《Chip》发表清华大学张巍团队最新成果：在硅光量子芯片上实现偏振纠缠贝尔态的产生和操纵
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近日，清华大学张巍团队的最新研究成果「Generation and dynamic manipulation of frequency degenerate polarization entangled Bell states by a silicon quantum photonic circuit」发表于全球新锐综合性研究期刊Chip，第一作者为博士生刘东宁。该研究成果在硅光量子芯片上实现了偏振纠缠贝尔态的产生和操纵。
量子纠缠是实现量子信息功能的重要资源。偏振纠缠贝尔态则是在量子光学和光量子信息实验中应用最为广泛的纠缠双光子量子态。这类量子态可以通过自发参量下转换（SPDC）和自发四波混频（SFWM）等参量非线性光学过程产生，通过单侧光子的偏振控制来操纵，并且通过基于线性光学的贝尔状态测量（BSM）装置进行区分。因此，它们是量子信息的理想载体，在量子通信和光量子信息处理的研究中有重要应用。简单可靠的实现产生偏振纠缠贝尔态的量子光源一直是光量子器件领域重要的研究课题。特别是应用于光纤量子通信的光通信波段量子光源尤为引起人们关注。
基于硅光子集成的光量子芯片为实现这种量子光源提供了新方法。硅波导在光通信波段是良好的三阶非线性光学材料。仅仅利用几毫米长的硅波导就可以通过自发四波混频产生高质量的双光子量子态。此外，基于SOI的硅光子集成芯片上可以很方便实现大规模光学干涉仪阵列，这为光量子态的片上操控提供了手段。张巍教授团队利用硅光量子芯片技术实现了一种产生频率简并偏振纠缠贝尔态的量子光源新方案。
在这个量子光源芯片上，首先在四个长硅波导中利用自发的四波混频产生频率简并的关联双光子态。然后，这些双光子量子态分为两组，分别在两个片上分束器中相互干涉。通过调控这些量子干涉中的相位差，两个分束器输出的光子对将处于反聚束双光子态，即，处于这种量子态下的两个光子将分别从分束器的两个不同端口输出。由此，在两个分束器的输出端口处通过量子叠加实现了路径纠缠双光子态的产生。最后，成对的两个光子通过两个二维波导光栅耦合到两条光纤输出。在此过程中，芯片上的路径纠缠被转换为光纤中的偏振纠缠。输出的偏振纠缠双光子态可以通过芯片上的热移相器调控，由此这个量子光源芯片可以支持两个特定的偏振纠缠贝尔态输出， 和 。通过BSM实验验证了该芯片量子光源的输出量子态特性，表明基于线性光学的BSM可以成功分辨芯片输出的两个贝尔态。
「更重要的是，」张巍教授说，「输出的量子态可以动态操控，这将激发硅光量子芯片的新应用。」工作中利用BSM实验论证了芯片上热移相器对量子态动态调纵的瞬态过程。结果表明，输出量子态可以在 和 之间切换，调制速率达到10千赫兹。量子态动态操控的功能可以应用于基于贝尔态的量子信息编码，在量子通信和光量子信息处理中有重要前景。可以预见，量子态的片上动态操控将越来越受到关注，成为光量子芯片研究和应用的新切入点。
Generation and manipulation of polarization entangled Bell states are realized on a siliconphotonic chip
Quantum entanglement is an important resource for quantum information. Polarization entangled Bell states are the most widely used entangled biphoton states in quantum optics experiments. Photon pairs at these states can be generated by parametric nonlinear processes such as spontaneous parametric down-conversion and spontaneous four-wave mixing, manipulated by single-side polarization control and discriminated by Bell state measurement (BSM) based on linear optics. Hence, they are ideal carriers of quantum information and widely applied inquantum communications and photonic quantum information processing. Simple and reliable quantum light sources for polarization entangled Bell state generation are desired, especially at telecom band for quantum communications over optical fibers.
Quantum photonic circuits based on silicon photonics provide a promising way to realize quantum light sources for entanglement generation. Silicon waveguides are good nonlinear media at telecom band. By spontaneous four-wave mixing, high-quality biphoton states can be generated in silicon waveguides of several millimeters. Moreover, quantum state manipulation of these states can be realized by on-chip interferometers, which can be realized conveniently by silicon photonics based on SOI platform. In a new paper published on Chip, a team of researchers led by Professor Wei Zhang at Tsinghua University, China, proposed and demonstrated a clever silicon quantum photonic circuit for frequency-degenerate polarization entangled Bell states.
On the chip, frequency-degenerate biphoton states were generated in four long silicon waveguides by spontaneous four-wave mixing. Then, they interfered each other in two on-chip beam splitters. By properly adjusting the phase differences in these quantum interferences, photon pairs output from the two beam splitters would be at anti-bunched biphoton states, i.e., two photons in a pair at such a state would output from different ports of the beam splitter，respectively. As a result, a path-entangled biphoton state would be generated on the chip through quantum superposition. Finally, the two photons in a pair were coupled to two optical fibers through two two-dimensional waveguide gratings. In this process, path entanglement on the chip was converted to polarization entanglement in the optical fibers. The generated polarization entangled biphoton state could be controlled by on-chip thermal phase shifters, supporting the outputs of two specific polarization entangled Bell states, and .The experiment of BSM was carried to demonstrate the performance of this chip-based quantum light source, showing that its output Bell states could be discriminated by BSM successfully.
"More importantly", said Prof. Zhang, "its output state can be manipulated dynamically. It will inspire new applications of silicon photonic chips." The transient process of the dynamical manipulation by the on-chip thermal phase shifters was also demonstrated by the experiment of BSM. It showed that the output state of the chip could be switched between and , with a modulation rate on the order of ten kHz. This function could be used as quantum information encoding based on the Bell states, which has important applications in quantum communications and quantum information processing. It can be expected that on-chip dynamical manipulation of quantum states would attract more and more attention, as an important function of quantum photonic circuits based on silicon photonics, or other material platforms.
文章预印版：
https://doi.org/10.1016/j.chip.2021.100001
关于 Chip
Chip是全球唯一聚焦芯片类研究的综合性国际期刊，已入选由中国科协、教育部、科技部、中科院等单位联合实施的「中国科技期刊卓越行动计划高起点新刊项目」，为科技部鼓励发表「三类高质量论文」期刊之一。
Chip期刊由上海交通大学与Elsevier集团合作出版，并与多家国内外知名学术组织展开合作，为学术会议提供高质量交流平台。
Chip秉承创刊理念: All About Chip，旨在发表与芯片相关的各科研领域尖端突破，助力未来芯片科技发展。迄今为止，Chip已在其编委会汇集了来自12个国家的57名世界知名专家学者，其中包括多名中外院士及IEEE、ACM等知名国际协会终身会士（Fellow）。
Chip首刊将于2022年3月以完全开放获取形式发布。敬请期待！
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https://www.journals.elsevier.com/chip
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