Optimal Control of a SiGe-Quantum Bus for Coherent Electron Shuttling in the Presence of Material Defects*

Scaling up quantum processors to a large number of qubits to enable the implementation of error-correction algorithms is one of the major challenges on the path to realizing universal quantum computers. Spin qubits in gate-defined SiGe quantum dots provide excellent prospects for scalability, however the lithographic processing, signal routing and wiring of large qubit arrays at a small footprint poses a significant challenge [1]. One possible solution could be to separate the qubit register into small dense qubit arrays, which are interconnected by a quantum bus that allows for coherent transfer of quantum information by physically moving electrons between distant nodes along a channel [2]. Limitations in qubit shuttling arise from the interaction of the electron with material defects within the channel that can cause non-adiabatic transitions to excited states. As excited (orbital) states have a modified effective g-factor, this results in an accumulation of a random phase which finally diminishes the fidelity. In this contribution, we theoretically explore the capabilities for bypassing defect centers using optimally engineered control signals that allow for a quasi-adiabatic passage of the electron through the channel without reducing the velocity. Our approach is based on quantum optimal control theory and Schrödinger wave packet propagation using realistic potential landscapes.

References:

[1] Vandersypen et al., npj Quantum Inf. 3, 34 (2017)

[2] Langrock et al., PRX Quantum 4, 020305 (2023)