Editing Density matrix (section)

== History ==
The formalism of density operators and matrices was introduced in 1927 by [[John von Neumann]]<ref>{{Citation | last = von Neumann | first = John  | year = 1927 | author-link = John von Neumann  |title=Wahrscheinlichkeitstheoretischer Aufbau der Quantenmechanik | journal = Göttinger Nachrichten | volume = 1|pages= 245–272|url=https://eudml.org/doc/59230}}</ref> and independently, but less systematically, by [[Lev Landau]]<ref name=landau1927>{{cite book |doi=10.1016/B978-0-08-010586-4.50007-9 |chapter=The Damping Problem in Wave Mechanics (1927)|title=Collected Papers of L.D. Landau |pages=8–18 |year=1965 |isbn=978-0-08-010586-4 }}</ref> and later in 1946 by [[Felix Bloch]].<ref name="Ugo ">{{cite journal |doi=10.1007/BF03001661 |title=Density matrices as polarization vectors |journal=Rendiconti Lincei |volume=6 |issue=2 |pages=123–130 |year=1995 |last1=Fano |first1=Ugo |s2cid=128081459 }}</ref> Von Neumann introduced the density matrix in order to develop both quantum statistical mechanics and a theory of quantum measurements. The name density matrix itself relates to its classical correspondence to a [[phase-space]] [[probability measure]] (probability distribution of position and momentum) in classical [[statistical mechanics]], which was introduced by [[Eugene Wigner]] in 1932.<ref name="fano1957" />

In contrast, the motivation that inspired Landau was the impossibility of describing a subsystem of a composite quantum system by a state vector.<ref name="landau1927" />