  Single electron spins confined in silicon quantum dots hold great promise as
a quantum computing architecture with demonstrations of long coherence times,
high-fidelity quantum logic gates, basic quantum algorithms and device
scalability. While single-shot spin detection is now a laboratory routine, the
need for quantum error correction in a large-scale quantum computing device
demands a quantum non-demolition (QND) implementation. Unlike conventional
counterparts, the QND spin readout imposes minimal disturbance to the probed
spin polarization and can therefore be repeated to extinguish measurement
errors. However, it has remained elusive for an electron spin in silicon as it
involves exquisite exposure of the system to the external circuitry for readout
while maintaining the coherence and integrity of the qubit. Here we show that
an electron spin qubit in silicon can be measured in a highly non-demolition
manner by probing another electron spin in a neighboring dot Ising-coupled to
the qubit spin. The high non-demolition fidelity (99% on average) enables over
20 readout repetitions of a single spin state, yielding an overall average
measurement fidelity of up to 95% within 1.2 ms. We further demonstrate that
our repetitive QND readout protocol can realize heralded high-fidelity (>
99.6%) ground-state preparation. Our QND-based measurement and preparation,
mediated by a second qubit of the same kind, will allow for a new class of
quantum information protocols with electron spins in silicon without
compromising the architectural homogeneity.
