  Understanding and protecting the coherence of individual quantum systems is a
central challenge in quantum science and technology. Over the last decades, a
rich variety of methods to extend coherence have been developed. A
complementary approach is to look for naturally occurring systems that are
inherently protected against decoherence. Here, we show that pairs of identical
nuclear spins in solids form intrinsically long-lived quantum systems. We study
three carbon-13 pairs in diamond and realize high-fidelity measurements of
their quantum states using a single NV center in their vicinity. We then reveal
that the spin pairs are robust to external perturbations due to a unique
combination of three phenomena: a clock transition, a decoherence-free
subspace, and a variant on motional narrowing. The resulting inhomogeneous
dephasing time is $T_2^* = 1.9(3)$ minutes, the longest reported for
individually controlled qubits. Finally, we develop complete control and
realize an entangled state between two spin-pair qubits through projective
parity measurements. These long-lived qubits are abundantly present in diamond
and other solids, and provide new opportunities for quantum sensing, quantum
information processing, and quantum networks.
