  With the long coherence time, the fixed-frequency transmon qubit is a
promising qubit modality for quantum computing. Currently, diverse qubit
architectures that utilize fixed-frequency transmon qubits have been
demonstrated with high-fidelity gate performance. Nevertheless, the relaxation
times of transmon qubits can have large temporal fluctuations, causing
instabilities in gate performance. The fluctuations are often believed to be
caused by nearly on-resonance couplings with sparse two-level-system (TLS)
defects. To mitigate their impact on qubit coherence and gate performance, one
direct approach is to tune the qubits away from these TLSs. In this work, to
combat the potential TLS-induced performance fluctuations in a tunable-bus
architecture unitizing fixed-frequency transmon qubits, we explore the
possibility of using an off-resonance microwave drive to effectively tuning the
qubit frequency through the ac-Stark shift while implementing universal gate
operations on the microwave-dressed qubit. We show that the qubit frequency can
be tuned up to 20 MHz through the ac-stark shift while keeping minimal impacts
on the qubit control. Besides passive approaches that aim to remove these TLSs
through more careful treatments of device fabrications, this work may offer an
active approach towards mitigating the TLS-induced performance fluctuations in
fixed-frequency transmon qubit devices.
