[论文解读] Why I am optimistic about the silicon-photonic route to quantum computing

Terry Rudolph|arXiv (Cornell University)|Jul 28, 2016

Photonic and Optical Devices被引用 4

一句话总结

该论文认为，大规模硅基光子量子计算的关键在于实现高效、近确定性的三光子纠缠态源，从而通过线性光学实现可扩展的簇态量子计算。一旦此类光源得以实现，该架构仅需每光子恒定数量的光学元件，利用当前CMOS兼容的光子集成电路即可实现，并有望将误码率控制在$10^{-8}$以下。

ABSTRACT

This is a short overview explaining how building a large-scale, silicon-photonic quantum computer has been reduced to the creation of good sources of 3-photon entangled states (and may simplify further). Given such sources, each photon need pass through a small, constant, number of components, interfering with at most 2 other spatially nearby photons, and current photonics engineering has already demonstrated the manufacture of thousands of components on two-dimensional semiconductor chips with performance that allows the creation of tens of thousands of photons entangled in a state universal for quantum computation. At present the fully-integrated, silicon-photonic architecture we envisage involves creating the required entangled states by starting with single-photons produced non-determistically by pumping silicon waveguides (or cavities) combined with on-chip filters and nanowire superconducting detectors to herald that a photon has been produced. These sources are multiplexed into being near-deterministic, and the single photons then passed through an interferometer to non-deterministically produce small entangled states - necessarily multiplexed to near-determinism again. This is followed by a `ballistic' scattering of the small-scale entangled photons through an interferometer such that some photons are detected, leaving the remainder in a large-scale entangled state which is provably universal for quantum computing implemented by single-photon measurements. There are a large number of questions regarding the optimum ways to make and use the final cluster state, dealing with static imperfections, constructing the initial entangled photon sources and so on, that need to be investigated before we can aim for millions of qubits capable of billions of computational time-steps. The focus in this article is on the theoretical side of such questions.

研究动机与目标

• 论证构建可扩展硅基光子量子计算机的主要挑战在于高效生成三光子纠缠态。
• 证明一旦此类光源可用，其余架构将简化为线性光学干涉仪，而该技术已可通过当前光子集成电路（PIC）技术实现。
• 强调该方法无需量子存储器，并通过每光子仅使用恒定数量的元件（无论计算规模大小）来最小化资源开销。
• 倡导针对损耗容错图态、复用方案以及抗错误簇态架构的理论研究，以适用于光子系统。
• 强调所提出的架构与现有CMOS制造工艺兼容，从而可实现未来扩展至数百万个元件。

提出的方法

• 该架构利用泵浦硅波导或微腔实现非确定性单光子源，结合片上滤波器和超导纳米线探测器实现后选择（heralding）。
• 单光子通过复用实现近确定性操作，随后通过干涉仪生成小规模纠缠态，再经复用实现近确定性。
• 通过一种‘弹道’散射过程，将这些小规模纠缠态分布至大型干涉仪中，光子探测将剩余光子投影至大规模、通用的簇态。
• 该簇态用于测量型量子计算，其中Hadamard门和S门通过现场预准备的图态实现。
• 提出损耗容错图态（如“疯狂图”）以在不依赖量子存储器或长延迟的情况下抵御光子丢失。
• 采用复用技术抑制随机噪声并提高资源效率，理论分析表明误码率有望低于$10^{-8}$。

实验结果

研究问题

• RQ1 是否可在不依赖量子存储器或长延迟的前提下，将高度损耗容错的量子码集成至弹道光子架构中？
• RQ2 是否存在不放大状态制备过程中随机Pauli噪声的方法，以生成所需的簇态图？
• RQ3 ‘疯狂图’架构是否可借鉴树码原理，提升量子中继器的性能？
• RQ4 如何最优地复用非确定性光源，以在最小资源开销下实现近确定性的三光子纠缠？
• RQ5 理论框架如何适配硅光子学的独特约束，特别是噪声与可扩展性方面？

主要发现

• 可扩展硅基光子量子计算的主要挑战在于实现高效、近确定性的三光子纠缠态；一旦解决，其余架构将变得极为直接。
• 当前光子集成电路技术已能在一个芯片上制造数千个元件，性能足以生成数万个纠缠光子，支持通用量子计算。
• 理论分析表明，硅光子系统中的随机误码率可低至$10^{-8}$，远低于容错量子计算所需的$10^{-6}$阈值。
• 最终簇态中每个功能量子比特所需物理光子少于20个，仅有两个光子通过潜在噪声较大的有源元件。
• 通过复用的片上单光子源与干涉仪态生成，可实现无需量子存储器的可扩展、模块化簇态制备。
• 理论框架如“疯狂图”态在损耗容错方面展现出前景，且可被适配以提升量子中继器性能。