van der Lugt Tein, Barrett Jonathan, Chiribella Giulio
Department of Computer Science, University of Oxford, Wolfson Building, Parks Road, Oxford, OX1 3QD, United Kingdom.
Perimeter Institute for Theoretical Physics, Waterloo, ON, N2L 2Y5, Canada.
Nat Commun. 2023 Sep 19;14(1):5811. doi: 10.1038/s41467-023-40162-8.
Quantum theory is compatible with scenarios in which the order of operations is indefinite. Experimental investigations of such scenarios, all of which have been based on a process known as the quantum switch, have provided demonstrations of indefinite causal order conditioned on assumptions on the devices used in the laboratory. But is a device-independent certification possible, similar to the certification of Bell nonlocality through the violation of Bell inequalities? Previous results have shown that the answer is negative if the switch is considered in isolation. Here, however, we present an inequality that can be used to device-independently certify indefinite causal order in the quantum switch in the presence of an additional spacelike-separated observer under an assumption asserting the impossibility of superluminal and retrocausal influences.
量子理论与操作顺序不确定的情形是兼容的。对这类情形的实验研究均基于一种称为量子开关的过程，这些研究在实验室所用设备的假设条件下，证明了不确定的因果顺序。但是，能否进行与通过违反贝尔不等式来认证贝尔非局域性类似的、与设备无关的认证呢？先前的结果表明，如果单独考虑量子开关，答案是否定的。然而，在此我们提出一个不等式，在断言超光速和逆因果影响不可能存在的假设下，该不等式可用于在存在额外的类空分离观察者的情况下，对量子开关中的不确定因果顺序进行与设备无关的认证。
