With the rapid development of quantum technologies, optical circuits become more and more complex. Usually, several classical and quantum optical devices are needed in an optical circuit, which requires high stability. Free-space optics cannot meet the requirement of quantum information for further development any more. However, photonic integrated circuits (PICs) inherit the advantages of both integrated electronic circuits and free-space optics, such as low power consumption, high stability, high integration density, strong anti-jamming capability, and so on. So PICs attract more and more attention in recent years. In free-space optics, information is usually encoded on the two spin eigenstates of photons , whereas the routes of waveguide are used for encoding information in PICs. Therefore, in order to communicate information between free-space optics and PICs, it is essential that information is transformed between these two encoding schemes.
The research group, led by Prof. Guangcan Guo, from Key Lab of Quantum Information, University of Science and Technology of China, proposed a novel method to transform information from spin encoding to route encoding. It is known that the photon spin state, i.e., the polarization of light, could affect the properties of the excited surface plasmon polaritons (SPPs). By manipulating the structure of the medium and the spin state of the excitation light, the propagation mode of SPPs can be controlled. Based on this mechanism, a spin-route encoding convertor (SREC) with special array structure on the center was designed, so that SPPs could be steered to a particular waveguide according to the spin states of the excitation light. For example, SPPs can be output from the left (right) waveguide with the input of left (right) polarization. Accordingly, the transformation from spin encoding to route encoding is realized. It is reported in Chinese Optics Letters Vol. 12, No. 7, 2014 (http://www.osapublishing.org/col/abstract.cfm?uri=col-12-7-072401).
This method is based on the properties of SPPs depending on the excitation polarization. SPPs can break the diffraction limit and propagate along nanostructure. In addition, the SREC can be directly integrated on the chip, which can make the circuit more compact. It thus has a potential application in integrated photonics.
The SREC has proved to be theoretically feasible by this work. Future work will be focused on the experimental verification. Because the current micro-nano processing technology can meet the precision requirement of the designed SREC, and the optical detection sensitivity can reach single photon level, the transformation of information from spin encoding to route encoding is expected to be experimentally realizable.
中国科学技术大学郭光灿院士课题组提出了一种从自旋编码转换为路径编码的信息处理方法。光子的自旋态，即光的偏振，会影响所激发的表面等离子体激元（SPPs）的性质。通过调节介质结构和激发光子的自旋态，可以得到不同传播模式的SPPs。根据这个原理，设计特定的阵列介质结构，使不同自旋态光子激发的SPPs耦合到不同的波导路径中，如输入左旋偏振态的光子可以在左边波导中输出SPPs，输入右旋偏振态的光子则在右边波导中输出SPPs，从而实现信息从自由空间光学的自旋编码与集成光学的路径编码之间的转换。该方法是基于SPPs的性质对光子偏振的依赖性，SPPs能够突破衍射极限而束缚在微纳结构中传播，而且自旋-路径编码转换器可以直接集成在芯片上，提高器件的集成度，这在集成光学的研究中具有潜在的应用价值。相关研究成果发表在Chinese Optics Letters 2014年第7期上（http://www.opticsinfobase.org/col/abstract.cfm?uri=col-12-7-072401）。