Indefinite Causal Order in a Quantum Switch.

  title={Indefinite Causal Order in a Quantum Switch.},
  author={Kaumudibikash Goswami and Christina Giarmatzi and Michael Kewming and Fabio Costa and Cyril Branciard and Jacquiline Romero and A. G. White},
  journal={Physical review letters},
  volume={121 9},
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[over ^] and B[over ^] 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… 

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