Coherent control of a high-spin-nucleus detected via microwave photon counting - Part 1

Nuclear spins in the solid state are a promising platform for quantum computing due to the large coherence they exhibit. These spins are weakly coupled to their environment and a common way to address them is to couple them to an individual paramagnetic impurity. This ancilla qubit is then used for controlling and reading-out the state of the nuclear spin. Here, we present a proof-of-principle quantum computing platform using a Nb 9/2-spin system coupled via hyperfine interaction to a nearby Er³⁺ ion in a CaWO₄ crystal. The electron spin is coupled to a superconducting resonator to enhance the spin's microwave fluorescence, measured with a Single Microwave Photon Detector (SMPD) at 10 mK. We use stimulated Raman driving to coherently control the state of the nuclear spin. The techniques presented here are not limited to this specific system and many other choices of substrate and paramagnetic impurity can be envisaged. Our results pave the way for quantum computing using high-spin systems and demonstrate the potential of nuclear spins in the solid state for storing quantum information.