  In this paper, the quantum fluctuations of the relative velocity of
constituent solitons in a Gross-Pitaevskii breather are studied. The breather
is confined in a weak harmonic trap. These fluctuations are
monitored,indirectly, using a two-body correlation function measured at a
quarter of the harmonic period after the breather creation. The results of an
ab initio quantum Monte Carlo calculations, based on the Feynman-Kac path
integration method, are compared with the analytical predictions using the
recently suggested approach within the Bogoliubov approximation, and a good
agreement is obtained.
