  A theory of quantum state reduction is advanced. It is based on two
principles: (1) Gauge decomposition; (2) Maximum entropy. To wit: (1) The
reduction decomposition of a state vector is the Schmidt decomposition with
respect to the states of a set of (dressed) gauge boson modes; (2) The
reduction instant is that of the maximum entropy of a resulting mixed state.
The theory determines states undergoing the reduction, its instant, resulting
pure states and their probabilities. Applications: (Polarized) photon
absorption and transmission, emission, particle detection, reduction of a
superposition of states, nonintegral photon states, photon and matter-photon
entanglement, processes with weak bosons, and the role of gluons.
