3D integration of a hybrid quantum dot circuit-QED device for fast gate dispersive charge readout and coherent spin-photon coupling

Hybrid circuit quantum electrodynamics (cQED) aims at coupling various quantum degrees of freedom, among which are spin and charge degrees of freedom in gate defined quantum dots, phonons or magnons... with quantized electromagnetic fields in superconducting microwave cavities to investigate fundamental physics questions or for quantum computation and simulation. However, low microwave losses, key for many hybrid cQED experiments, are challenging to achieve given the often exotic and/or complex material stacks (e.g. semiconducting material, ferromagnets, or piezoelectric materials) required to host the various quantum degrees of freedom. In this work, we present a 3D-integration process to overcome this challenge for semi-industrial silicon MOS spin qubits. The process is based on dense indium bump interconnects at a pitch of 10 $\mathrmμ$m and superconducting thin films of Niobium Nitride (NbN). First, we report on DC and RF interconnect properties that demonstrate a high galvanic interconnection yield and internal quality factors above $10^{5}$ in the single photon regime for NbN resonators interrupted by a single indium bump interconnect. Eventually, we fabricated a 3D-integrated hybrid circuit quantum electrodynamics (cQED) device based on a semi-industrial MOS hole double quantum dot and a high impedance NbN resonator. For this device, we report a cavity internal quality factor above 10000 and demonstrate record sensitivity for gate-based dispersive readout of the charge degree of freedom with an SNR of 100 in 300 ns. Finally, we demonstrate strong spin-photon coupling of $g_{s}/2π$ = 75 MHz, which highlights the viability of 3D-integration for quantum dot based hybrid spin circuit quantum electrodynamics and opens to high-fidelity spin readout and microwave photon-based remote spin qubit entanglement.