  We propose an optical scheme to generate a superposition of coherent states
with enhanced size adopting an interferometric setting at the single-photon
level currently available in the laboratory. Our scheme employs a nondegenerate
optical parametric amplifier together with two beam splitters so that the
detection of single photons at the output conditionally implements the desired
superposition of second-order photonic operations. We analyze our proposed
scheme by considering realistic on-off photodetectors with nonideal efficiency
in heralding the success of conditional events. A high-quality performance of
our scheme is demonstrated in view of various criteria such as quantum
fidelity, mean output energy, and measure of quantum interference.
