Process tomography, the experimental characterization of physical processes, is a central task in science and engineering. Here we investigate the axiomatic requirements that guarantee the in-principle feasibility of process tomography in general physical theories. Specifically, we explore the requirement that process tomography should be achievable with a finite number of auxiliary systems. We show that this requirement is satisfied in every theory equipped with universal extensions, that is, correlated states from which all other correlations can be generated with non-zero probability. We show that universal extensions exist in two cases: (1) theories permitting conclusive state teleportation, and (2) theories satisfying three properties of Causality, Pure Product States, and Purification. In the second case, the existence of universal extensions follows from a symmetry property of Purification, whereby all pure bipartite states with the same marginal on one system are interconvertible. Crucially, our results hold even in theories that do not satisfy Local Tomography, the property that the state of any composite system can be identified from the correlations of local measurements. Summarizing, the existence of universal extensions, without any additional requirement of Local Tomography, is a sufficient guarantee for the characterizability of physical processes using a finite number of auxiliary systems.
翻译:物理过程的实验性剖析过程,即物理过程的实验性定性,是科学和工程的一项核心任务。在这里,我们调查了在一般物理理论中保证过程剖析过程在原则上的可行性的逻辑要求。具体地说,我们探索了程序剖析应能够用数量有限的辅助系统来实现的要求。我们表明,在具备普遍扩展的每一个理论中,即所有单一的两边国家都满足了这一要求,也就是说,所有具有非零概率的所有其他关联都可以从中产生。我们表明,在两种情况下存在着普遍扩展:(1) 允许最终国家遥测的理论,和(2) 满足Causity、纯产品国家和纯化的三种特性的理论。在第二种情况下,普遍扩展的存在产生于纯化的对称属性,即所有在一个系统中具有相同边际的纯双方国家都是可相互转换的。我们的结果甚至存在于不满足本地地形学的理论中,任何复合系统的状况都可以从地方测量的相互关系中确定。归结,存在普遍扩展,而无需对本地地形学的任何额外要求,使用固定的物理特性是充分的保证。