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

TL;DR

This study demonstrates long electron spin coherence times in atomic hydrogen encapsulated in methyl-free POSS cages, revealing minimal decoherence mechanisms and potential for quantum technology applications.

Contribution

First measurement of electron spin coherence times in methyl-free H@POSS cages, eliminating methyl rotation effects and analyzing decoherence mechanisms at low temperatures.

Findings

Achieved electron spin coherence times up to 280 μs at 90 K.

Identified linear dependence of decoherence rate on hydrogen concentration.

Suppressed nuclear spin diffusion using dynamical decoupling methods.

Abstract

Encapsulated atomic hydrogen in cube-shaped octa-silsesquioxane (POSS) cages of the Si$_8$O$_{12}$R$_8$ type (where R is an organic group) is the simplest alternative stable system to paramagnetic endohedral fullerenes (N@C$_{60}$ or P@C$_{60}$) that have been regarded as key elements of spin-based quantum technologies. Apart from common sources of decoherence like 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 relaxation properties of the smallest methyl-free derivative of this family with R=H, namely H@T$_8$H$_8$. We suppress nuclear spin diffusion by applying dynamical decoupling methods and we measure electron spin coherence times $T_2$ up to 280 $\pm$ 76 $\mu$s at $T=90$ K. We observe a linear dependence of the decoherence rate $1/T_2$ on trapped hydrogen concentrations ranging between 9$\times 10^{14}$ cm$^{-3}$ and 5$\times 10^{15}$ cm$^{-3}$ which we attribute to the spin dephasing mechanism of instantaneous diffusion and a nonuniform spatial distribution of encapsulated H atoms.