Quantum light to realize metasurface computer-generated holography
Fig. 1. Metasurface holographic experimental setup using quantum light source and raster scan.
Fig. 2. Imaging results. The first line is the simulation result, the second line is the result of laser illumination and CCD imaging, and the third line is the result of quantum light source illumination and raster scan imaging.
Metasurfaces have gradually represented powerful abilities to manipulate the amplitude, phase, polarization as well as orbital angular momentum of output light with subwavelength resolution. Due to the subwavelength pixel size, the reconstructed images of metasurface holography can obtain higher resolution and large field of view compare to traditional holography based on spatial light modulator. Meanwhile, the unwanted diffraction orders can also be eliminated.
Benefiting from the powerful multi-dimensional light field manipulation ability provided by the metasurfaces, multiple independent holograms can be encoded into single metasurface and enhance the information capabilities successfully. Besides, the metasurface holographic imaging method also paves the way to realize beam shaping in miniaturized optical systems. Therefore, metasurface holography is of the great research value.
With the development of quantum information, it is a natural idea to apply the metasurface holography technology to the compact quantum information systems. The single photon light source, as a typical quantum light source, has a special property: wave-particle duality. The spatial distribution of single-photon wave packets could be described by the quantum wave function. When quantum observation is performed on a single photon, the quantum wave function will collapse to a certain eigenstate.
Most of the current research on metasurface holography imaging are realized under laser illmumination. However, acquiring holographic images with single-photon light source illumination has not been realized. In addition, how wave-particle duality is reflected in the interaction of single photon with the metasurface also needs to be further investigated.
To solve the above problems. The research group led by Prof. An-Ning Zhang cooperated with Prof. Ling-Ling Huang from the Beijing Institute of Technology. They worked together to reduce the loss of quantum light source by optimizing the experimental light path and using quantum state modulation theory to realize metasurface holography based on quantum light source. The relevant research results are published in Photonics Research, Volume. 10, Issue 11, 2022 (Jia-Zhi Yang, Rui-Zhe Zhao, Zhe Meng, Jian Li, Qing-Yuan Wu, Ling-Ling Huang, An-Ning Zhang. Quantum metasurface holography[J]. Photonics Research, 2022, 10(11): 2607).
Based on a modified Gerchberg–Saxton algorithm, multiple holograms with quantified phase relations are generated and encoded into different polarization channels of single metasurface. The reconstructed holographic images are successfully obtained under laser illumination firstly. Then a metasurface holographic experimental device under quantum light source illumination as well as raster scan detection is built, as shown in Fig. 1.
The quantum light source used in this work is heralded single photon source. One signal photon is used to predict the existence of another idle photon and can be detected by a detector. In addition, we employ another detector to monitor the photon count rate reflected by the polarization beam splitter before the metasurface. By adjusting the reflected photon count rate to the minimum, we achieved high photon utilization.
By controlling the polarization states of input and output, the reconstructions of images in different areas can be realized, and the results are shown in Fig. 2. For a single photon, its propagation satisfies the description of the quantum wave function. The choice of the input and output polarization state is to project the quantum wave function to a specific measurement basis. As a result, the quantum state will collapse. It will cause a change in the quantum wave function of the single photon, not just a strength reduction.
As shown in Fig. 2, the reconstructed images under quantum light source are basically consistent with the results under laser illumination. This result shows that the quantum light will interact with the entire metasurface sample when passing through it. In other words, a single photon will be modulated by the entire metasurface sample. This phenomenon satisfies the description of quantum wave function, indicating that quantum wave function can be modulated by subwavelength structure. This conclusion will effectively expand the application of metasurface in quantum control.
Professor Ling-Ling Huang believes that the use of quantum light sources in metasurface computer-generated holography is of great significance to metasurfaces fields and quantum imaging fields. Metasurface has attracted much attention in recent years due to its special subwavelength structure and light field manipulation ability. Quantum light sources have also received more and more attention with the development of quantum technology. Combining quantum light sources and metasurface holography will expand the application field of quantum light sources, and is expected to provide new ideas and visions for the design of metasurface.
“超表面+全息术”：量子光源实现超表面计算全息
超表面通常由单层亚波长尺寸的金属或介质纳米天线阵列构成，能够对出射光的振幅、相位、偏振态、频率等物理量进行灵活的调控。相比于基于空间光调制器的传统全息方法，基于超表面的全息方法具有再现像分辨率高、视场角大以及不存在高级衍射级次串扰等优点。
受益于超表面强大的多维度光场调控能力，多幅独立的全息图能够编码于同一超表面之中，进而通过复用的方式提高全息图的信息容量。同时，超表面全息成像方法也为在小型化光学系统当中实现光束整形提供了新的思路。
随着量子信息科学技术的发展，将具有小型化优势的超表面全息成像用于量子信息科学是一个自然的想法。单光子是量子信息科学中的典型量子光源，具有波粒二象性。单光子波包的空间分布满足量子波函数描述，对单光子进行量子探测，单光子波包将量子塌缩到某一确定位置。目前的超表面全息成像研究大多使用激光照明，基于单光子光源照明的超表面全息成像尚未实现，单光子与超表面相互作用时如何体现波粒二象性也有待于进一步研究。
为解决以上问题，且实现基于量子光源的超表面全息成像，北京理工大学张安宁教授课题组和黄玲玲教授课题组合作，优化实验光路，减少量子光源的损耗，通过量子态调控实现了基于量子光源的多通道超表面全息成像，并提高了量子成像结果的对比度。相关研究成果发表于Photonics Research 2022年第11期（Jia-Zhi Yang, Rui-Zhe Zhao, Zhe Meng, Jian Li, Qing-Yuan Wu, Ling-Ling Huang, An-Ning Zhang. Quantum metasurface holography[J]. Photonics Research, 2022, 10(11): 2607）。
该工作首先利用改进的Gerchberg–Saxton算法，生成了多幅相位相关联的全息图。利用双折射介质超表面结合偏振旋转矩阵，将多幅全息图编码到同一超表面的不同偏振通道之中。在使用激光照明的情况下测试了样品的全息成像功能，然后搭建了使用量子光源照明和扫描探测的超表面全息实验装置，如图1所示。
该工作中使用的量子光源是预报式单光子源，依靠一个信号光子来预报另一个闲置光子的存在，因此需要对信号光子进行探测。另外该工作还使用一个探测器来探测输入光经过偏振分束器反射的光子，通过调节反射光计数率到最低来确保输入到超表面上的光最强，保证了高的光子利用率。
通过控制输入和输出的偏振态，可以实现不同通道的展示，如图2所示。对于单光子来说，其传播过程满足量子波函数的描述，对于输入和输出偏振态的控制实际上是将单光子的波函数投影到了特定的测量基下，这种投影测量的结果就是量子态坍缩，经过投影测量的单光子的波函数发生了改变，而不仅仅是强度的减弱。
图2表明量子光源的成像结果和激光光源的结果基本一致。这一结果说明光量子在经过超表面时会和整个超表面发生作用，单个光子会受到整个超表面的调制。这一现象满足量子波函数的描述，说明量子波函数可以被亚波长结构的超表面调制。该结论将有效的拓展超表面在量子调控中的应用。
黄玲玲教授表示：“将量子光源用于超表面计算全息中对于超表面和量子成像领域都有重要意义。超表面因其亚波长的特殊结构和优秀的光场调节能力近年来备受瞩目，而量子光源随着量子技术的发展也越来越受到重视，将量子光源和超表面全息结合将拓展量子光源的应用领域，同时有望为超表面的设计提供新的思路和视野。”