Optimal Control of Conveyor-Mode Spin-Qubit Shuttling in a Si/SiGe Quantum Bus in the Presence of Charged Defects 

Schematic illustration of the Si-QuBus device. The clavier gate electrodes on the top surface generate a moving array of QD potentials that allow to transport the electron spin-qubit in a conveyor-mode along the channel.

Lasse Ermoneit, Burkhard Schmidt, Thomas Koprucki, Jürgen Fuhrmann, Tobias Breiten, Arnau Sala, Nils Ciroth, Ran Xue, Lars R. Schreiber, and Markus Kantner 

Spin-qubit shuttles are novel functional elements in modular architectures of semiconductor quantum processors, that have the capability of solving the scalability problem. Such coherent quantum links serve to interconnect different processor units and enable the transfer of quantum information over longer distances across the chip by physical transport of electrons. The shuttling fidelity is limited by hardly avoidable material defects and fabrication imperfections, which can cause spin dephasing. In this paper, we present a numerical simulation framework for conveyor-mode spin-qubit shuttling in Si/SiGe and investigate the impact of charged defects in the channel on the orbital state dynamics of the transported electron. Quantum optimal control theory is employed to engineer control pulses that enable nearly deterministic passage of the electron through the channel by minimizing the accumulated energy uncertainty. The resulting control pulses facilitate quasi-adiabatic driving of the electron by circumventing critical regions in the channel without reducing the shuttling speed. Moreover, we demonstrate that trailing electrons subject to the same control pulse at a defect-free segment of the channel are not disturbed by the control. The theoretical results serve as a guideline for fine-tuning the controls in spin-qubit shuttling experiments.