Łukasz Cywiński

Institute of Physics, Polish Academy of Sciences
Talk title
Theory of spin qubit shuttling: Si vs GaAs and chains of tunnel-coupled dots vs a travelling dot
Abstract Invited

Coherent communication between quantum registers separated by ∼ 10 μm distances
is one of the requirements for scalability of architectures based on gated quantum dots
[1]. Moving a spin qubit across this distance, e.g. along a chain of tunnel-coupled dots [2],
is one of possible solutions. I will discuss our recent theoretical work on such a dot-to-dot
adiabatic charge transfer in presence of realistic amount of charge noise and electron-
phonon coupling [3,4]. The main result is that unless tunnel couplings (defined here as
equal to the gap at the anticrossing of states localized in the two dots) are larger than ≈ 50
μeV, charge noise-induced excitations will be a factor limiting the adiabatic character
of electron transfer across a chain of ∼ 100 QD. I will discuss the close relationship
between non-adiabatic (and thus non-deterministic) character of charge transfer and spin
dephasing caused by dot- and state-specific g-factors, and highlight the differences between
GaAs and Si-based quantum dots caused by stronger electron-phonon coupling in the
former. Finally, I will discuss an alternative qubit shuttling scheme in which gates are
used to create a moving quantum dot [5,6]. I will argue that this scheme is more robust to
electrostatic disorder compared to the one based on pre-existing tunnel coupled quantum
dots, and I will discuss how the valley-orbit coupling in presence of interface roughness
will be the main source of spin qubit dephasing in this scheme [6].

This work has been funded by the National Science Centre (NCN), Poland under
QuantERA program, Grant No. 2017/25/Z/ST3/03044. Project Si-QuBus has received
funding from the QuantERA ERA-NET Cofund in Quantum Technologies implemented
within the European Union’s Horizon 2020 program.

[1] L.M.K. Vandersypen, H. Bluhm, J. S. Clarke, A. S. Dzurak, R. Ishihara, A. Morello,
D. J. Reilly, L. R. Schreiber, and M. Veldhorst, Interfacing spin qubits in quantum dots
and donors—hot, dense, and coherent, npj Quantum Inf. 3, 1 (2017); J. M. Boter et al,
The spider-web array–a sparse spin qubit array, preprint arXiv:2110.00189 (2021).
[2] A.R. Mills, D.M. Zajac, M.J. Gullans, F.J. Schupp, T.M. Hazard, and J.R. Petta,
Shuttling a single charge across a one-dimensional array of silicon quantum dots, Nat.
Commun. 10, 1063 (2019).
[3] J.A. Krzywda and L. Cywiński, Adiabatic electron charge transfer between two quan-
tum dots in presence of 1/f noise, Phys. Rev. B 101, 035303 (2020).
[4] J.A. Krzywda and L. Cywiński, Interplay of charge noise and coupling to phonons in
adiabatic electron transfer between quantum dots, Phys. Rev. B 104, 075439 (2021).
[5] I. Seidler, T. Struck, R. Xue, N. Focke, S. Trellenkamp, H. Bluhm, and L. R. Schreiber,
Conveyor-mode single electron shuttling in Si/SiGe for a scalable quantum computing ar-
chitecture, preprint arXiv:2108.00879 (2021).
[6] V. Langrock, J.A. Krzywda, N. Focke, I. Seidler, L.R. Schreiber, and L. Cywiński,
Blueprint of a scalable spin qubit shuttle device for coherent mid-range qubit transfer in
disordered Si/SiGe/SiO 2 , preprint arXiv:2202.11793 (2022).

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