  In the signal-processing literature, a frame is a mechanism for performing
analysis and reconstruction in a Hilbert space. By contrast, in quantum theory,
a positive operator-valued measure (POVM) decomposes a Hilbert-space vector for
the purpose of computing measurement probabilities. Frames and their most
common generalizations can be seen to give rise to POVMs, but does every
reasonable POVM arise from a type of frame? In this paper we answer this
question using a Radon-Nikodym-type result.

Functional Analysis