Density matrix (mixed quantum state)
Matriz de densidad (estado cuántico mixto)
- Date
- 1927 · confidence high
- Place
- University of Berlin, Berlin, Germany
- Credited to
- John von Neumann (independently, also Lev Landau)
- Field
- Cosmos & Earth
⚠️ Disputed: the sources disagree.
What was discovered
Von Neumann introduced the density operator (matrix) to describe quantum states that are not pure -statistical mixtures, or subsystems of a larger composite system- thereby formulating quantum statistical mechanics and a theory of measurement. Landau reached an equivalent idea the same year by a different route.
Wikidata also lists Felix Bloch, who gave his own formulation in 1946, almost two decades later; the term 'density' for this operator was fixed by Paul Dirac in 1931.
What it led to
- The modern formulation of quantum statistical mechanics
- Quantum measurement theory and decoherence
- Widespread use in quantum information (mixed states, entanglement)