“在量子物理中，因果关系并不总是一个事件引起另一个事件那样简单。量子力学中的不可思议之处在于事件可以在没有固定顺序的情况下发生。” 来自澳大利亚昆士兰大学（University of Queensland）工程量子系统卓越中心（Centre of Excellence for Engineered Quantum Systems）的博士雅基?罗梅罗（Jacqui Romero）说：“比如你每天去上班，需要乘火车和公交车。一般来说，你可能先乘火车再转乘公交车，或是反过来。而在我们的实验中，乘火车和坐公交可以同时发生。这就叫做‘不确定因果顺序’。这种因果关系在我们的平日生活中是无法观测到的。”
[摘要]Quantum mechanics allows events to happen with no definite causal order： this can be verified by measuring a causal witness， in the same way that an entanglement witness verifies entanglement。 Here， we realize a photonic quantum switch， where two operations^Aand^Bact in a quantum superposition of their two possible orders。 The operations are on the transverse spatial mode of the photons； polarization coherently controls their order。 Our implementation ensures that the operations cannot be distinguished by spatial or temporal position―further it allows qudit encoding in the target。 We confirm our quantum switch has no definite causal order by constructing a causal witness and measuring its value to be 18 standard deviations beyond the definite-order bound。
