The nonclassical nature of a multi-photon wave packet is a key resource for overcoming the limitations of the present optical technologies governed by classical wave optics. Specifically, quantum entanglement of photons, which is the most striking nonclcassical feature of the multi-photon wave packet, plays a pivotal role in quantum optical technologies, including quantum information and communication, quantum imaging and quantum metrology.

One of our goals is to clarify the role of quantum entanglement in light-matter interactions at the single-photon level. Although there have been some reports on nonlinear light-matter interactions under extremely weak light conditions, almost all of these works focus on nonlinear optical phenomena caused by the differences in photon statistics between classical and nonclassical lights. However, it is predicted that optical nonlinear effects are sensitive to the temporal correlations of a multi-photon wave packet in a fully quantum mechanical treatment of nonlinear light-matter interactions, and that frequency correlation, or frequency entanglement, strongly affects the temporal correlation among the constituent photons in a multi-photon wave packet as a result of Fourier duality.

In order to further develop studies on light-matter interactions at the single-photon level, we are exploiting novel technologies for controlling and detecting quantum nature of light.