Nuclear spin-wave quantum register for a solid-state qubit.

Solid-state nuclear spins surrounding individual, optically addressable qubits are a crucial resource for quantum networks, computation and simulation. Although hosts with sparse nuclear spin baths are typically chosen to mitigate qubit decoherence, developing coherent quantum systems in nuclear-spin-rich hosts enables exploration of a much broader range of materials for quantum information applications. The collective modes of these dense nuclear spin ensembles provide a natural basis for quantum storage; however, using them as a resource for single-spin qubits has thus far remained elusive. Here, by using a highly coherent, optically addressed Yb qubit doped into a nuclear-spin-rich yttrium orthovanadate crystal, we develop a robust quantum control protocol to manipulate the multi-level nuclear spin states of neighbouring V lattice ions. Via a dynamically engineered spin-exchange interaction, we polarize this nuclear spin ensemble, generate collective spin excitations, and subsequently use them to implement a quantum memory. We additionally demonstrate preparation and measurement of maximally entangled Yb-V Bell states. Unlike conventional, disordered nuclear-spin-based quantum memories, our platform is deterministic and reproducible, ensuring identical quantum registers for all Yb qubits. Our approach provides a framework for utilizing the complex structure of dense nuclear spin baths, paving the way towards building large-scale quantum networks using single rare-earth ion qubits.