  We report the creation of a wide range of quantum states with controllable
degrees of entanglement and entropy using an optical two-qubit source based on
spontaneous parametric downconversion. The states are characterised using
measures of entanglement and entropy determined from tomographically determined
density matrices. The Tangle-Entropy plane is introduced as a graphical
representation of these states, and the theoretic upper bound for the maximum
amount of entanglement possible for a given entropy is presented. Such a
combination of general quantum state creation and accurate characterisation is
an essential prerequisite for quantum device development.
