For 10 points, name this function that describes a quantum system with mixed states.
There is only one objectively real way to interpret this function according to the GHJW theorem, which states that any two ensemble preparations of this function can be related by a unitary transformation. A linear, completely positive map from the set of these functions onto itself is a quantum operation. Deriving this function's Kraus representation in the limit of infinitesimal time produces a master equation that includes a summation of quantum jumps. This function is the Weyl transform of the Wigner quasi-probability distribution. This function's values correspond to Bloch vectors in Hilbert space, and its components are called populations or coherences based on whether they lie on or off the diagonal. The von Neumann entropy is equal to the negative trace of the product of this function with its logarithm, and the expectation value of an operator is equal to the trace of the operator's product with this function. For 10 points, name this function that describes a quantum system with mixed states.
There is only one objectively real way to interpret this function according to the GHJW theorem, which states that any two ensemble preparations of this function can be related by a unitary transformation.
density matrix [or density operator]