  We analyze the linear optical realization of number-sum Bell measurement and
number-state manipulation by taking into account the realistic experimental
situation, specifically imperfectness of single-photon detector. The present
scheme for number-state manipulation is based on the number-sum Bell
measurement, which is implemented with linear optical elements, i.e., beam
splitters, phase shifters and zero-one-photon detectors. Squeezed vacuum states
and coherent states are used as optical sources. The linear optical Bell state
detector is formulated quantum theoretically with a probability operator
measure. Then, the fidelity of manipulation and preparation of number-states,
particularly for qubits and qutrits, is evaluated in terms of the quantum
efficiency and dark count of single-photon detector. It is found that a high
fidelity is achievable with small enough squeezing parameters and coherent
state amplitudes.
