从黑盒子到因果律:寻找量子物理的信息原理

From Black Boxes to Causal Orders:The Quest for Information-Theoretical Principles of Quantum Physics

自20世纪初期量子物理诞生以来,对它物理意义的争论就没有停止过。这种现象出现的一部分原因是量子物理的理论框架更多的是对其背后数学空间的描述,而并未向人们揭示数学描述背后的物理原理。近些年来,随着量子信息的发展与量子技术的逐渐成熟,量子物理在信息处理等方面展现出超越经典系统的优越性。在量子信息中,基于器件无关思想的黑盒模型成为了探索量子物理背后逻辑的重要工具,黑盒模型作为一种只考虑实验室输入输出信息的抽象概念,具有不受限于特定理论表达的特点。寻找量子物理的信息原理之路大致有两条岔道:一方面,对黑盒内部物理原理与机制的研究产生了广义概率论。广义概率论中的黑盒内部可能遵守经典概率论、量子理论甚至是超越量子理论的未知物理理论,但黑盒之间总是存在全局的因果关系。另一方面,如果确定黑盒内部的运行原理可以用经典或量子理论描述,但并不假设不同的黑盒之间遵守确定的因果关系,则可以得到遵守经典或量子理论的数学描述,却并不能用全局确定因果关系来解释的现象。纵观对量子物理的信息原理的探索,可以看出用符合物理直觉的原理重构量子理论并不是一件容易的事。厘清量子物理背后的逻辑也许既需要对黑盒内部的运行机理有准确的刻画,又要对黑盒输入输出组合的因果关系有新的理解。

Arguments about the interpretation and meaning of quantum theory have persisted since its birth in the early 20th century to the present day. The difficulty of agreeing on fundamental aspects of quantum theory partly stems from its mathematical formulation, which obfuscated possible physical principles hiding therein. In recent years, quantum theory has shown its great potential in speeding up many information processing tasks such as computation and communication. Black box models, which are device-independent abstractions of input-output correlations of information-processing tasks, provide a valuable tool in our quest for information-theoretical principles of quantum physics. The quest for information theoretical principles of quantum physics diverges in two paths. On one hand, the internal mechanism of black boxes can be inferred from observing the their input-output correlations, while assuming they obey a global causal order. Such mechanisms can be classical probability theory,quantum theory or even some unknown theory with stronger-than-quantum correlations, all without violating the principles of relativity. On the other hand, it is possible that the inputs and outputs of several boxes are connected in a way such that separating the connections into causally distinct parts is impossible, while each black box obeys the laws of classical or quantum physics internally. A review of process in both paths shows that it is not easy to reconstruct quantum theory from information-theoretical principles. Understanding the physical intuition behind quantum theory may require both an accurate characterization of the internal mechanisms of black boxes and a deeper understanding of the causal orders that may arise from them.