  Superconducting qubits are a promising platform for building a larger-scale
quantum processor capable of solving otherwise intractable problems. In order
for the processor to reach practical viability, the gate errors need to be
further suppressed and remain stable for extended periods of time. With recent
advances in qubit control, both single- and two-qubit gate fidelities are now
in many cases limited by the coherence times of the qubits. Here we
experimentally employ closed-loop feedback to stabilize the frequency
fluctuations of a superconducting transmon qubit, thereby increasing its
coherence time by 26\% and reducing the single-qubit error rate from $(8.5 \pm
2.1)\times 10^{-4}$ to $(5.9 \pm 0.7)\times 10^{-4}$. Importantly, the
resulting high-fidelity operation remains effective even away from the qubit
flux-noise insensitive point, significantly increasing the frequency bandwidth
over which the qubit can be operated with high fidelity. This approach is
helpful in large qubit grids, where frequency crowding and parasitic
interactions between the qubits limit their performance.
