Design and Simulation of BiCMOS Readout Chain for Dispersive Reflectometry of Superconducting Transmon Qubits

The development of scalable superconducting quantum processors requires highly integrated control and readout electronics capable of operating under stringent noise, power, and bandwidth constraints. In this context, this thesis investigates the design and simulation of a cryogenic microwave readout architecture for superconducting transmon qubits, implemented in BiCMOS technology and based on reflectometry. The work begins with the analysis of the behaviour of semiconductor devices at cryogenic temperatures, with particular emphasis on the electrical parameters relevant to the design of low-noise microwave circuits. Based on this analysis, two low-noise amplifier architectures are designed and simulated: a cascode based amplifier and a common-emitter multi-stage amplifier. Their performance is evaluated in terms of gain, noise figure, input and output matching, stability, bandwidth, and power consumption, with the aim of identifying their suitability for cryogenic qubit-readout applications. The thesis also addresses the design and simulation of a cryogenic integrated circulator and its associated support circuitry. In particular, the proposed architecture includes digital and mixed-signal building blocks such as a frequency divider and an active balun, which provide the control and signal-conversion functions required for the operation and interfacing of the microwave readout system. At the device and quantum-model level, Josephson junctions and superconducting transmon qubits have been modelled in Verilog-A. These models reproduce the relevant nonlinear and state-dependent behaviour of the quantum devices while enabling their integration into a conventional circuit-simulation environment. The Josephson-junction model is used to describe the nonlinear superconducting element, whereas the transmon model captures the qubit-dependent interaction with the readout resonator. Finally, all the developed models and circuit blocks have been integrated into a complete simulation environment to reproduce the reflectometric readout of the transmon qubit. The qubit is considered as a static system occupying one of two possible states, and the corresponding microwave response of the readout chain is analysed. The resulting simulations demonstrate how the qubit state is encoded in the reflected signal and assess the capability of the proposed BiCMOS architecture to distinguish the two states through amplitude and phase variations. The study therefore provides a circuit-level framework for investigating the co-design of cryogenic electronics and superconducting quantum devices, supporting the development of compact and scalable readout interfaces for future quantum processors.