Microtechnology and Nanoscience

Title: <span style="font-size: 12.0pt; font-family: 'Aptos',sans-serif; mso-fareast-font-family: Aptos; mso-fareast-theme-font: minor-latin; mso-bidi-font-family: Aptos; color: black; mso-ansi-language: SV; mso-fareast-language: SV; mso-bidi-language: AR-SA;">Implementation of Qubit Reset for Fixed-Frequency Transmons in Tunable-Coupler Architectures
Date and time: 8 November 2024, 15.00 Location: TBA
Supervisor: Liangyu Chen, Giovanna Sammarco Tancredi&nbsp; Examiner: Jonas Bylander Opponents: Yuchao He
Abstract: Unconditional and fast qubit reset is a key element to decrease algorithm computation time, as the lifetime of the physical qubits continuously grows. For example, in quantum error correction (QEC), fast qubit reset in ancilla qubits is highly desired to accelerate the surface code algorithm. This thesis reports a qubit reset protocol utilizing a tunable coupler to transfer excitation from the qubit to the dedicated readout resonator, in an architecture consisting of fixed-frequency transmons pairwise coupled by tunable-couplers. The reset pulse is designed and optimized based on the Roland-Cerf protocol for resetting the |e⟩-state adiabatically in a two-level system (TLS) with an adiabatic pulse, demonstrating an improvement in reset fidelity compared to linear pulse in simulation. By changing the pulse shape, the evolution follows the shortcut-to-adiabaticity (STA) path within some parameter regions, enabling faster and better qubit reset. For resetting |f ⟩-state, the numerical results also give adiabatic and STA pulse shapes similar to that given by the Roland-Cerf protocol in a two-level system, thus enabling us to model the |f ⟩-state reset model as an approximate TLS system. We verify our theoretical prediction by running the reset protocols on a 25-qubit chip. The experiment results show fast reset operations while keeping low rest errors, verifying the validity of the proposed pulses. However, the presence of other qubits limits the reset fidelity and therefore frequency separation, between coupled qubits, should be a parameter to be carefully considered at the design stage.

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8 November 2024

15.00 Zixian Yan