The fundamental notion of fixed order between two events is challenged by formulations of quantum mechanics that makes no reference to a global time. The indefinite causal order arising from these formalisms can be realised using the quantum switch, a device where two operations can act in two different orders simultaneously. Similar to entanglement verification which employ entanglement witness, we verify indefinite causal order using a causal witness. In our experiment, we use polarisation as a control for the order of the operations that act on the spatial mode of a photon. We measure a causal witness value of -0.16±0.013, below the threshold of zero, thus demonstrating indefinite causal order in the quantum switch.
Jacquiline Romero, Kaumudibikash Goswami, Christina Giarmatzi, Fabio Costa, Cyril Branciard, and Andrew G. White, "Spatial modes for testing indefinite causal order
," Proc. SPIE 10549, Complex Light and Optical Forces XII, 1054908 (Presented at SPIE OPTO: January 30, 2018; Published: 22 February 2018); https://doi.org/10.1117/12.2292732.
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