Exact quantum recovery: Knill–Laflamme conditions
Research context
postgraduate concept · established · As of 2026-10-10
A code subspace with projector P exactly corrects a specified span of errors E-a precisely when P E-a† E-b P=c-ab P for every error pair.
Key knowledge
A code subspace with projector P exactly corrects a specified span of errors E-a precisely when P E-a† E-b P=c-ab P for every error pair. The matrix c is independent of the encoded state, so the environment cannot learn which logical state was encoded from those errors. Correcting a chosen error set differs from correcting arbitrary system-wide noise, and syndrome recovery operations can themselves introduce faults.
Values and equations
P E_a† E_b P=c_ab P for all a,b
- P
- Orthogonal projector onto the code subspace
- E_a,E_b
- Errors in the specified correctable span
- c_ab
- State-independent coefficients
Conditions and scope
- Exact finite-dimensional subspace recovery and a declared error span are assumed.
- Approximate recovery, subsystem codes and noisy syndrome measurements require additional conditions.
Sources · 1
References
Sources retain their recorded scope. A source link alone does not establish verification of every statement.
Evidence and review
Author checked this English statement, displayed equation where present and stated hypotheses against these source locations: §7.2 equation 7.17, CSS/stabilizer codes, distance, §7.18 surface-code thresholds and resource caveats; draft textbook chapter. The check is bounded to the recorded claims; no complete proof reconstruction, software proof replay or universal literature completeness is claimed.
2026-10-10 · Author source-body/statement/equation/domain check, with graph associations scoped individually. Separate independent content/source/inventory review is required before branch closure.
Entry details
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