The University of Science and Technology of China has made important progress in

Academician Guo Guangcan of the University of Science and Technology of China has made important progress in the research of integrated photonic chip quantum devices. Zou Changling and Li Ming research groups of this team proposed a general method for artificially synthesizing optical nonlinear processes. High efficiency synthetic high-order nonlinear processes were experimentally observed in the integrated chip microcavity, and their application potential in the cross band quantum entanglement light source was demonstrated. Relevant achievements were published online in the international academic journal Nature Communication on October 20 under the title of "Synthetic five wave mixing in an integrated microcavity for visible telecom entry generation".

Since the advent of laser, nonlinear optical effects have been widely used in optical imaging, optical sensing, frequency conversion and precision spectroscopy. For the emerging quantum information processing, it is also the core element to realize quantum entanglement light source and quantum logic gate operation. However, due to the intrinsic property that the nonlinear susceptibility of materials decreases exponentially with order, the application of optical nonlinearity is mainly limited to second-order and third-order processes, and high-order processes involving multiple photons are rarely studied. On the one hand, low order processes limit the performance of traditional nonlinear and quantum devices, such as the scalability of quantum light sources; On the other hand, people are also curious about the novel nonlinear and quantum physical phenomena contained in high-order nonlinear processes.

The nonlinear interaction between photons can be enhanced by using micro nano optical structures on integrated photonic chips, which has become a research hotspot in the field of integrated optics and nonlinear optics. Zou Changling's research team, Li Ming, and others have been committed to the research of integrated photonic chip quantum devices for a long time, expanding the micro cavity enhanced nonlinear photonics, proposing and confirming the synergistic effect of multiple nonlinear processes in the micro cavity [PRL 126, 133601 (2021); PRA 98, 013854 (2018)], and opening up a new way for quantum devices with fewer photons or even single photons at room temperature [PRL 129, 043601 (2022); PRApplied 13, 044013 (2020)]. At this stage, the research group has been able to increase the decay rate of nonlinear interaction intensity with order from 10-10 to 10-5. Even so, it is still a challenge to observe high efficiency nonlinear effects with order greater than three experimentally on integrated photonic chips.

Figure 1. Schematic diagram of artificially synthesized nonlinear five wave mixing
In response to this problem, Li Ming and others have found another way to propose a novel theory of nonlinear process synthesis, that is, using the inherent strong second-order, third-order and other low order effects of materials, by manually adjusting the nonlinear optical network formed by cascading multiple low order processes to achieve any form, any order of nonlinear photon interaction. This method avoids the nonlinear response of the material to be modified at the atomic scale, and only needs to control the geometry of the micro/nano devices to achieve an efficient, reconfigurable high-order nonlinear process.

Figure 2. Characterization of nonlinear processes
Using the integrated aluminum nitride optical microcavity, the team experimentally manipulated the second-order sum frequency process and the third-order four wave mixing process simultaneously to synthesize higher-order fourth-order nonlinear processes. Experiments show that the synthetic process is more than 500 times stronger than the inherent fourth order nonlinear effect of the material. If the quality factor of the microcavity is further improved, the enhancement multiple can reach more than 10 million.

The team applied the artificially synthesized fourth order nonlinearity to generate quantum entanglement light sources across the visible communication band. The coherence of the synthetic process is verified by measuring the time energy entanglement between the cross band photons. Compared with the traditional generation method of cross band quantum entanglement light source, this work greatly reduces the difficulty of phase matching, and only requires a single pump laser in the communication band, showing the advantages and application potential of artificial synthesis of nonlinear processes. The reviewers highly affirmed the innovation of the work ("it should be published in Nature communication for its innovation qualities").

Wang Jiaqi and Yang Yuanhao, doctoral students of the Key Laboratory of Quantum Information, Chinese Academy of Sciences, are the co first authors of the paper, and Li Ming, associate researcher, and Professor Zou Changling are the corresponding authors of the paper. This research has been supported by the key R&D plan of the Ministry of Science and Technology of the People's Republic of China, the National Natural Science Foundation of the People's Republic of China, the Natural Science Foundation of Anhui Province and the basic research business fees of central universities.

Paper link: https://www.nature.com/articles/s41467-022-33914-5

Source: Guangxingtianxia