  Based on the exact results obtained by Bethe ansatz, we demonstrate the
existence of stable bound pair (BP) wave packet in Bose Hubbard model with
arbitrary on-site interaction U. In large-U regime, it is found that an
incoming single-particle (SP) can coherently pass through a BP wave packet and
leave a coherent shift in the position of it. This suggests a simple scheme for
constructing a BP charge qubit to realize a quantum switch, which is capable of
controlling the coherent transport of one and only one photon in a
one-dimensional waveguide.
