Qubits in isotopically enriched 28Si

• Applicant: Professor Dr. Michael Oestreich
• Subject Area: Experimental Condensed Matter Physics
• Term: since 2022
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 496720564

Isotopically enriched 28Si is a fascinating material system for spin-based quantum devices. Electron and nuclear qubits using shallow donors in phosphorous doped 28Si offer extremely long spin relaxation and dephasing times and are thereby among the most promising candidates for a semiconductor quantum information technology. One of the major challenges of quantum logic in 28Si:P is of course the electron and nuclear spin initialization which has been achieved by Yang et al. by selective optical pumping of the phosphorous donor transitions. The linewidths of these optical transitions are among the narrowest linewidths of shallow donor ensembles in semiconductors and the experimentally demonstrated degree of optical spin initialization is rather high. However, the degree of spin polarization is well below the desirable 100 % so far which is most likely linked to the observation of spectral holes burned into the donor transitions.The first goal of this proposal is a profound understanding of the non-trivial physics of this hole burning dynamics. The group of Thewalt observed in continuous wave experiments minimal hole burning linewidths which are a factor of four larger than calculated from the donor bound trion recombination time. The cause is unknown but might be attributed to a combination of (a) interaction with optically excited free electrons and holes, (b) fluctuating internal electric fields due to unavoidable, unintentional, p-type background doping, and (c) electron-electron interaction. We will apply a special time-resolved pump-probe absorption setup to determine quantitatively the different contributions and use this information (a) to optimize the optically induced spin polarization to well above 99% and (b) to write energetically separated spin sub-ensembles into the donor bound electron ensemble. The second goal of this proposal is entanglement of macroscopic donor spin ensembles. We will use the optimized spin initialization and write two, spatially separated, macroscopic spin ensembles into a high quality 28Si sample. Afterwards, we entangle these spin ensembles by an optical pulse and verify the entanglement by a spin noise like technique. Entanglement of a donor-bound electron spin ensemble with the respective nuclear spin ensemble has been demonstrated before but entanglement of spatially separated macroscopic donor electron spin ensembles has not been established in 28Si so far, to the best of our knowledge.Phosphorous donors in 28Si are excellent candidates for qubits but single optical defects which are isolated and bright and emitting at identical wavelengths in the telecom bands would enable additionally the implementation of scalable silicon quantum photonics. Very recent experiments on carbon implanted Si report on such single photon emitter but with strongly varying wavelengths. The search for single photon emitters with identical wavelengths in high quality 28Si will be the third task of this proposal.

• DFG Programme: Research Grants