Multiplying it by its natural logarithm and finding the trace of the product gives the von Neumann entropy, and the trace of this object is 1.
Stephen White developed an iterative method using this object's namesake renormalization group, in which it is used to project a candidate ground state onto a series of blocks. If the projection operator is P sub n, taking a measurement creates a new instance of this object equal to the sum over all n of P sub n times this object times P sub n. The expectation value of the measurement of an observable is equal to the trace of the product of this object with the observable. Multiplying it by its natural logarithm and finding the trace of the product gives the von Neumann entropy, and the trace of this object is 1. For pure states, this object is idempotent, and for those states it can be replaced by a wavefunction. For 10 points, name this operator which is represented as a Hermitian semi-definite matrix used to represent a mixed state of a quantum system.
Stephen White developed an iterative method using this object's namesake renormalization group, in which it is used to project a candidate ground state onto a series of blocks.
density matrix [or density operator]