tetthetsoperatorer
The term "tetthetsoperatorer" is a Swedish word that translates to "density operators" in English. In quantum mechanics, a density operator is a mathematical object that describes the state of a quantum system. It is particularly useful for representing mixed states, which are probabilistic combinations of pure states, or for dealing with subsystems of larger quantum systems.
A density operator is a positive-semidefinite Hermitian operator with a trace of one. For a pure state,
The expectation value of an observable A in a state described by the density operator ρ is given