  The construction of photon-photon quantum phase gate based on photonic
nonlinearity has long been a fundamental issue, which is vital for
deterministic and scalable photonic quantum information processing. It requires
not only strong nonlinear interaction at the single-photon level, but also
suppressed phase noise and spectral entanglement for high gate fidelity. In
this paper, we propose that high-quality factor microcavity with strong
$\chi^{(2)}$ nonlinearity can be quantized to anharmonic energy levels and be
effectively treated as an artificial atom. Such artificial atom has a size much
larger than the photon wavelength, which enables passive and active
ultra-strong coupling to traveling photons. High-fidelity quantum control-phase
gate is realized by mediating the phase between photons with an intermediate
artificial atom in a photonic molecule structure. The scheme avoids the
two-photon emission and thus eliminates the spectral entanglement and quantum
phase noises. Experimental realization of the artificial atom can be envisioned
on the integrated photonic chip and holds great potential for
single-emitter-free, room-temperature quantum information processing.
