Horodecki Paweł, Ramanathan Ravishankar
International Centre for Theory of Quantum Technologies, University of Gdańsk, Wita Stwosza 63, 80-308, Gdańsk, Poland.
Faculty of Applied Physics and Mathematics, National Quantum Information Centre, Gdańsk University of Technology, Gabriela Narutowicza 11/12, 80-233, Gdańsk, Poland.
Nat Commun. 2019 Apr 12;10(1):1701. doi: 10.1038/s41467-019-09505-2.
The ubiquitous no-signaling constraints state that the probability distributions of outputs of any subset of parties in a Bell experiment are independent of remaining parties' inputs. These constraints are considered to form ultimate limits for physical correlations and led to the fields of post-quantum cryptography, randomness generation besides identifying information-theoretic principles underlying quantum theory. Here we show that while these constraints are sufficient, they are not necessary to enforce relativistic causality in multi-party correlations, i.e., the rule that correlations do not allow casual loops. Depending on the space-time coordinates of the measurement events, causality only imposes a subset of no-signaling conditions. We first consider the n-party Bell experiment (n > 2) and identify all configurations where subsets of the constraints suffice. Secondly, we examine the implications for device-independent cryptography against an eavesdropper constrained only by relativity, detailing among other effects explicit attacks on well-known randomness amplification and key distribution protocols.
无处不在的无信号约束表明，在贝尔实验中，任何子集的参与方输出的概率分布都与其余参与方的输入无关。这些约束被认为构成了物理相关性的最终限制，并导致了后量子密码学、随机性生成等领域的发展，同时也揭示了量子理论背后的信息论原理。在这里，我们表明，虽然这些约束是充分的，但对于在多方关联中强制实施相对论因果律而言，它们并非必要条件，即关联不允许因果循环的规则。根据测量事件的时空坐标，因果律仅施加了无信号条件的一个子集。我们首先考虑n方贝尔实验（n>2），并确定约束子集足够的所有配置。其次，我们研究了针对仅受相对论约束的窃听者的设备无关密码学的影响，详细说明了对著名的随机性放大和密钥分发协议的明确攻击等其他影响。
