Recent advances in SPDC-based quantum light sources based on two-dimensional layered materials have been made by a team led by Prof. REN Xifeng from the University of Science and Technology of China (USTC), collaborating with Prof. QIU Chengwei and Dr. GUO Qiangbing from the National University of Singapore (NUS). The result was published in Nature.

Miniaturization and integration are strategies broadly employed in optical quantum systems to enhance their scalability and stability, thus providing a path to scalable and practical solutions for optical quantum computing and quantum communications.

In this study, researchers reported a van der Waals crystal (NbOCl2) featuring monolayer-like excitonic behavior in bulk form, indicating a verified weak interlayer electronic coupling. Theoretical calculations imply that such weak interlayer coupling derived from the strong ionic Nb-Cl bond in the crystal.

SHG (second-harmonic generation) is the lowest-order nonlinear optical process where the second order nonlinear optical susceptibility is responsible for the generation of light at second-harmonic frequency. Despite having a high second-order nonlinear susceptibility, conventional 2D materials (e.g. WS2) show a decreasing SHG response as the layer number increases, while the scalable SHG intensity in NbOCl2 is up to three orders of magnitude higher than that in monolayer WS2.

Notably, this newly-reported crystal flake is as thin as 46nm. The strong second-order nonlinearity of crystal NbOCl2 enables a spontaneous parametric down-conversion (SPDC) process, a second-order nonlinear process in which a photon from a strong pump laser is converted to a photon pair, which means a detection of one photon of the pair heralds the presence of the other.

The discovery by those scientists makes crystal NbOCl2 both the thinnest, and the first two-dimensional SPDC source ever reported.

The result not only provides an integrable quantum light source for optical quantum information technology, but also opens a new direction in the study of optical nonlinearity in two-dimensional materials.

(Written by QIU Rong, edited by TONG Xinyang, USTC News Center)