SiGe quantum wells with oscillating Ge concentrations for quantum dot qubits

McJunkin, Thomas; Harpt, Benjamin; Feng, Yong; Losert, Merritt P.; Rahman, Rajib; Dodson, J. P.; Wolfe, M. A.; Savage, D. E.; Lagally, M. G.; Coppersmith, S. N.; Friesen, Mark; Joynt, Robert; Eriksson, M. A.

doi:10.48550/arxiv.2112.09765

Cited by 4 publications

(14 citation statements)

References 30 publications

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“…One of them suggests back-gates for controlling the electric field across the QD independent from the QD filling [108]. Another approach uses engineering of the Ge profile across the Si/SiGe heterostructure [74,109]. A third method relies on increasing random fluctuations of alloy composition (Ge concentration in Si QW), which statistically increases average E VS,0 [71], but at the cost of larger variance.…”

Section: Discussionmentioning

confidence: 99%

“…On the other hand, the perturbations having characteristic length scales comparable to lattice spacing (e.g. atomic steps at interface of the quantum well [72]) can severely affect not only E VS , but also the composition of valley eigenstates [46,60,61,71,73,74]. Driving the spatial degrees of freedom of the electron in presence of such perturbations can lead to transitions between the two valley states.…”

Section: Introductionmentioning

confidence: 99%

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Blueprint of a scalable spin qubit shuttle device for coherent mid-range qubit transfer in disordered Si/SiGe/SiO$_2$

Langrock¹,

Krzywda²,

Focke³

et al. 2022

Preprint

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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 between qubit registers has been suggested to solve the signal fan-out problem. Here we present a blueprint of such a ∼ 10 µm long link, called a spin qubit shuttle, which is based on connecting an array of gates into a small number of sets. The number of these gate sets and thus the number of required control signal is independent of the link distance to coherently shuttle the electron qubit. 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 of the conveyor shuttling mode, motional narrowing, the interplay between orbital and valley excitation and relaxation in presence of g-factors that depend on orbital and valley state of the electron, and effects from spin-hotspots are discussed in detail. We find that a transfer fidelity of 99.9 % is feasible in Si/SiGe at a speed of ∼10 m/s, if the average valley splitting and its inhomogeneity stay within realistic bounds. Operation at low global magnetic field ≈ 20 mT and material engineering towards high valley splitting is favourable to reach high transfer fidelities.

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Section: Discussionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Blueprint of a scalable spin qubit shuttle device for coherent mid-range qubit transfer in disordered Si/SiGe/SiO$_2$

Langrock¹,

Krzywda²,

Focke³

et al. 2022

Preprint

View full text Add to dashboard Cite

show abstract

“…The orbital energy gap is determined by the electrostatic confinement of the quantum dot. The valley energy gap can be made large in a Si-MOS quantum dot with a strong vertical field to force the electron against the interface [64,65] and can be made large in Si/SiGe devices with alloy engineering [66,67].…”

Section: Two-qubit Error Channelsmentioning

confidence: 99%

“…This is not unreasonable. For Si/SiGe quantum dots, the fixed alloy composition largely dictates the valley phase [66,67,[71][72][73][74][75][76]. For Si-MOS quantum dots, the position of the oxide interface largely dictates the valley phase, given a sufficient vertical electric field [19,64,65].…”

Section: B Correlated ẑ ⊗ ẑmentioning

confidence: 99%

The remarkable prospect for quantum-dot-coupled tin qubits in silicon

Witzel¹,

Lutz²,

Luhman³

2022

Preprint

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Spin-1 2 119 Sn nuclei in a silicon semiconductor could make excellent qubits. Nuclear spins in silicon are known to have long coherence times. Tin is isoelectronic with silicon, so we expect electrons can easily shuttle from one Sn atom to another to propagate quantum information via a hyperfine interaction that we predict, from all-electron linearized augmented plane wave density functional theory calculations, to be roughly ten times larger than intrinsic 29 Si. A hyperfine-induced electronuclear controlled-phase (e-n-CPhase) gate operation, generated (up to local rotations) by merely holding an electron at a sweet-spot of maximum hyperfine strength for a specific duration of time, is predicted to be exceptionally resilient to charge/voltage noise. Diabatic spin flips are suppressed with a modest magnetic field (> 15 mT for < 10 −6 flip probabilities) and nuclear spin bath noise may be avoided via isotopic enrichment or mitigated using dynamical decoupling or through monitoring and compensation. Combined with magnetic resonance control, this operation enables universal quantum computation.

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“…Indeed, a number of publications consider it as a challenge to be overcome or to do away with (see, e.g., Refs. [1,[8][9][10]), whereas a second group of publications (see, e.g., Refs. [11][12][13][14][15]) considers it as a potential resource to be further explored.…”

Section: Introductionmentioning

confidence: 99%

Valleytronic full configuration-interaction approach: An application to the excitation spectra of Si double-dot qubits

Yannouleas

Landman

2022

Preprint

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A study of the influence of strong electron-electron interactions and Wigner-molecule (WM) formation on the spectra of 2e singlet-triplet double-dot Si qubits is presented based on a full configuration interaction (FCI) approach that incorporates the valley degree of freedom (VDOF) in the context of the continuous (effective mass) description of semiconductor materials. Our FCI solutions correspond to treating the VDOF as an isospin in addition to the regular spin. A major advantage of our treatment is its capability to assign to each energy curve in the qubit's spectrum a complete set of good quantum numbers for both the spin and the valley isospin. This allows for the interpretation of the Si double-dot spectra according to an underlying SU(4) ⊃ SU(2) × SU(2) group-chain organization. Considering parameters in the range of actual experimental situations, we demonstrate for the first time in a double-dot qubit that, in the (2,0) charge configuration and compared to the expected large, and dot-size determined, single-particle (orbital) energy gap, the strong e − e interactions drastically quench the spin-singlet−spin-triplet energy gap, E^⊕_ST, within the same valley, making it competitive to the small energy gap, E_V , between the two valleys. We present results for both the E^⊕_ST < E_V and E^⊕_ST > E_V cases, which have been reported to occur in different experimental qubit devices. In particular, we investigate the spectra as a function of detuning and demonstrate the strengthening of the all-important avoided crossings due to a lowering of the interdot barrier and/or the influence of valley-orbit coupling. We further demonstrate, as a function of an applied magnetic field, the emergence of avoided crossings in the (1,1) charge configuration due to the more general spin-valley coupling, in agreement with experiments. The valleytronic FCI method formulated and implemented in this paper, and demonstrated for the case of two electrons confined in a tunable double quantum dot, offers also a most effective tool for analyzing the spectra of Si qubits with more than two wells and/or more than two electrons, in field-free conditions, as well as under the influence of an applied magnetic field. Furthermore, it can also be straightforwardly extended to the case of bilayer graphene quantum dots.

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SiGe quantum wells with oscillating Ge concentrations for quantum dot qubits

Cited by 4 publications

References 30 publications

Blueprint of a scalable spin qubit shuttle device for coherent mid-range qubit transfer in disordered Si/SiGe/SiO$_2$

Blueprint of a scalable spin qubit shuttle device for coherent mid-range qubit transfer in disordered Si/SiGe/SiO$_2$

The remarkable prospect for quantum-dot-coupled tin qubits in silicon

Valleytronic full configuration-interaction approach: An application to the excitation spectra of Si double-dot qubits

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