Photon-Photon Quantum Phase Gate in a Photonic Molecule with χ (2 ) Nonlinearity

The quantum logic between single photons lies at the foundation of deterministic, scalable quantum information processing. However, practical implementation suffers from weak optical nonlinearity, and gate fidelity is intrinsically limited by phase noise and spectral mixing. The authors address these concerns by utilizing an ultrahigh-$Q$ photonic microcavity with ${\ensuremath{\chi}}^{2}$ nonlinearity. Two-photon spontaneous emission is thoroughly suppressed by shutting off the coupling channels between this artificial atom and the continuum states of a waveguide. This promising scheme for room-temperature operation is almost within reach of current experiments, and can be generalized to other systems.