  Solid-state electronic spin systems such as nitrogen-vacancy (NV) color
centers in diamond are promising for applications of quantum information,
sensing, and metrology. However, a key challenge for such solid-state systems
is to realize a spin coherence time that is much longer than the time for
quantum spin manipulation protocols. Here we demonstrate an improvement of more
than two orders of magnitude in the spin coherence time ($T_2$) of NV centers
compared to previous measurements: $T_2 \approx 0.5$ s at 77 K, which enables
$\sim 10^7$ coherent NV spin manipulations before decoherence. We employed
dynamical decoupling pulse sequences to suppress NV spin decoherence due to
magnetic noise, and found that $T_2$ is limited to approximately half of the
longitudinal spin relaxation time ($T_1$) over a wide range of temperatures,
which we attribute to phonon-induced decoherence. Our results apply to
ensembles of NV spins and do not depend on the optimal choice of a specific NV,
which could advance quantum sensing, enable squeezing and many-body
entanglement in solid-state spin ensembles, and open a path to simulating a
wide range of driven, interaction-dominated quantum many-body Hamiltonians.
