What is Your Logical Qubit?
summary
Survey of recent demos lists concrete claims and limits: small codes (9‑qubit, 5‑qubit NV, bosonic cat/GKP) show coherence gains ≈1–2× or fault‑tolerant Clifford injections; Quantinuum/Honeywell reports logical SPAM below physical; Google reports ~3% per syndrome extraction cycle at distance 3 (not under threshold); neutral‑atom demo runs 16 patches and reports CNOT error dropping from ~20–40% to ~7–10% when joint decoding. Theory point: single‑round stabilizer readout can make logical teleportation fail with probability ~p, so measurement repetition and clear operational definitions of “logical error rate” matter.
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chapters (11)
- 0:00Logical qubits introduction
- 1:04Why logical qubits needed
- 4:42Physical vs logical qubit challenges
- 8:22Early quantum error correction
- 13:55Bosonic codes progress
- 20:30Threshold theorem in error correction
- 29:33Google’s surface code limits
- 32:05Harvard/QuEra neutral atom demo
- 43:22Measurement fidelity importance
- 45:37Logical qubit demos summary
- 49:20Q&A
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