Introduction to Quantum Physics and Information Processing

Chapter 5

Mixed States, Open Systems, and

the Density Operator

The formalism for quantum systems developed so far applies to what are

called pure states. A system in a pure state is completely specified by the

state vector. A complete set of experimental tests will determine the system

state fully: we have maximal knowledge of the system. For example, for a spin

system, we can find a particular orientation of an SG machine such that the

state is in its + or − port. This also means that the state is an eigenvector of

some operator, or is always a linear combination of the computational basis

states.

As opposed to this, as in most practical cases, we only ...