  Nanofabricated quantum bits permit large-scale integration but usually suffer
from short coherence times due to interactions with their solid-state
environment. The outstanding challenge is to engineer the environment so that
it minimally affects the qubit, but still allows qubit control and scalability.
Here we demonstrate a long-lived single-electron spin qubit in a Si/SiGe
quantum dot with all-electrical two-axis control. The spin is driven by
resonant microwave electric fields in a transverse magnetic field gradient from
a local micromagnet, and the spin state is read out in single-shot mode.
Electron spin resonance occurs at two closely spaced frequencies, which we
attribute to two valley states. Thanks to the weak hyperfine coupling in
silicon, Ramsey and Hahn echo decay timescales of 1us and 40us, respectively,
are observed. This is almost two orders of magnitude longer than the intrinsic
timescales in III-V quantum dots, while gate operation times are comparable to
those achieved in GaAs. This places the single-qubit rotations in the
fault-tolerant regime and strongly raises the prospects of quantum information
processing based on quantum dots.
