The Schmidt number as a universal entanglement measure

The class of local invertible operations is defined, and the invariance of entanglement under such operations is established. For the quantification of entanglement, universal entanglement measures are defined, which are invariant under local invertible transformations. They quantify entanglement in a very general sense. It is shown that the Schmidt number is a universal entanglement measure, which is most important for the general amount of entanglement. For special applications, pseudo-measures are defined to quantify the entanglement useful for a certain quantum task. The entanglement quantification is further specified by operational measures, which include the observables accessible by a given experimental setup.