Time-Delocalized Local Measurements in an Indefinite Causal Order
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Quantum Physics arXiv:2604.11878 (quant-ph) [Submitted on 13 Apr 2026] Title:Time-Delocalized Local Measurements in an Indefinite Causal Order Authors:Yann Valibouse, Martí Cladera-Rosselló, Michael Antesberger, Patrick Lima, Philip Walther, Lee A. Rozema View a PDF of the paper titled Time-Delocalized Local Measurements in an Indefinite Causal Order, by Yann Valibouse and 5 other authors View PDF HTML (experimental) Abstract: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: 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 arXiv-issued DOI via DataCite Submission history From: Yann Valibouse [view email] [v1] Mon, 13 Apr 2026 18:00:01 UTC (4,996 KB) Full-text links: Access Paper: View a PDF of the paper titled Time-Delocalized Local Measurements in an Indefinite Causal Order, by Yann Valibouse and 5 other authorsView PDFHTML (experimental)TeX Source view license Current browse context: quant-ph new | recent | 2026-04 References & Citations INSPIRE HEP NASA ADSGoogle Scholar Semantic Scholar export BibTeX citation Loading... BibTeX formatted citation × loading... Data provided by: 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 Code, Data and Media Associated with this Article alphaXiv Toggle alphaXiv (What is alphaXiv?) Links to Code Toggle CatalyzeX Code Finder for Papers (What is CatalyzeX?) DagsHub Toggle DagsHub (What is DagsHub?) GotitPub Toggle Gotit.pub (What is GotitPub?) Huggingface Toggle Hugging Face (What is Huggingface?) ScienceCast Toggle ScienceCast (What is ScienceCast?) Demos Demos Replicate Toggle Replicate (What is Replicate?) Spaces Toggle Hugging Face Spaces (What is Spaces?) Spaces Toggle TXYZ.AI (What is TXYZ.AI?) Related Papers Recommenders and Search Tools Link to Influence Flower Influence Flower (What are Influence Flowers?) Core recommender toggle CORE Recommender (What is CORE?) Author Venue Institution Topic About arXivLabs arXivLabs: experimental projects with community collaborators arXivLabs is a framework that allows collaborators to develop and share new arXiv features directly on our website. Both individuals and organizations that work with arXivLabs have embraced and accepted our values of openness, community, excellence, and user data privacy. arXiv is committed to these values and only works with partners that adhere to them. Have an idea for a project that will add value for arXiv's community? Learn more about arXivLabs. Which authors of this paper are endorsers? | Disable MathJax (What is MathJax?)