[Semiconductor Quantum Computer – External Research Review] XOSO Qubits Leveraging Hole Spins and Strong Spin–Orbit Interaction – A New Proposal in Silicon/Germanium
Introduction
While our research primarily focuses on electron-spin semiconductor quantum dot qubits,
there is a growing body of work exploring hole-spin-based approaches.
A paper posted on arXiv in October 2024,
“Exchange-Only Spin-Orbit Qubits in Silicon and Germanium” (Stefano Bosco et al., QuTech),
proposes a new XOSO (Exchange-Only Spin-Orbit) qubit architecture that uses hole-spin quantum dots in silicon/germanium and exploits strong spin–orbit interaction (SOI).
By using SOI for complete electrical control, this approach eliminates the need for magnetic field gradients or complex pulse synchronization—often required in conventional Exchange-Only (XO) designs—and enables single-step, low-leakage two-qubit gates.
XOSO Qubit Structure and Encoding
Conceptual Structure Diagram
[QD1]──J12──[QD2]──J23──[QD3]
↑ Hole spin ↑ Hole spin ↑ Hole spin
(SOI active) (SOI active) (SOI active)
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Quantum dots (QD1–QD3) each confine a single hole spin
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Exchange interactions
J_{12} J_{23} -
SOI tilts each spin’s quantization axis, enabling anisotropic exchange interactions
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Qubits are encoded in the three-spin exchange-only subspace:
- Computational basis: total spin quantum number
S=1/2 - Leakage states:
S=3/2
- Computational basis: total spin quantum number
Comparison with Conventional XO
| Feature | Conventional XO (Electron Spin) | XOSO (Hole Spin + SOI) |
|---|---|---|
| Main material | Si/SiGe, GaAs (electrons) | Si/SiGe, Ge (holes) |
| Spin control | Exchange-only (magnetic gradients not required in principle, but often added) | All-electric control via strong SOI |
| Rotating frame | Sometimes required | Not required (SOI provides control axes) |
| Gate layout | 3 dots with plunger + barrier gates | 3 dots with SOI-enabled simplified wiring |
| Two-qubit gates | Multi-step exchange pulses | Single-step low-leakage gates |
| Leakage suppression | Pulse design & multi-step control | Natural suppression via encoding + SOI |
| Implementation issue | Small g-factor anisotropy makes all-electric control difficult | Requires uniform SOI and high-quality material |
| Scalability | Many control lines, complex routing | Fewer gates, easier scaling |
| Operation speed | μs–ns (depending on control) | ns-scale operation possible |
| Noise sensitivity | Sensitive to detuning noise | First-order insensitivity at charge symmetry point |
Note: Magnetic Field Gradients & Microwaves in Electron XO
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In principle, not required: A three-dot electron-spin system can implement all gates purely via exchange interactions
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Reasons added in practice:
- To simplify single-qubit control
- To shorten gate times
- To suppress leakage
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As a result, some implementations integrate micromagnets or ESR microwave lines
Scalability Considerations
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Reduced wiring and control complexity
- XOSO removes the need for magnetic gradients or microwave lines, increasing wiring density
- No need for complex multi-gate pulse synchronization hardware
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Fabrication compatibility
- Hole-spin devices in Ge or SiGe have strong CMOS process compatibility
- SOI strength and stability depend on wafer quality
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Performance
- SOI enables fast gate operations
- However, it may introduce additional electrical noise channels, requiring careful shielding and layout design
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Large-scale challenges
- Crosstalk from electric fields and device variability must be managed
- Multi-chip and modular interconnect strategies will be essential
Conclusion
The XOSO qubit architecture demonstrates that hole spins and strong spin–orbit coupling can remove the need for magnetic structures and multi-step controls often added to XO systems.
While the physical properties and control mechanisms differ from electron-spin approaches,
a comparative and complementary evaluation of both paths is essential for achieving truly scalable semiconductor quantum processors.