  Measures are introduced to quantify the degree of superposition in mixed
states with respect to orthogonal decompositions of the Hilbert space of a
quantum system.
  These superposition measures can be regarded as analogues to entanglement
measures, but can also be put in a more direct relation to the latter. By a
second quantization of the system it is possible to induce superposition
measures from entanglement measures. We consider the measures induced from
relative entropy of entanglement and entanglement of formation. We furthermore
introduce a class of measures with an operational interpretation in terms of
interferometry. We consider the superposition measures under the action of
subspace preserving and local subspace preserving channels. The theory is
illustrated with models of an atom undergoing a relaxation process in a
Mach-Zehnder interferometer.
