「在量子物理中,因果關係並不總是一個事件引起另一個事件那樣簡單。量子力學中的不可思議之處在於事件可以在沒有固定順序的情況下發生。」 來自澳大利亞昆士蘭大學(University of Queensland)工程量子系統卓越中心(Centre of Excellence for Engineered Quantum Systems)的博士雅基·羅梅羅(Jacqui Romero)說:「比如你每天去上班,需要乘火車和公交車。一般來說,你可能先乘火車再轉乘公交車,或是反過來。而在我們的實驗中,乘火車和坐公交可以同時發生。這就叫做『不確定因果順序』。這種因果關係在我們的平日生活中是無法觀測到的。」
[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 ^A and ^B act 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。