Single-atom spin qubits in silicon

Spin qubits in silicon are excellent candidates for scalable quantum information processing (QIP) due to their long coherence times and the enormous investment in silicon MOS technology. Here I discuss qubits based upon single phosphorus (P) dopant atoms in Si [1]. Projective readout of such qubits had proved challenging until single-shot measurement of a single donor electron spin was demonstrated [2] using a silicon single electron transistor (Si-SET) and the process of spin-to-charge conversion. The measurement gave readout fidelities \textgreater\ 90{\%} and spin lifetimes T$_{\mathrm{1e}}$ \textgreater\ 6 s [2], opening the path to demonstration of electron and nuclear spin qubits in silicon. Integrating an on-chip microwave transmission line enabled single-electron spin resonance (ESR) of the P donor electron. We used this to demonstrate Rabi oscillations of the electron spin qubit, while a Hahn echo sequence revealed electron spin coherence times T$_{\mathrm{2e}}$ \textgreater\ 0.2 ms [3]. This time is expected to become much longer in isotopically enriched $^{28}$Si devices. We also achieved single-shot readout of the $^{31}$P nuclear spin (with fidelity \textgreater\ 99.6{\%}) by monitoring the two hyperfine-split ESR lines of the P donor system. By applying (local) NMR pulses we demonstrated coherent control of the nuclear spin qubit, giving a coherence time T$_{\mathrm{2n}}$ \textgreater\ 60 ms. \\[4pt] [1] B.E. Kane, \textit{Nature} \textbf{393}, 133 (1998). \newline [2] A. Morello et al., \textit{Nature} \textbf{467}, 687 (2010). \newline [3] J.J. Pla et al., \textit{Nature} \textbf{489}, 541 (2012).