Belief propagation with quantum messages for quantum-enhanced classical communications
Belief propagation with quantum messages for quantum-enhanced classical communications
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DOI：
10.1038/s41534-021-00422-1
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发表时间：
2020-03
影响因子：
7.6
通讯作者：
Narayanan Rengaswamy;K. Seshadreesan;S. Guha;H. Pfister
中科院分区：
文献类型：
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作者：
Narayanan Rengaswamy;K. Seshadreesan;S. Guha;H. Pfister
对于天基激光通信，当每个接收到的光脉冲的平均光子数远小于1时，执行单个脉冲检测的接收器与共同作用于调制脉冲长码字块的量子最优“联合检测接收器”之间的通信容量存在很大差距；这种效应通常被称为“超加性能力”。在本文中，我们考虑了已知具有大超加性容量的最简单场景：具有相干态二相移键控（BPSK）调制的纯损耗信道。两个BPSK状态可以在概念上映射到一个量子比特的两个非正交状态，用每个脉冲平均光子数的函数的内积来描述。利用这一映射，我们基于最近提出的“带量子信息的信念传播”（BPQM）思想，推导出联合探测接收机量子电路的显式结构。我们量化了它在Dolinar接收器上的性能改进，Dolinar接收器执行最佳脉冲逐脉冲检测，这代表了最佳的“经典”方法。我们对该方案进行了严格的分析，并证明该方案达到了用树因子图识别长度为5的二进制线性码的8 （BPSK）码字的最小平均错误概率的量子极限。我们的研究结果表明，BPQM接收器可以达到这种bpsk调制的纯损耗信道的Holevo容量。此外，我们的接收器电路为量子霸权实验提供了一种替代方案，针对特定应用，可以在能够执行基于猫的通用量子比特逻辑的小型专用光子量子计算机上实现。
For space-based laser communications, when the mean photon number per received optical pulse is much smaller than one, there is a large gap between communications capacity achievable with a receiver that performs individual pulse-by-pulse detection, and the quantum-optimal “joint-detection receiver” that acts collectively on long codeword-blocks of modulated pulses; an effect often termed “superadditive capacity”. In this paper, we consider the simplest scenario where a large superadditive capacity is known: a pure-loss channel with a coherent-state binary phase-shift keyed (BPSK) modulation. The two BPSK states can be mapped conceptually to two non-orthogonal states of a qubit, described by an inner product that is a function of the mean photon number per pulse. Using this map, we derive an explicit construction of the quantum circuit of a joint-detection receiver based on a recent idea of “belief-propagation with quantum messages”(BPQM). We quantify its performance improvement over the Dolinar receiver that performs optimal pulse-by-pulse detection, which represents the best “classical” approach. We analyze the scheme rigorously and show that it achieves the quantum limit of minimum average error probability in discriminating 8 (BPSK) codewords of a length-5 binary linear code with a tree factor graph. Our result suggests that a BPQM receiver might attain the Holevo capacity of this BPSK-modulated pure-loss channel. Moreover, our receiver circuit provides an alternative proposal for a quantum supremacy experiment, targeted at a specific application that can potentially be implemented on a small, special-purpose, photonic quantum computer capable of performing cat-basis universal qubit logic.