Development of a coaxial circuit QED architecture for quantum computing

Superconducting circuit QED is a promising approach for building a quantum computer. In order to realise superconducting circuits at a sufficient scale for useful near-term applications, an architecture with an extensible design is required which implements good connectivity between qubits, and allows for selective readout and control of the qubits without introducing detrimental crosstalk or decoherence. This thesis describes the development of a new coaxial circuit QED architecture that fulfils this requirement of extensibility by incorporating out-of-plane wiring into the sample holder. Single-qubit unit cells consisting of a transmon qubit and readout resonator with coaxial geometries are fabricated on opposing sides of a substrate, and selective control and readout of the qubits is achieved via a capacitance to coaxial wiring built into the device enclosure. Unit cells of qubit and resonator can be arranged in a 2D array without modification of the wiring scheme. A single-qubit unit cell of this architecture is used to implement dispersive circuit QED, and a full characterisation of the Hamiltonian is performed. The device is shown to have parameters comparable to those found in other approaches, such as a coupling between qubit-resonator of ~100 MHz and a coherence time of order ~10 µs. The extension of this scheme to 2D arrays of qubits is then presented, and realizations of two-qubits gates are demonstrated with fidelities all above 87% on a four qubit device. Further evaluations are performed on multi-qubit devices, including a characterisation of the drive isolation of the mode-matched drive ports, finding values in the range of 50 dB and 30 dB for measurement and control respectively. Similarly, the cross coupling between circuits is shown to have values ~2% of the coupling within a unit cell. The effective circuit temperatures are measured, finding typical values of ~100 mK, and the techniques of spin-locking and T2 spectroscopy are employed to probe the noise environment. Finally the architecture is extended to incorporate frequency tuning of qubits with gradiometric SQUID loops by way of off-chip flux bias lines (FBLs). These lines are used to tune qubits with a signal isolation of >99%. Furthermore, the ability of these FBLs to dynamically control the qubit frequency is shown by demonstrating switching of the frequency on a nanosecond time-scale, and parametric driving over a frequency range of gigahertz.