Qubits, density states and measurement probabilities
Research context
postgraduate concept · established · As of 2026-10-10
A qubit pure state is a normalized vector in C² modulo global phase; a mixed state is a positive trace-one density operator.
Key knowledge
A qubit pure state is a normalized vector in C² modulo global phase; a mixed state is a positive trace-one density operator. A POVM consists of positive effects summing to the identity, with outcome probability tr(rho E-m). A density matrix can represent many ensemble decompositions, so it does not identify a unique hidden classical preparation. Quantum measurements and noncommuting observables cannot be modeled by one arbitrary classical sample space without further structure.
Values and equations
rho≥0; tr rho=1; E_m≥0; sum_m E_m=I; p(m)=tr(rho E_m)
- rho
- Quantum density operator
- E_m
- POVM effect for outcome m
- I
- Identity operator
Conditions and scope
- Finite-dimensional states and normalized measurement operators are specified.
- The outcome law alone does not specify the post-measurement state; an instrument supplies that operation.
Sources · 2
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: Qubits, density operators, ensembles, von Neumann entropy and no-cloning; CPTP/Kraus maps, unitary dilation, generalized measurements and reduced-state channels. 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
Permanent link to this entry ↗- Record identity
- research-info-quantum-state-measurement
- Revision
- research.1
- Structured reference record
- Postgraduate & research reference