Semi-device-independent certification of indefinite causal order in a photonic quantum switch

Huan Cao (Corresponding author), Jessica Bavaresco, Ning Ning Wang, Lee A. Rozema, Chao Zhang, Yun Feng Huang (Corresponding author), Bi Heng Liu, Chuan Feng Li (Corresponding author), Guang Can Guo, Philip Walther

Publications: Contribution to journalArticlePeer Reviewed


Quantum processes are transformations that act on quantum operations. Their study led to the discovery of the phenomenon of indefinite causal order: some quantum processes, such as the quantum switch, act on independent quantum operations in such a way that the order in which the operations are acted upon not only cannot be determined but is simply undefined. This is the property that we experimentally certify in this work. We report an experimental certification of indefinite causal order that relies only on the characterization of the operations of a single party. We do so in the semi-device-independent scenario with the fewest possible assumptions of characterization of the parties’ local operations in which indefinite causal order can be demonstrated with the quantum switch. To achieve this result, we introduce the concept of semi-device-independent causal inequalities and show that the correlations generated in a photonic quantum switch, in which all parties are able to collect local outcome statistics, achieve a violation of this inequality of 224 standard deviations. This result consists of the experimental demonstration of indefinite causal order with the fewest device-characterization assumptions to date.
Original languageEnglish
Pages (from-to)561-568
Number of pages8
Issue number5
Publication statusPublished - 20 May 2023

Austrian Fields of Science 2012

  • 103025 Quantum mechanics

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