Quantum causal inference with extremely light touch

IF 6.6 1区 物理与天体物理 Q1 PHYSICS, APPLIED
Xiangjing Liu, Yixian Qiu, Oscar Dahlsten, Vlatko Vedral
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Abstract

We give a causal inference scheme using quantum observations alone for a case with both temporal and spatial correlations: a bipartite quantum system with measurements at two times. The protocol determines compatibility with five causal structures distinguished by the direction of causal influence and whether there are initial correlations. We derive and exploit a closed-form expression for the spacetime pseudo-density matrix (PDM) for many times and qubits. This PDM can be determined by light-touch coarse-grained measurements alone. We prove that if there is no signalling between two subsystems, the reduced state of the PDM cannot have negativity, regardless of initial spatial correlations. In addition, the protocol exploits the time asymmetry of the PDM to determine the temporal order. The protocol succeeds for a state with coherence undergoing a fully decohering channel. Thus coherence in the channel is not necessary for the quantum advantage of causal inference from observations alone.

Abstract Image

极轻触摸的量子因果推理
我们给出了一个单独使用量子观测的因果推理方案,用于同时具有时间和空间相关性的情况:具有两次测量的二部量子系统。该协议确定与五种因果结构的兼容性,这些结构由因果影响的方向和是否存在初始相关性来区分。我们推导并利用了多次、多量子位的时空伪密度矩阵(PDM)的封闭表达式。该PDM可以仅通过轻触式粗粒度测量来确定。我们证明了如果两个子系统之间没有信号,那么无论初始空间相关性如何,PDM的约简状态都不可能具有负性。此外,该协议利用PDM的时间不对称性来确定时间顺序。该协议对于经历完全退相干信道的相干状态是成功的。因此,通道中的相干性对于仅从观测中进行因果推理的量子优势来说是不必要的。
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来源期刊
npj Quantum Information
npj Quantum Information Computer Science-Computer Science (miscellaneous)
CiteScore
13.70
自引率
3.90%
发文量
130
审稿时长
29 weeks
期刊介绍: The scope of npj Quantum Information spans across all relevant disciplines, fields, approaches and levels and so considers outstanding work ranging from fundamental research to applications and technologies.
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