Blueprint of a Scalable Spin Qubit Shuttle Device for Coherent Mid-Range Qubit Transfer in Disordered Si/SiGe/SiO2

Silicon spin qubits stand out due to their very long coherence times, compatibility with industrial fabrication, and prospect to integrate classical control electronics. To achieve a truly scalable architecture, a coherent mid-range link that moves the electrons between qubit registers has been suggested to solve the signal fan-out problem. Here, we present a blueprint of such a link of 10 mu m length, called a spin qubit shuttle, which is based on connecting an array of gates into a small number of sets. To control these sets, only a few voltage control lines are needed and the number of these sets and thus the number of required control signals is independent of the length of this link. We discuss two different operation modes for the spin qubit shuttle: a qubit conveyor, i.e., a potential minimum that smoothly moves laterally, and a bucket brigade, in which the electron is transported through a series of tunnel-coupled quantum dots by adiabatic passage. We find the former approach more promising considering a realistic Si/SiGe device, including potential disorder from the charged defects at the Si/SiO2 layer, as well as typical charge noise. Focusing on the qubit transfer fidelity in the conveyor shuttling mode, we discuss in detail motional narrowing, the interplay between orbital and valley excitation and relaxation in the presence of g factors that depend on orbital and valley state of the electron, and effects from spin hotspots. We find that a transfer fidelity of 99.9% is feasible in Si/SiGe at a speed of approximately 10 m/s, if the average valley splitting and its inhomogeneity stay within realistic bounds. Operation at low global magnetic field approximately equal to 20 mT and material engineering towards high valley splitting is favorable for reaching high fidelities of transfer.

Automated summary

Silicon spin qubits have long coherence times, are compatible with industrial fabrication, and contain the potential to integrate classical control electronics. A blueprint for a coherent mid-range link, called a spin qubit shuttle, has been presented to address the signal fan-out problem between qubit registers. The shuttle consists of an array of gates connected into sets, requiring only a few voltage control lines to control these sets, and is independent of the length of the link. Two operation modes, qubit conveyor and bucket brigade, have been discussed, with the former being more viable in a realistic Si/SiGe device. Transfer fidelity of 99.9% is feasible at a speed of approximately 10 m/s, given realistic bounds on valley splitting and its inhomogeneity.