  Flux-tunable qubits are a useful resource for superconducting quantum
processors. They can be used to perform cPhase gates, facilitate fast reset
protocols, avoid qubit-frequency collisions in large processors, and enable
certain fast readout schemes. However, flux-tunable qubits suffer from a
trade-off between their tunability range and sensitivity to flux noise.
Optimizing this trade-off is particularly important for enabling fast,
high-fidelity, all-microwave cross-resonance gates in large, high-coherence
processors. This is mainly because cross-resonance gates set stringent
conditions on the frequency landscape of neighboring qubits, which are
difficult to satisfy with non-tunable transmons due to their relatively large
fabrication imprecision. To solve this problem, we realize a coherent,
flux-tunable, transmon-like qubit, which exhibits a frequency tunability range
as small as 43 MHz, and whose frequency, anharmonicity and tunability range are
set by a few experimentally achievable design parameters. Such a weakly tunable
qubit is useful for avoiding frequency collisions in a large lattice while
limiting its susceptibility to flux noise.
