3D-integrated off-chip flux tuning of coaxial transmons with low crosstalk

Flux tuning is an important ingredient in superconducting quantum circuits, particularly for the operation of fast, high-fidelity entangling gates. Conventional circuit QED implementations use on-chip flux bias lines (FBLs) to achieve fast local biasing of circuit elements, such as transmon qubits and tunable couplers. These FBLs can suffer from high levels of non-local crosstalk due to uncontrolled current flow in the ground plane of the device, as well as substrate heating. Here we demonstrate a 3D-integrated architecture for off-chip local flux tuning of gradiometric coaxial transmon qubits, with low crosstalk and large bandwidth. The high selectivity of our FBL design is achieved by optimally matching its geometry and orientation to the circuit on the chip. We characterize flux crosstalk, bandwidth and noise properties of this system, and show that it is compatible with the operation of fast entangling interactions, such as parametrically activated gates. Additionally, this FBL implementation considerably reduces device complexity by removing the need for bonding, ground planes and air bridges on the substrate.