A team led by Prof. Xifeng Ren from the University of Science and Technology of China (USTC), in collaboration with Prof. Chengwei Qiu and Dr. Qiangbing Guo from the National University of Singapore (NUS), has recently made advancements in SPDC-based quantum light sources depending on two-dimensional layered materials. Nature published the findings.
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A path to practical and scalable solutions for optical quantum computing and quantum communications is made possible by the widely used miniaturization strategies and integration in optical quantum systems.
Researchers in this study reported a van der Waals crystal (NbOCl2) with bulk excitonic behavior resembling a monolayer, implying a confirmed weak interlayer electronic coupling. According to theoretical calculations, the crystal’s strong ionic Nb-Cl bond is where this weak interlayer coupling originated.
Second-harmonic generation, or SHG, is the lowest-order nonlinear optical process in which the generation of light with a second-harmonic frequency is caused by a second-order nonlinear optical susceptibility.
Conventional 2D materials, such as WS2, exhibit a decreasing SHG response as the layer count rises despite having a high second-order nonlinear susceptibility, whereas the scalable SHG intensity in NbOCl2 is up to three orders of magnitude higher than that in monolayer WS2.
Notably, this recently discovered crystal flake is only 46 nm thick. The 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, is made possible by the strong second-order nonlinearity of the crystal NbOCl2. This means that the detection of one photon of the pair signals the presence of the other.
The discovery makes crystal NbOCl2 the thinnest and first-ever two-dimensional SPDC source to be reported.
The outcome opens a new avenue for the investigation of optical nonlinearity in two-dimensional materials and also offers an integrable quantum light source for optical quantum information technology.
Guo, Q., et al. (2023) Ultrathin quantum light source with van der Waals NbOCl2 crystal. Nature. doi:10.1038/s41586-022-05393-7.