For the first time, scientists have realized a quantum light source based on a new two-dimensional material that is nonlinear

Ren Xifeng, academician of the Chinese Academy of Sciences and professor of the team of Guo Guangcan, professor of the University of Science and Technology of China, and Professor Qiu Chengwei and Dr.
Guo Qiangbing of the National University of Singapore have made important progress
in the research of nonlinear quantum light sources for two-dimensional materials.
On January 4, the research results were published in Nature under the title Ultrathin quantum light source with van der Waals NbOCl₂crystal
.

Miniaturization and integration are ideal solutions to solve the problems of poor stability and non-scalability of space optical quantum systems, and are also the only way
for optical quantum computing and quantum communication to become large-scale and practical.
As an indispensable part of quantum optical systems, the miniaturization of quantum light sources has always been the focus of
research.
In the early stage of cooperation with Nanjing University, Ren Xifeng introduced metasurfaces into the field of quantum information, integrated metalens arrays and nonlinear optical crystals, realized 100-path parametric downconversion, and prepared ultra-high-dimensional quantum entangled states and multiphoton sources [Science 368, 1487 (2020)
].

In order to further improve the integration of quantum light sources, Ren Xifeng cooperated with the National University of Singapore and others to use the nonlinear process of NbOCl₂, a new two-dimensional material, for the first time to realize an ultra-thin quantum light source with a thickness as low as 46nm
.

The intralayer crystal structure of two-dimensional materials is stable, while the interaction force between atomic layers is much
weaker.
Based on this characteristic, single-layer two-dimensional materials can maintain the stability of physical properties while maintaining the thickness of the atomic scale, so that the two-dimensional materials can be stably and flexibly directly coupled with various micro-nanoscale optical devices, so they are widely used in
various important components of integrated photonic chips.
Although common two-dimensional materials (_WS2,_{WSe 2}, etc.
) have large second-order nonlinear coefficients, the thickness of the single layer is too thin (<1nm), resulting in a low
overall nonlinear signal strength.
If the number of layers of the material is increased, the second-order nonlinear process will be weakened or even disappeared
due to the spatial symmetry caused by the stacking of multiple layers.

A novel_NbOCl2 material
was used in this study.
It has a high second-order nonlinear coefficient characteristic of common single-layer two-dimensional materials, and more importantly, its weak interlayer electron coupling and asymmetric
spatial structure.
This characteristic makes its second-order nonlinear signal intensity increase with the increase of the number of layers of the two-dimensional material, which can exceed the WS2 frequency doubling intensity of the single-layer two-dimensional material by more than
two orders of magnitude.

In this study, the spontaneous parametric downconversion process
of multilayer NbOCl2 two-dimensional materials was further tested.
A continuous laser with a wavelength of 404 nm (maximum pump power of 59 mW) was used to pump the two-dimensional material and collect the parametric light
around 808 nm generated by the downconversion process.
The second-order association function g(2) test results far exceed 2, proving that the process produces photon pairs
with nonclassical associations.
At the same time, the variation relationship between the intensity of parametric optical signals and the thickness of two-dimensional materials was measured, and the experimental results were completely consistent
with theoretical expectations.
It is worth noting that experiments have confirmed that the material with a thickness as low as 46 nm can also prepare quantum light sources, which is the thinnest nonlinear quantum light source
currently reported internationally.
This research provides an integrable quantum light source for the study of optical quantum information, and opens up a new direction
for the nonlinear study of two-dimensional materials.

The research work is supported
by the Ministry of Science and Technology, the National Natural Science Foundation of China, the Chinese Academy of Sciences, Anhui Province, and the University of Science and Technology of China.
Researchers from the University of Chinese Academy of Sciences participated in the research
.

Figure 1.
Structural test of_NbOCl2 crystal, monolayer thickness of about 0.
65 nm

Figure 2.
Testing of frequency-doubling second-order nonlinear processes of_NbOCl2 two-dimensional materials

Figure 3.
Quantum light source based on_NbOCl2 two-dimensional materials