  The problem of on-demand generation of entanglement between single-atom
qubits via a common photonic channel is examined within the framework of
optical interferometry. As expected, for a Mach-Zehnder interferometer with
coherent laser beam as input, a high-finesse optical cavity is required to
overcome sensitivity to spontaneous emission. We show, however, that with a
twin-Fock input, useful entanglement can in principle be created without
cavity-enhancement. Both approaches require single-photon resolving detectors,
and best results would be obtained by combining both cavity-feedback and
twin-Fock inputs. Such an approach may allow a fidelity of $.99$ using a
two-photon input and currently available mirror and detector technology. In
addition, we study interferometers based on NOON states and show that they
perform similarly to the twin-Fock states, yet without the need for
high-precision photo-detectors. The present interferometrical approach can
serve as a universal, scalable circuit element for quantum information
processing, from which fast quantum gates, deterministic teleportation,
entanglement swapping $etc.$, can be realized with the aid of single-qubit
operations.
