Implementation of qubit reset for fixed-frequency transmons in tunable-coupler architectures

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 reset errors, verifying the validity of the proposed pulses [1]. 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.