  Evolution of a Bose-Einstein condensate subject to a time-dependent external
perturbation can be described by a time-dependent Bogoliubov theory: a
condensate initially in its ground state Bogoliubov vacuum evolves into a
time-dependent excited state which can be formally written as a time-dependent
Bogoliubov vacuum annihilated by time-dependent quasiparticle annihilation
operators. We prove that any Bogoliubov vacuum can be brought to a diagonal
form in a time-dependent orthonormal basis. This diagonal form is tailored for
simulations of quantum measurements on an excited condensate. As an example we
work out phase imprinting of a dark soliton followed by a density measurement.
