UCLA

To build up large-scale quantum systems, four challenges in the existing hardware realization need to be addressed: (1) I/O management, (2) latency and phase matching in a large system footprint, (3) the heat and power dissipation as the qubit count scales up, and (4) transmission of delicate quantum information. To minimize these issues, there are efforts using different architectures, such as CryoCMOS, 3D stacking, and Josephson-Junction-based (JJ-based) approach. Superconducting Silicon Interconnect Fabric (Superconducting-IF) is yet another promising approach, which is an advanced fine-pitch and wafer-scale cryogenic packaging platform. The proposed architecture, which aims to co-integrate superconducting qubit control/readout with qubits in a compact way, and the demonstration of the Superconducting-IF are detailed. The key enabler of Superconducting-IF is the low-temperature assembly technology, the Au interlayer bonding method, which is demonstrated to be fine-pitch (≤ 10 um), mechanically robust (> 30 MPa), electrically reliable, and quantum-compatible (< 150℃). The Au interlayer with Nb superconducting interconnects has a transition temperature at 9 K and is demonstrated to be a low-temperature-compatible process having a high critical current for integrated superconducting processing applications. On the Superconducting-IF platform, heterogeneity and flexibility are both demonstrated regarding the die sizes, the inter-die spacing, and the configuration. For future transmission of delicate quantum information, Superconducting RF optimization, including insertion loss and crosstalk, is implemented. All the simulated and measured results on short superconducting links (≤ 500 um) with 2 and 5 um line/space are far below the quantum limitation, meaning the superconducting interconnects are suitable to carry future inter-dielet delicate quantum communication. This dissertation achieves the following intellectual contributions: (1) the first combination of heterogeneous integration and advanced packaging with quantum applications, (2) the first proposal of the architecture of integrated system-on-wafer quantum hardware, (3) the first demonstration of the Au interlayer bonding to be fine-I/O pitch, mechanically robust, electrically reliable, and quantum compatible, and (4) the first achievement of low-loss, low-crosstalk, and 20 GHz-broadband RF capability on superconducting short links through advanced packaging. To sum up, it is promising to use the Superconducting-IF for future large-scale quantum systems to realize the full strength of quantum computing.