  We propose and theoretically investigate an unambiguous Bell measurement of
atomic qubits assisted by multiphoton states. The atoms interact resonantly
with the electromagnetic field inside two spatially separated optical cavities
in a Ramsey-type interaction sequence. The qubit states are postselected by
measuring the photonic states inside the resonators. We show that if one is
able to project the photonic field onto two coherent states on opposite sites
of phase space, an unambiguous Bell measurement can be implemented. Thus our
proposal may provide a core element for future components of quantum
information technology such as a quantum repeater based on coherent multiphoton
states, atomic qubits and matter-field interaction.
