Entanglement between two intrinsically coherence-protected spin qubits

Understanding and protecting the coherence of individual quantum systems is a central challenge in quantum science and technology. A variety of methods to protect coherence have been demonstrated, including clock states, dynamical decoupling, quantum error correction, isotopic purification, and decoherence-protected subspaces. Here we introduce a new type of long-lived quantum system: a pair consisting of two identical coupled nuclear spins. These spin pairs naturally combine clock states with decoherence-protected subspaces making them intrinsically robust to external perturbations. We study three carbon-13 nuclear spin pairs and realize high-fidelity control and single-shot readout using a single NV center in their vicinity. We demonstrate a long inhomogeneous dephasing time, T2* = 113(18) seconds. Finally, we demonstrate complete control over these qubits by preparing an entangled state of two spin pairs through projective quantum parity measurements. The long-lived spin pairs demonstrated here are naturally abundant in diamond and other solid-state systems, and provide new opportunities for quantum bits, quantum networks and precision measurements.