Valley splitting and spin shuttling in Si/SiGe heterostructures

Coherent coupling between distant qubits is needed for any scalable quantum computing scheme. In quantum dot systems, one proposal to achieve long distance coupling is the coherent transfer of electron spins across a heterostructure, called spin shuttling [1,2,3,4]. In this talk, we examine how the valley degree of freedom poses challenges for spin shuttling in Si/SiGe heterostructures. We show that, for most known devices, valley splitting is dominated by alloy disorder. In such devices, pockets of low valley splitting are distributed throughout the heterostructure. While such pockets may be small in size, an electron is likely to encounter one on a long enough shuttling path. At these spots, inter-valley tunneling leads to dephasing of the spin wavefunction, substantially deteriorating the shuttling fidelity. We demonstrate how heterostructure modifications and fine-tuning strategies can be used to mitigate this problem. In particular, we consider varying the heterostructure composition, modulating the vertical electric field, tuning the dot position within the shuttling path, modulating the shuttling velocity, and modulating the orbital energy of the dot. We show that combinations of these strategies can improve shuttling fidelity by several orders of magnitude, putting shuttling fidelities within the error correction threshold.

[1] Xue et al., arXiv:2306.16375 (2023)

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

[3] Künne and Willmes et al., arXiv:2306.16348 (2023)

[4] Struck et al., arXiv:2307.04897 (2023)