Fast-flux tunable drive filter for superconducting qubits enabling control, reset, and thermometry.

We present an experimental implementation of a tunable-frequency drive filter that addresses the challenges of fast control, long coherence, and reduced thermal load in superconducting-qubit architectures that traditionally use weakly-coupled drive lines. The filter integrates a series dc-SQUID array into a quarter-wave stopband structure on the qubit's drive line, enabling \textit{in situ} tuning of the qubit--drive coupling with fast-flux pulses on the tens-of-nanoseconds scale. This fast tunability supports protected control, reset, and precision thermometry on a single device.

We demonstrate: (i) dynamic control of relaxation, tuning $T_1$ from $\sim 200~\text{ns}$ to $>150~\mu\text{s}$, accompanied by a comparable ($\sim 10^3$) modulation of the Rabi frequency; (ii) modest chip-level drive power ($\lesssim -110~\text{dBm}$) for $40~\text{ns}$ single-qubit gates with $0.999$ standard randomized-benchmarking fidelity and $0.9999$ idle-gate fidelity---a $10\times$ improvement over a conventional line at equal coupling strength; (iii) sub-$\mu\text{s}$ reset achieving ground-state populations of $>97\%$, $95\%$, and $91\%$ when initialized in $\lvert \text{e}\rangle$, $\lvert \text{f}\rangle$, and $\lvert \text{h}\rangle$, respectively; and (iv) qubit thermometry with noise-equivalent temperature $\approx 0.6~\text{mK}/\sqrt{\text{Hz}}$ at $\sim 65~\text{mK}$, approaching the Cram\'er--Rao bound at higher temperatures.

Together, these results show that a tunable stopband on the drive line can resolve fast gates with long lifetimes while lowering required control power, providing a practical path to scalable, low-dissipation operation and new metrological capabilities in superconducting quantum processors.