  Electron spins confined in quantum dots are an attractive system to realize
high-fidelity qubits owing to their long coherence time. With the prolonged
spin coherence time, however, the control fidelity can be limited by systematic
errors rather than decoherence, making characterization and suppression of
their influence crucial for further improvement. Here we report that the
control fidelity of Si/SiGe spin qubits can be limited by the microwave-induced
frequency shift of electric dipole spin resonance and it can be improved by
optimization of control pulses. As we increase the control microwave amplitude,
we observe a shift of the qubit resonance frequency, in addition to the
increasing Rabi frequency. We reveal that this limits control fidelity with a
conventional amplitude-modulated microwave pulse below 99.8%. In order to
achieve a gate fidelity > 99.9%, we introduce a quadrature control method, and
validate this approach experimentally by randomized benchmarking. Our finding
facilitates realization of an ultra-high fidelity qubit with electron spins in
quantum dots.
