  We study the electron spin decoherence of encapsulated atomic hydrogen in
octasilsesquioxane cages induced by the 1H and 29Si nuclear spin bath. By
applying the Carr-Purcell-Meiboom-Gill (CPMG) pulse sequence we significantly
suppress the low-frequency noise due to nuclear spin flip-flops up to the point
where a maximum T2 = 56 us is observed. Moreover, dynamical decoupling with the
CPMG sequence reveals the existence of two sources of high-frequency noise:
first, a fluctuating magnetic field with the proton Larmor frequency,
equivalent to classical magnetic field noise imposed by the 1H nuclear spins of
the cage organic substituents, and second, decoherence due to entanglement
between the electron and the inner 29Si nuclear spin of the cage.
