# Photonic, Spin & Topological Qubits: State of the Field (October 2026)

Research dossier · v1 · updated 2026-10-30 · 4,876 words · 42 sources · 26 claims · verification: verified

An expert briefing, current to late October 2026, on three qubit families outside the superconducting and trapped-atom mainstream: photonic qubits built from fusion-based and continuous-variable/GKP architectures, semiconductor spin qubits in silicon and germanium, and Majorana-based topological qubits. Silicon-photonic platforms report component fidelities above 99% when photon detection is conditioned on, and the platform's authors identify optical loss as the main remaining gap. Silicon spin qubits have reached about 99% single-qubit control fidelity in research devices, and 2026 work centres on control electronics and shuttling. Majorana tetron preprints report parity lifetimes from microseconds to about 20 seconds, but whether these devices host topological zero modes remains contested.

## Contents

- Executive summary
- Background and core concepts
- Timeline of key milestones
- State of the art
  - Photonic qubits: fusion-based architectures
  - Continuous-variable photonics and GKP
  - Silicon and germanium spin qubits
  - Majorana-based topological qubits
  - Quantitative benchmarks
- Open problems and active debates
- Major programs and players
- Outlook: what to watch (next 12-18 months)
- Key papers
- Glossary

## Sample findings

- A 2025 silicon-photonics platform reports dual-rail state preparation and measurement at 99.98% ± 0.01%, conditional on photon detection and excluding loss.
- The same platform reports Hong–Ou–Mandel visibility of 99.50% ± 0.25%, two-qubit fusion fidelity of 99.22% ± 0.12% and chip-to-chip interconnect fidelity of 99.72% ± 0.04%, and the abstract states that these figures are conditional on photon detection and do not include loss.

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