Quantum instruments and conditional states
Research context
postgraduate concept · established · As of 2026-10-10
An instrument associates outcome a with a completely positive trace-nonincreasing map ℐ_a.
Key knowledge
An instrument associates outcome a with a completely positive trace-nonincreasing map ℐ_a. With Kraus operators M_aμ, ℐ_a(ρ)=Σ_μM_aμρM_aμ†. Its trace is the outcome probability; division by that probability gives the conditional state. Ignoring the outcome gives Σ_aℐ_a(ρ), a trace-preserving channel.
Values and equations
p(a)=Tr ℐ_a(ρ); ρ_a=ℐ_a(ρ)/p(a)
- ℐ_a
- Completely positive outcome map
- p(a)
- Outcome probability
- ρ,ρ_a
- Initial and conditional density operators
Conditions and scope
- Σ_aμM_aμ†M_aμ=I; normalize only for outcomes with positive probability.
- The normalized conditional update is nonlinear because of conditioning; the subnormalized operation remains linear.
Sources · 1
References
Sources retain their recorded scope. A source link alone does not establish verification of every statement.
Evidence and review
Inspected §§3.1.2, 3.2.1, 3.2.4, 3.2.6, 3.5 and 3.6: probabilities, operations, complete positivity, semigroups and coloured noise; source check dated 2026-10-10. Scope is limited to the cited statements, equations and assumptions, not the whole publication.
2026-10-10 · Author source inspection of the displayed claim, model assumptions and equations; independent exact-content and branch-inventory review is a separate gate. Contemporary status is a dated snapshot, not a promise of ongoing monitoring.
Entry details
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- research-physics-quantum-instrument
- Revision
- research.1
- Structured reference record
- Postgraduate & research reference