Abstract
One of the most promising approaches to building a large-scale quantum computer is to encode qubits in the spin degree of freedom in silicon. Spin states in silicon have been shown to maintain coherence for timescales on the order of 10 milliseconds, allowing plenty of time for quantum operations to be performed. Small-scale devices have recently demonstrated single and two-qubit fidelities above the threshold for quantum error-correction protocols. The next challenge is to develop devices compatible with industrial fabrication that achieve similar performance, which can then be scaled up by exploiting VLSI capabilities of modern foundries.This thesis focuses on the problem of qubit readout, which is performed by spin-to-charge conversion in conjunction with a charge sensor. Gate-based dispersive readout is among the most scalable of approaches, for the reason that it does not require doped regions near to the qubit. The technique is performed by connecting an electrical resonator to a nearby gate electrode, then charge transitions can be detected via shifts in the resonant frequency. Both semi-classical and circuit-QED approaches are used to model the sensitivity. A tunable resonator is constructed and used to investigate the predicted conditions for optimal sensitivity, finding that impedance matching, low internal losses, and high characteristic impedance are desired. A superconducting resonator is then developed with a novel inductive coupling to the input line, which lends naturally to frequency multiplexing. Using this superconducting resonator, the circuit-QED regime is explored, measuring a charge-photon coupling strength sufficient for strong coupling. Fast charge detection is demonstrated in 50 nanoseconds, measuring a signal to noise ratio of 3.3. Finally, a method for spin manipulation in industrial nanowire devices is explored. To this end, electrical tunability of the valley-splitting is demonstrated via voltage applied to the silicon handle wafer or back-gate.
| Date of Award | 12 May 2022 |
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| Original language | English |
| Awarding Institution |
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| Supervisor | Ruth Oulton (Supervisor), M. Fernando Gonzalez-Zalba (Supervisor) & Edmund G H Harbord (Supervisor) |