Tsinghua University realizes large-scale high-performance quantum light source

Single photon source is an important quantum light source and one of the cores of quantum information technology. In quantum key distribution of quantum confidential communication, single photon source is essential for secure transmission of information using quantum secret key distribution protocol; in quantum computing, extremely high purity single photon source is required to meet applications such as all-optical quantum repeaters. However, the current single-photon source preparation technology is far from meeting the needs of various quantum technology applications, especially in the controlled large-scale preparation of high-purity single-photon sources, facing various difficulties and challenges.

In view of this, Prof. Cunzheng Ning's team at the Department of Electronics of Tsinghua University has combined the controllable large-scale fabrication capability of laser processing and the excellent properties of two-dimensional broadband semiconductor material boron nitride (hBN) to solve several key problems of current single-photon light sources and achieve the spatially controllable large-scale fabrication of high-purity, high-brightness single-photon light sources.

Hexagonal boron nitride (hBN) is a new broadband semiconductor that has received great attention in recent years due to its layer-like structure, easy peeling into thin layers or even single molecular layer thickness, excellent material properties and easy integration with other two-dimensional materials. It has been found that single-photon sources can be prepared in hexagonal boron nitride at room temperature, and their good stability and easy integration make them the most promising single-photon sources for practical applications. Currently, researchers usually generate single-photon sources by high-temperature annealing of hBN, chemical etching, electron beam/ion beam/neutron irradiation, and stress induction. Although all these methods can prepare single photons in hBN, they all have certain drawbacks, such as low brightness or purity of the generated single-photon sources, low yield of single-photon sources or high requirements for the processed equipment and processes. Therefore, it is necessary to explore a simple and efficient method to prepare high-quality single-photon sources.

Laser processing of single-photon sources is done by irradiating defects generated by ultrashort intense pulses as a light-emitting source, which has the advantage of being spatially controllable and can be produced on a large scale, but the previous practice has efficiency and purity problems due to thermal effects that are not well resolved. The main approach of this research is to optimize the single-pulse parameters by spatially given points for only single-pulse irradiation, which effectively avoids the thermal effect and low purity problems. The experimental team achieved efficient fabrication of a single photon source on a thin layer of hBN by single-pulse femtosecond laser irradiation. For every 100 single-pulse femtosecond laser irradiation positions, 43 single-photon sources can be produced, which is the highest yield among the current top-down fabrication methods. The purity and brightness of the single-photon source are very high. The minimum value of the second-order correlation function g2(0), which measures the single-photon index, is 0.06 ± 0.03, and the highest single-photon emission intensity is 8.69 Mcps, which is one of the brightest single-photon sources produced so far. In addition, the processing of materials by femtosecond laser direct writing technology is based on nonlinear processes such as multiphoton absorption, which can break the diffraction limit to induce the production of artificially controllable and highly spatially resolved micro and nano structures without the need for expensive micro and nano processing equipment and process conditions, and can be used for mass production.

Fig. 1 Arrays of defect structures of different sizes (a) and the corresponding photoluminescence images (b)(c) were prepared. (d) shows the statistically obtained single-photon source yields at different sizes

Figures 1a to c show the defect patterns and the corresponding photoluminescence images for four different sizes. Figure 1d shows the percentage of the total number of defects that produce single-photon (g2(τ) < 0.5) defects, i.e., the single-photon yield. The yield of the single-photon source increases with increasing defect pattern size, reaching the highest yield of 42.9% in a 3.0-μm-sized defect pattern, which is the highest of all top-down fabrication methods available. The single-photon source yield then gradually decreases as the size increases further. The black curve in Fig. 2a shows the photon emission rate of the single-photon source versus the pump power, and after fitting it, we can get the saturation photon emission rate of 8.69 Mcps, which is the highest brightness among the single-photon sources fabricated using top-down processing techniques so far.

Figure 2. (a) Photon emission rate versus pump power for the highest brightness single-photon source produced. (b) Luminous peak of a typical single-photon source and its second-order correlation function (inset)

Figure 2b shows a typical luminescence peak of a fabricated single-photon source with a luminescence peak wavelength around 540 nm. The inset shows its second-order correlation function, where g2(0) is 0.06 ± 0.03, indicating the high purity of the single-photon source. This is because the study utilized a single-pulse femtosecond laser, which avoids material damage caused by the thermal effects of high heavy-frequency pulsed lasers. In addition to the above-mentioned advantages of efficient fabrication of high-quality single-photon sources, the method is non-damaging to the substrate and has the capability of large-scale fabrication, which opens an effective way for the application of hBN single-photon sources in quantum integrated optical chips.

The results were recently published in the journal ACS Nano under the title "Large-Scale, High-Yield Laser Fabrication of Bright and Pure Single-Photon Emitters at Room Temperature in Hexagonal Boron Nitride", and were presented by Lin Gan, an assistant researcher in the Department of Electrical Engineering at Tsinghua University, and Yang Yang Zhang, a Ph. Yang Yang Zhang, a 2020 PhD student, is the co-first author, and Cunzheng Ning, a professor of electrical engineering at Tsinghua University, is the corresponding author. The work was performed by the Department of Electronic Engineering of Tsinghua University and the Institute of Integrated Circuits and Optoelectronic Chips of Shenzhen University of Technology.

This research was supported by the National Natural Science Foundation of China and the Beijing Natural Science Foundation of China.

Link to the paper：

https://pubs.acs.org/doi/10.1021/acsnano.2c04386