Kato Kentaro
Quantum ICT Research Institute, Tamagawa University, 6-1-1 Tamagawagakuen, Machida 194-8610, Tokyo, Japan.
Entropy (Basel). 2022 Apr 21;24(5):581. doi: 10.3390/e24050581.
Modern optical communication technology can realize a large-scale multilevel (or -ary) optical signal. Investigating the quantum mechanical nature of such a large-scale -ary optical signal is essential for a unified understanding of quantum information science and optical communication technology. This article focuses on the quantum-mechanical non-orthogonality for a collection of pure quantum states and proposes a non-orthogonality index based on the least squares error criterion in quantum detection theory. First, we define the index for linearly independent signals, and the proposed index is analyzed through numerical simulations. Next, the index is applied to a highly large-scale -ary phase-shift keying (PSK) coherent state signal. Furthermore, the index is compared with the capacity of the pure state channel with the PSK signal. As a result, it is shown that a highly large-scale -ary PSK coherent state signal exhibits a quantum nature even when the signal transmission power is very high. Thus, the theoretical characterization of a highly large-scale -ary coherent state signal based on the proposed index will be the first step toward a better understanding of cutting-edge optical communication technologies such as the quantum stream cipher Y00.
现代光通信技术能够实现大规模多电平（或多进制）光信号。研究这种大规模多进制光信号的量子力学本质对于统一理解量子信息科学和光通信技术至关重要。本文聚焦于一组纯量子态的量子力学非正交性，并基于量子检测理论中的最小二乘误差准则提出了一种非正交性指标。首先，我们为线性独立信号定义该指标，并通过数值模拟对所提出的指标进行分析。接下来，将该指标应用于高度大规模多进制相移键控（PSK）相干态信号。此外，将该指标与具有PSK信号的纯态信道容量进行比较。结果表明，即使信号传输功率非常高，高度大规模多进制PSK相干态信号仍表现出量子特性。因此，基于所提出的指标对高度大规模多进制相干态信号进行理论表征将是朝着更好地理解诸如量子流密码Y00等前沿光通信技术迈出的第一步。
