[论文解读] Quantum-message-passing receiver for quantum-enhanced classical communications

Narayanan Rengaswamy, Henry D. Pfister+2 more|arXiv (Cornell University)|Mar 9, 2020

Optical Network Technologies被引用 2

一句话总结

本文提出了一种用于纯衰减信道中相干态BPSK调制光通信的量子消息传递（BPQM）接收机，利用光子量子电路对多个脉冲进行联合检测。该方法在分辨长度为5的二元线性码的8个码字时，实现了最小误码概率的量子极限，显著优于经典单脉冲检测方法，表明使用小型光子量子计算机有望实现Holevo容量。

ABSTRACT

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---measured in bits communicated per pulse---achievable with a receiver that detects (converts from optical to electrical domain) each modulated pulse one at a time, versus with the quantum-optimal joint-detection receiver that acts on a long codeword comprised of $n$ modulated pulses; an effect often termed superadditive capacity. The action of this receiver cannot be described as the detection of each individual pulse, interspersed with classical feedforward and soft-information post-processing. 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 single qubit, described by an inner product that is a function of the mean photon number of each BPSK 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) [arXiv:1607.04833]. We analyze this 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. We quantify its performance improvement over the (Dolinar) receiver that optimally detects one pulse at a time. Our result suggests that a BPQM-receiver might attain the Holevo capacity, the quantum limit of classical communication capacity, of this BPSK-modulated pure-loss channel. This suggests a new application for a small, special-purpose, photonic quantum computer capable of so-called cat-basis universal qubit logic.

研究动机与目标

• 通过设计一种联合检测接收机，弥合低光子数光通信中经典极限与量子极限之间的差距。
• 解决相干态BPSK调制的纯衰减信道中存在超加性容量增益的问题，其中单脉冲检测并非最优。
• 基于信念传播与量子信息的BPQM框架，实现对码字的量子电路联合检测。
• 证明BPQM接收机在分辨BPSK码字时可达到最小误码概率的量子极限。
• 探索使用小型专用光子量子计算机实现光通信中Holevo容量的可行性。

提出的方法

• 将BPSK调制的光脉冲映射为内积依赖于平均光子数的非正交量子比特态。
• 基于BPQM框架构建量子电路，实现对n=5个脉冲码字的联合检测。
• 利用二元线性码的树状因子图表示来构建消息传递协议。
• 采用猫态基通用量子比特逻辑实现光子电路中的必要量子操作。
• 以最小平均误码概率作为性能指标，分析接收机性能。
• 将BPQM接收机的误码性能与Dolinar接收机（最优单脉冲检测）进行对比。

实验结果

研究问题

• RQ1 基于BPQM的接收机是否能在分辨长度为5的BPSK码字时达到最小误码概率的量子极限？
• RQ2 在BPSK调制的纯衰减信道中，BPQM接收机的误码性能与Dolinar接收机相比如何？
• RQ3 BPQM接收机在低光子数光通信中能多大程度上利用超加性容量增益？
• RQ4 使用猫态基通用量子比特操作实现光子量子电路以达到Holevo容量是否可行？
• RQ5 BPQM框架能否扩展至更一般的光通信场景以实现量子容量极限？

主要发现

• BPQM接收机在分辨长度为5的二元线性码的8个BPSK码字时，实现了最小平均误码概率的量子极限。
• 与Dolinar接收机相比，性能增益显著，证明了联合检测相对于单脉冲检测的优势。
• BPQM接收机的误码概率与理论量子极限完全一致，证实了其在给定码与信道下的最优性。
• 结果表明，BPQM接收机有可能实现BPSK调制纯衰减信道的Holevo容量。
• 结果表明，使用猫态基通用量子比特逻辑的小型专用光子量子计算机实现此类接收机具有可行性。
• 明确的量子电路构造为光子实现量子增强型接收机提供了实用蓝图。