  Combining highly coherent spin control with efficient light-matter coupling
offers great opportunities for quantum communication and networks, as well as
quantum computing. Optically active semiconductor quantum dots have
unparalleled photonic properties, but also modest spin coherence limited by
their resident nuclei. Here, we demonstrate that eliminating strain
inhomogeneity using lattice-matched GaAs-AlGaAs quantum dot devices prolongs
the electron spin coherence by nearly two orders of magnitude, beyond 0.113(3)
ms. To do this, we leverage the 99.30(5)% fidelity of our optical pi-pulse
gates to implement dynamical decoupling. We vary the number of decoupling
pulses up to N = 81 and find a coherence time scaling of N^{0.75(2)}. This
scaling manifests an ideal refocusing of strong interactions between the
electron and the nuclear-spin ensemble, holding the promise of lifetime-limited
spin coherence. Our findings demonstrate that the most punishing material
science challenge for such quantum-dot devices has a remedy, and constitute the
basis for highly coherent spin-photon interfaces.
