  We present a theory of measurement-induced interference for weakly
interacting Bose-Einstein condensed (BEC) gases. The many-body state resulting
from the evolution of an initial fragmented (Fock) state can be approximated as
a continuous superposition of Gross-Pitaevskii (GP) states; the measurement
breaks the initial phase symmetry, producing a distribution pattern
corresponding to only one of the GP solutions. We discuss also analytically
solvable models, such as two-mode on-chip adiabatic recombination and soliton
generation in quasi one-dimensional condensates.
