Photon-photon interaction mediated by a virtual photon in nonlinear photonic circuits
Abstract: Photons are excellent quantum information carriers due to their ability to travel long distances through a vacuum without interacting with each other. However, the lack of interactions hinders their use in quantum information processing. The nonlinear optical processes are crucial for enabling photon-photon interactions. Such nonlinearities can arise from the post-selection of photons of photon detection, which is inherently probabilistic and inefficient. Atom-like media can also be used to produce highly nonlinear optical responses at single-photon levels, but the photon wavelengths and bandwidths are restricted by atomic transitions. The bulk optical nonlinearity of materials has attracted growing interest due to its integrated, solid-state form and compatibility with telecom quantum communication technologies. Nevertheless, due to the small nonlinear coefficient of bulk materials, the nonlinear effect is only observable under intense pumps. Observing nonlinear effects at single-photon levels still remains a significant challenge.
Here, we demonstrate the direct interaction between individual photons via χ(2) bulk nonlinearity implemented in integrated photonic circuits. In our scheme, the input two-photon state undergoes an elastic quantum scattering mediated by a virtual second-harmonic photon and gained a π phase shift relative to the linear transmission. The quantum scattering strength is boosted by the high-Q nano-cavity made from InGaP - a material with a substantial χ(2) coefficient. By engineering the direct transmission of the tunable photonic circuit, we can modify the output state, which is the interference between the quantum-scattered and direct-transmitted photons, allowing us to generate various exotic second-order correlations. The observed photon repulsion, photon attraction, and photon tunneling effects clearly demonstrate the existence of photon-photon interactions. Our approach represents a novel approach to realizing single-photon level interactions in bulk nonlinear systems, which is both fundamentally fascinating and holds great potential for quantum technologies such as quantum non-demolition measurements and the creation of quantum states.