Appearance of causality in process matrices when performing fixedbasis measurements for two parties
Abstract
The recently developed framework for quantum theory with no global causal order allows for quantum processes in which operations in local laboratories are neither causally ordered nor in a probabilistic mixture of definite causal orders. The causal relation between the laboratories is described by the process matrix. We show that, if the inputs of the laboratories are measured in a fixed basis, one can introduce an effective process matrix which is operationally indistinguishable from the original one. This effective process matrix can be obtained by applying the von NeumannLüders update rule for nonselective measurements to the original process matrix and, in the bipartite case, it is compatible with a definite causal order. The latter extends the original Oreshkov et al. proof where one considers that both the measurement of the input and the repreparation of the output are performed in a fixed basis.
 Publication:

Physical Review A
 Pub Date:
 June 2016
 DOI:
 10.1103/PhysRevA.93.062324
 arXiv:
 arXiv:1601.06620
 Bibcode:
 2016PhRvA..93f2324B
 Keywords:

 Quantum Physics
 EPrint:
 Phys. Rev. A 93, 062324 (2016)