Time-Delocalized Local Measurements in an Indefinite Causal Order

Quantum Physics [Submitted on 13 Apr 2026] Time-Delocalized Local Measurements in an Indefinite Causal Order Yann Valibouse, Martí Cladera-Rosselló, Michael Antesberger, Patrick Lima, Philip Walther, Lee A. Rozema Processes with indefinite causal order (ICO), such as the quantum switch, are an emerging resource for quantum tasks and a fundamental test bed for studies of temporal correlations in quantum mechanics. A limitation of past photonic implementations of the quantum switch, however, is their inability to perform measurements inside the switch without either destroying the superposition of causal orders or delaying readout until the after the quantum switch. Measurements where the results are read out locally are needed for several applications of ICO, but also for a loophole-free verification of ICO. Here, we overcome past limitations by introducing a \mathit{local} measurement scheme and coupling the photon in the switch to a \mathit{time-delocalized} ancilla system. We experimentally realize this protocol using a photonic quantum switch with post-selected linear optical logic gates. Our method ensures that the measurement apparatus interacts with the system at two distinct times and yet yields a single outcome. We use a quantum eraser measurement to preserve the ICO, which we certify by measuring a causal witness and finding a negative value of \mathcal{C}_W \approx -0.305 (1). Furthermore, by explicitly realizing a time-delocalized ancilla system, our protocol not only enables a new class of quantum switch protocols requiring local readout, but also provides a general method for path-coherence-preserving measurements with broad applications beyond ICO. Comments: 12 pages, 5 figures Subjects: Quantum Physics (quant-ph) Cite as: arXiv:2604.11878 [quant-ph]   (or arXiv:2604.11878v1 [quant-ph] for this version)   https://doi.org/10.48550/arXiv.2604.11878 Focus to learn more Submission history From: Yann Valibouse [view email] [v1] Mon, 13 Apr 2026 18:00:01 UTC (4,996 KB) Access Paper: HTML (experimental) view license Current browse context: quant-ph < prev   |   next > new | recent | 2026-04 References & Citations INSPIRE HEP NASA ADS Google Scholar Semantic Scholar Export BibTeX Citation Bookmark Bibliographic Tools Bibliographic and Citation Tools Bibliographic Explorer Toggle Bibliographic Explorer (What is the Explorer?) Connected Papers Toggle Connected Papers (What is Connected Papers?) Litmaps Toggle Litmaps (What is Litmaps?) scite.ai Toggle scite Smart Citations (What are Smart Citations?) Code, Data, Media Demos Related Papers About arXivLabs Which authors of this paper are endorsers? | Disable MathJax (What is MathJax?)