Hardware efficient methods for high fidelity quantum state measurements are crucial for superconducting qubit experiments, as qubit numbers grow and feedback and state reset begin tobe employed for quantum error correction. We present a 3D re-entrant cavity filter designed for frequency-multiplexed readout of superconducting qubits. The cavity filter is situated out of the plane of the qubit circuit and capacitively couples to an array of on-chip readout resonators in a manner that can scale to large qubit arrays. The re-entrant cavity functions as a large-linewidth bandpass filter with intrinsic Purcell filtering. We demonstrate the concept with a four-qubit multiplexed device.
Correlated errors in superconducting circuits due to nonequilibrium quasiparticles are a notable concern in efforts to achieve fault tolerant quantum computing. The propagation of quasiparticlescausing these correlated errors can potentially be mediated by phonons in the substrate. Therefore, methods that decouple devices from the substrate are possible solutions, such as isolating devices atop SiN membranes. In this work, we validate the compatibility of SiN membrane technology with high quality superconducting circuits, adding the technique to the community’s fabrication toolbox. We do so by fabricating superconducting coplanar waveguide resonators entirely atop a thin (∼110 nm) SiN layer, where the bulk Si originally supporting it has been etched away, achieving a suspended membrane where the shortest length to its thickness yields an aspect ratio of approximately 7.4×103. We compare these membrane resonators to on-substrate resonators on the same chip, finding similar internal quality factors ∼105 at single photon levels. Furthermore, we confirm that these membranes do not adversely affect the resonator thermalization rate. With these important benchmarks validated, this technique can be extended to qubits.