Quantum causality violates classical intuitions of cause and effect and is a unique quantum feature different from other quantum phenomena such as entanglement and quantum nonlocality. In order to avoid the detection loophole in quantum causality, we initiate the study of the detection efficiency requirement for observing quantum causality. We first show that previous classical causal inequalities require detection efficiency at least 95.97% (89.44%) to show violation with quantum (nonsignaling) correlations. Next we derive a classical causal inequality I_{222} and show that it requires lower detection efficiency to be violated, 92.39% for quantum correlations and 81.65% for nonsignaling correlations, hence substantially reducing the requirement on detection. Then we extend this causal inequality to the case of multiple measurement settings and analyze the corresponding detection efficiency. After that, we show that previous quantum causal inequalities require detection efficiency at least 94.29% to violate with nonsignaling correlations. We subsequently derive a quantum causal bound J_{222} that has a lower detection efficiency requirement of 91.02% for violation with nonsignaling correlations. Our work paves the way towards an experimental demonstration of quantum causality and shows that causal inequalities significantly differ from Bell inequalities in terms of the detection efficiency requirement.
Vacuum Texture: A New Interpretation of Quantum Mechanics and a New Loophole for Bell's Inequality Measurements that preserves Local Realism and Causality
Testing random-detector-efficiency countermeasure in a commercial system reveals a breakable unrealistic assumption
在过去的十年里，人们一直在努力调和量子密钥分发（QKD）的理论安全性与现实不完美的实现。提出了可实施的对策来修补发现的漏洞。然而，某些反措施并不像预期的那样有力。在本文中，我们提出了一个具体的例子ID量子的随机检测器效率对策检测器盲攻击。作为第三方测试人员，我们发现这种对策的第一个工业实现对原始的致盲攻击有效，但对修改后的致盲攻击并不免疫。然后，我们实现并测试了这个对策的一个完整版本，其中包含一个安全证明[C。C. W. Lim等人，IEEE Journal of Selected Topics in Quantum Electronics，21，6601305（2015）]。我们发现，它仍然是容易受到修改的盲攻击，因为一个假设的硬件特性的证明依赖失败的实践。
惠勒的延迟选择实验研究了波粒二象性的不确定性和测量仪器在量子理论中所扮演的角色。由于与经典物理学的不一致，人们普遍认为不可能用隐变量理论重现延迟选择实验。最近的研究表明，这种假设是错误的，事实上惠勒的延迟选择实验可以用因果二维隐变量理论来解释[R。Chaves，G. B. Lemos，and J. Pienaar，Phys. Rev. Lett. 120，190401（2018）]。在这里，我们进行了一个实验的设备无关的延迟选择实验使用的光子状态是空间分离的，并证明了一个无漏洞版本的延迟选择协议是符合量子理论，但不符合任何因果关系的二维隐变量理论。这挽救了惠勒的思想实验，并表明因果关系可以用来检验量子理论，以补充贝尔和莱格特-加格检验。
