, microwave-to-optical quantum transducers^15,16, and single-spin detection¹⁷, while they should also be observable in a range of different materials with anisotropic hyperfine interactions.