Long Electron Spin Coherence Times of Atomic Hydrogen Trapped in Silsesquioxane Cages

Encapsulated atomic hydrogen in cube-shaped octasilsesquioxane (POSS) cages of the Si8O12R8 type (where R is an organic group) is one of the simplest alternative stable systems to paramagnetic endofullerenes that have been regarded as key elements of spin-based quantum technologies. Apart from common sources of decoherence such as nuclear spin and spectral diffusion, all H@POSS species studied so far suffer from additional shortening of T-2 at low temperatures due to methyl group rotations. Here we eliminate this factor for the first time by studying the smallest methyl-free derivative with R = H, namely, H@T8H8. By applying dynamical decoupling methods, we measure electron spin coherence times T-2 up to 280 +/- 76 mu s at T = 90 K and observe a linear dependence of the decoherence rate 1/T-2 on trapped hydrogen concentrations, which we attribute to the spin dephasing mechanism of instantaneous diffusion and a nonuniform spatial distribution of encapsulated H atoms.