Homogeneous linewidth of the P31 bound exciton transition in silicon

The optical transitions of the shallow donor bound exciton associated with phosphorus in silicon are a subject of renewed interest due to the recent discovery that these transitions can be used to both read out and initialize the donor electron and nuclear spin in highly enriched S28i. The ultimate limit of these processes will be determined by the natural or homogeneous linewidth which we determine here using spectral hole burning. The observed 10 neV linewidth is only four times the limit set by the bound exciton lifetime.