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Electron spin coherence on a solid neon surface

A single electron floating on the surface of a condensed noble-gas liquid or solid can act as a spin qubit with ultralong coherence time, thanks to the extraordinary purity of such systems. Previous studies suggest that the electron spin coherence time on a superfluid helium (He) surface can exceed 100 s. In this paper, we present theoretical studies of the electron spin coherence on a solid neon (Ne) surface, motivated by our recent experimental realization of single-electron charge qubit on solid Ne. The major spin decoherence mechanisms investigated include the fluctuating Ne diamagnetic susceptibility due to thermal phonons, the fluctuating thermal current in normal metal electrodes, and the quasi-statically fluctuating nuclear spins of the $^{21}$Ne ensemble. We find that at a typical experimental temperature about 10 mK in a fully superconducting device, the electron spin decoherence is dominated by the third mechanism via electron-nuclear spin-spin interaction. For natural Ne with 2700 ppm abundance of $^{21}$Ne, the estimated inhomogeneous dephasing time $T_{2}^{*}$ is around 0.16 ms, already better than most semiconductor quantum-dot spin qubits. For commercially available, isotopically purified Ne with 1 ppm of $^{21}$Ne, $T_{2}^{*}$ can be $0.43$ s. Under the application of Hahn echoes, the coherence time $T_{2}$ can be improved to $30$ ms for natural Ne and $81$ s for purified Ne. Therefore, the single-electron spin qubits on solid Ne can serve as promising new spin qubits.