  We provide a simple physical picture of the loss of coherence between two
coherently split one-dimensional Bose-Einstein condensates. The source of the
dephasing is identified with nonlinear corrections to the elementary excitation
energies in either of the two independent condensates. We retrieve the result
by Burkov, Lukin and Demler [Phys. Rev. Lett. 98, 200404 (2007)] on the
subexponential decay of the cocherence for the large time, however, the scaling
of the characteristic decoherence time differs.
