Suppressing spectral diffusion of phosphorus donor electron spins in natural silicon using optical excitation*

Donor and defect electronic spins in solids are promising platforms for quantum technologies. Understanding how these potential electron spin qubit candidates decohere under different experimental conditions is key to enabling improved performance. The donor electron spins in phosphorus-doped silicon (Si:P) have some of the longest coherence times observed in solid-state spin systems. Natural silicon consists of 3 isotopes - ²⁸Si, ²⁹Si and ³⁰Si. While the 4.7% abundant ²⁹Si is a spin-1/2 nucleus, ²⁸Si and ³⁰Si are spin-0 nuclei.  Magnetic dipolar interactions between the ²⁹Si spins induce a fluctuating nuclear magnetic field at the site of the donor electron spin resulting in a decay of the electron spin coherence. This spectral diffusion due to the ²⁹Si spins is the dominant source of spin echo decay in lightly-doped natural Si:P samples at liquid helium temperatures in low magnetic fields (< 0.5 T). The elimination of the spin-1/2 ²⁹Si nuclei was seen to dramatically suppress spectral diffusion in low-field experiments on Si:P.

We characterize the spin-echo decays of phosphorus donor electron spins in lightly-doped natural silicon at liquid helium temperatures and high magnetic field (8.58 T) in the dark and under optical excitation.  The spectral diffusion times (T_SD) measured in the dark are observed to be significantly shorter and to vary more strongly with crystal orientation (in the external field) than previous measurements at low magnetic fields (0.35 T).   We also measured the echo decay as the wavelength of the optical excitation is swept across the band edge from 1047 nm to 1090 nm. At low optical powers, above-bandgap excitation is seen to increase the spectral diffusion time of the donor electron spins at most crystal orientations, while higher power optical excitation shortens both the T₁ and T₂ relaxation times of the donors significantly. Electron-nuclear double resonance experiments suggest that the optical excitation modulates the hyperfine interactions in the sample.