Silicon photonic devices for scalable quantum information applications

Quantum information science is a new frontier subject combining quantum mechanics and information science. The quantum nature of particle superposition, entanglement, and measurement is applicable for more efficient information processing, computation, transmission, and storage. In the last decades, quantum fundamental science was rapidly transformed into quantum technologies with huge resources invested by global academia, research centers, and industry. Quantum scientific research is moving from the stage of principle verification of quantum rules to the stage of practical device research and development governed by these rules.

Photon, as one ideal information carrier, has been widely used in quantum information processing and shows several unique advantages, such as fast transmission speed, low noise, multiple degrees of freedom for information encoding and high capacity. Besides, light has a wide range of applications in energy, communications, computation, and medical care. These increasingly mature industrial applications provide favorable supports for photonic quantum technologies.

To enhance the complexity of quantum optical experiments, optical quantum systems tend to use integrated photonic circuits. Compared to systems that use discrete optical components on an optical table, the integrated photonic devices enable localization and manipulation of photons at micro/nano scales, thus greatly improving the stability and scalability of quantum optical experiments and providing a complex, compact quantum photonics approach for quantum communications, sensing, and computing applications. Therefore, these integrated techniques will take quantum applications out of the laboratory and into large-scale and practical applications.

Multiple optical materials for quantum integrated photonic device, such as silica, silicon (Si), silicon nitride (SiN), lithium niobate (LN) have been developed. Among those materials, silicon photonics is a good candidate because of its easy preparation, high integration density, and excellent optical properties. Thanks to the leadership of classical silicon photonics in large-scale photonic integrated circuits, the performance of silicon-based devices also has been rapidly improved, and the potential for applications in scalable quantum information processing with silicon is being fully exploited.

The invited review article published in Photonics Research, Vol. 10, No. 10, 2022 (Lantian Feng, Ming Zhang, Jianwei Wang, Xiaoqi Zhou, Xiaogang Qiang, Guangcan Guo, Xifeng Ren. Silicon photonic devices for scalable quantum information applications [J]. Photonics Research, 2022, 10(10): A135-A153) provides a comprehensive overview on the latest research effort and state-of-the-art technologies on silicon photonic chips for scalable quantum applications.

1 Scalable technique for silicon photonic chips

Scalable quantum information processing needs the quantum photonic source, detector, logic operation and other core functions to be improved with high quality and be integrated on the same chip. For photonic source preparation, silicon waveguides have a strong third-order nonlinear response and can be directly used to prepare photon pair sources via a spontaneous four-wave mixing process. After pair generation, one of the photons is detected to herald the presence of its partner. Although each photon pair source is probabilistic, multiple heralded single photons can be dynamically switched to one single output mode, thus increasing the output probability. For example, a 100% enhancement was achieved by multiplexing photons from four temporal modes. On the other hand, the solid-state emitters at telecommunications wavelengths have been demonstrated. In particular, recently demonstrated G centers and T centers, which originate from carbon-related defects in silicon, can be directly integrated into silicon waveguides without hybrid integration for large-scale quantum photonic information applications.

For single-photon-level detection, superconducting nanowire single-photon detectors show excellent performance, such as near-unity system detection efficiency, GHz maximum count rate and picosecond-level temporal resolution. For non-cryogenic applications, other type detectors, such as germanium-on-silicon and InGaAs/InP single-photon detectors, are potential alternatives. In addition to these conventional devices, single-photon detection based on low-dimensional materials is emerging and has shown superior performance.

To adjust photon's frequency and spatial mode degree, integrated wavelength and mode division multiplexing techniques have been demonstrated. Structures like ring resonators, unbalanced MZIs, waveguide Bragg gratings, and arrayed waveguide gratings (AWGs) have been used for wavelength multiplexing and demultiplexing. Mode division multiplexing is an emerging technique that uses the high-order transverse waveguide modes of multimode waveguides to encode more information. Since multimode waveguides still support multiple wavelengths, this technique is compatible with wavelength division multiplexing to further increase the channel capacity. Modulators have achieved high speeds and can even operate at cryogenic temperatures.

Chip interconnection, which will play a key role in building large-scale quantum networks, has been further developed (Fig. 1). For example, grating couplers and end couplers both achieve less than 1 dB coupling loss. In addition to fabricating complex coupling structures on a chip, many other efficient approaches are also worth considering. For example, 3D-printed optical probes on the fiber end faces, photonic wire bonding and in situ 3D nanoprinting.

2 Scalable quantum information applications

The development of integration technology has promoted the progress of quantum applications. In terms of multiphoton and high dimensional applications (Fig. 2), the number of detected photon has achieved 8, the visibility of four-photon interference achieved 96% and 15-dimensional two-photon entanglement demonstrated. Besides, high-dimensional programmable quantum processor has been demonstrated recently. In terms of quantum error correction, the success rate was increased from 62.5% to 95.8% when running a phase-estimation algorithm using the error-correction program. In terms of quantum key distribution, multiple encoding protocols and even high-dimensional chip-to-chip key distribution have been realized. In addition, chip-to-chip quantum state teleportation have been demonstrated by using on-chip multi-photon entanglement.

3 Challenged and outlook

Despite the progress mentioned above, further improvements are needed in some areas for scalable quantum information applications, such as low-loss components, high-quality multiphoton entanglement generation, deterministic quantum operation and frequency conversion. With further upgrading of the fabrication technology, silicon photonics will have greater prospects in quantum information processing. Despite future quantum information processors are most likely to be hybrid with various materials elevated to the extreme, we believe that silicon photonics will play an important role.

硅光子芯片：量子信息应用的“多面手”

量子信息是一门结合量子力学和信息科学的新兴学科。粒子叠加，纠缠以及测量的量子本质能够用于更加有效的信息处理过程。近几十年来，全球学术界、研究中心和工业界等投入了大量资源，用于将量子力学基本原理转化为实用的量子技术。量子科学的研究也正从量子规则的原理验证阶段，逐渐走向由量子规则支配的实际器件研发阶段，即第二次量子革命。

光子作为一种特殊的信息载体，具有传输速度快、噪声低、信息编码自由度多、容量大等独特优点，在量子信息处理中得到了广泛应用。此外，光波在能源、通信、计算、医疗等方面的广泛应用也为光量子技术的发展提供了有力支持，使得光量子系统成为一个有前途的量子信息处理平台。

为了提高系统的复杂性，光量子系统倾向于使用光子集成电路来实现。与在光学平台上使用离散光学元件的系统相比，集成光子器件能够在微/纳米尺度上对光子进行操纵，从而大大提高了光量子系统的稳定性。光子集成技术是引领量子应用走出实验室，走向大规模和实用化的必经之路。

多种光波导材料如二氧化硅、硅、氮化硅、铌酸锂等被开发出来用于展示光量子应用。在这些材料中，硅光子材料以其制备简单、集成密度高和优良的光学性能而成为很好的候选材料。同时由于经典硅光子学在大规模光子集成电路中的领导地位，硅基器件的性能也得到了迅速提高，其在可扩展量子信息处理方面的应用潜力正在得到充分的发挥。

为了体现光量子系统的稳定性和可扩展性，中国科学技术大学中科院量子信息重点实验室任希锋教授、冯兰天副研究员、郭光灿教授和浙江大学张明博士、北京大学王剑威研究员、中山大学周晓祺教授、军事科学院国防科技创新研究院强晓刚研究员，受邀介绍了用于可扩展量子信息应用的硅光子芯片的相关研究成果和最新技术。该综述文章表于Photonics Research2022年第10期（Lantian Feng, Ming Zhang, Jianwei Wang, Xiaoqi Zhou, Xiaogang Qiang, Guangcan Guo, Xifeng Ren. Silicon photonic devices for scalable quantum information applications[J]. Photonics Research, 2022, 10(10): A135）。

1.硅光子芯片上的可扩展量子技术

光量子应用需要将光子产生，调控和探测等功能集成在芯片上，近些年相关技术均取得了一系列重要进展。在光源制备方面，硅波导中的自发四波混频过程被用于制备关联光子对，其中一个光子用于先验另一个光子的存在。

多个先验单光子源可以通过复用技术来增加光子产率，如通过复用四个时间模式的光子，实现了100%的亮度增强。通信波段的单光子源也被制造出来，特别是最近的G色心和T色心，利用硅中的缺陷来构造单光子源，因而不需要混合集成技术就能在硅器件中用于大规模光量子信息应用。

在光子探测方面，超导纳米线单光子探测器获得了极佳的性能，如接近100%的探测效率，GHz最大计数率和皮秒级别的时间分辨率。此外，一些不需要极低温的探测技术也发展起来，如硅锗探测器和铟镓砷探测器，以及利用低维材料的单光子探测技术也逐渐发展起来。

为了对光子波长、空间模式等性质进行调节，研究们者发展了片上波分复用和模分复用技术。波分复用技术利用级联干涉仪或级联环腔实现了波长复用和滤波等功能；而模分复用技术利用一根多模波导来传输多路信息，进一步增加了片上信息处理的复杂度。此外，也发展了相应的低温调控技术，多种低温调制器被开发出来。

如图1所示，片间互联技术得到了进一步发展，如常见的光栅耦合器和端面耦合器，均实现了小于1 dB的耦合损耗。此外，多种新型的耦合技术被开发出来，如3D垂直耦合，3D打印，键合，原位3D纳米打印等，这些新型器件均达到了较为理想的性能。

2.可扩展量子应用进展

集成技术的发展推动了量子应用的进展。在多光子和高维扩展方面（如图2所示），探测光子数达到了8个，片上多光子干涉可见度达到了96%，并实现了15维的双光子纠缠态制备，四维可编程量子处理器也被开发出来。

在量子纠错方面，利用图态的纠错功能被展示，相位评估算法的成功率从62.5%提升到了95.8%。在量子密钥分发方面，多种编码方案甚至高维片间密钥分发均得以实现。此外，利用多光子纠缠，芯片间的量子态远程传输功能也被展示。

3. 挑战和展望

尽管取得了许多重要进展，大规模实用化的光量子应用仍要求硅光子集成器件在多个方面提升性能。如获得更低损耗的元件、高质量多光子量子态制备、确定性的量子操作以及高效的频率转换等。随着制造技术的进一步升级，硅光子学在量子信息处理方面将有更大的前景。任希锋教授表示：“尽管未来的量子信息处理器很可能是多种材料的混合应用，我们相信硅光子器件将在其中发挥重要作用。”