  We propose and demonstrate a novel method to generate a large-amplitude
coherent-state superposition (CSS) via ancilla-assisted photon-subtraction. The
ancillary mode induces quantum interference of indistinguishable processes,
widening the controllability of quantum superposition at the conditional
output. We demonstrate the concept in the time domain, by a simple
time-separated two-photon subtraction from cw squeezed light. We observe the
largest CSS ever reported without any corrections, which will enable various
quantum information applications with CSS states.
