  Isolated spins in semiconductors provide a promising platform to explore
quantum mechanical coherence and develop engineered quantum systems. Silicon
has attracted great interest as a host material for developing spin qubits
because of its weak spin-orbit coupling and hyperfine interaction, and several
architectures based on gate defined quantum dots have been proposed and
demonstrated experimentally. Recently, a quantum dot hybrid qubit formed by
three electrons in double quantum dot was proposed, and non-adiabatic
pulsed-gate operation was implemented experimentally, demonstrating simple and
fast electrical manipulations of spin states with a promising ratio of
coherence time to manipulation time. However, the overall gate fidelity of the
pulse-gated hybrid qubit is limited by relatively fast dephasing due to charge
noise during one of the two required gate operations. Here we perform the first
microwave-driven gate operations of a quantum dot hybrid qubit, avoiding
entirely the regime in which it is most sensitive to charge noise. Resonant
detuning modulation along with phase control of the microwaves enables a pi
rotation time of less than 5 ns (50 ps) around X(Z)-axis with high fidelities >
93 (96) %. We also implement Hahn echo and Carr-Purcell (CP) dynamic decoupling
sequences with which we demonstrate a coherence time of over 150 ns. We further
discuss a pathway to improve gate fidelity to above 99 %, exceeding the
threshold for surface code based quantum error correction.
