Number filter for matter waves

In current Bose-Einstein condensate experiments, the shot-to-shot variation of the atom number fluctuates up to 10%. In here, we present a procedure to suppress such fluctuations by using a nonlinear $p\ensuremath{-}\ensuremath{\pi}\ensuremath{-}\overline{p}$ matter wave interferometer for a Bose-Einstein condensate with two internal states and a high beam-splitter asymmetry $(p\ensuremath{\ne}0.5)$. We analyze the situation for an inhomogeneous trap within the Gross-Pitaevskii mean-field theory, as well as a quantum mechanical Josephson model, which addresses complementary aspects of the problem and agrees well otherwise.