Hybrid 2D/3D integrated readout multiplexing in circuit QED

As the field of superconducting quantum circuits progresses to large scale devices, methods for readout multiplexing are required to reduce hardware overheads. Typically, reported architectures use on-chip integration to address readout resonators, which can introduce unwanted circuit crosstalk. Here we present a method to multiplex readout resonators out-of-plane, featuring off-chip 3D integration and Purcell filtering of qubit radiative decay. We demonstrate the method in a prototype device, performing multiplexed readout of a four transmon device in a single 1.6 GHz wide readout channel, characterising the readout crosstalk and qubit coherence. Measurements of T1>200 µs inside the filter band demonstrate the compatibility of this 3D-integrated multiplexing architecture with high coherence multi-qubit devices.