No-Go Theorem for Ancilla-Assisted Gaussian Enhancement in Passive-Unitary Estimation
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Quantum Physics arXiv:2606.00256 (quant-ph) [Submitted on 29 May 2026] Title:No-Go Theorem for Ancilla-Assisted Gaussian Enhancement in Passive-Unitary Estimation Authors:Zihao Gong, Saikat Guha View a PDF of the paper titled No-Go Theorem for Ancilla-Assisted Gaussian Enhancement in Passive-Unitary Estimation, by Zihao Gong and Saikat Guha View PDF HTML (experimental) Abstract:We study the maximum quantum Fisher information (QFI) for estimating a single parameter embedded in a generic multimode lossless passive Gaussian unitary using general Gaussian probes under a signal-energy constraint. Unlike previous work, which imposed a total energy constraint on the full probe, we constrain only the transmitted signal modes while allowing an arbitrary number of locally retained ancilla modes with arbitrarily large energy. We prove that this additional freedom does not increase the maximum achievable QFI; the optimum remains identical to that attainable without extra ancilla energy. The same conclusion also extends to the sequential setting under a total energy constraint. We also characterize the family of optimal probe states and show that entanglement is not necessary to attain the optimum in the lossless setting. This extends the result of Matsubara et al. to the physically motivated signal-energy-constrained scenario and establishes a no-go theorem for ancilla-assisted Gaussian enhancement in noiseless passive-unitary estimation. Subjects: Quantum Physics (quant-ph) Cite as: arXiv:2606.00256 [quant-ph] (or arXiv:2606.00256v1 [quant-ph] for this version) https://doi.org/10.48550/arXiv.2606.00256 Focus to learn more arXiv-issued DOI via DataCite (pending registration) Submission history From: Zihao Gong [view email] [v1] Fri, 29 May 2026 18:42:05 UTC (23 KB) Full-text links: Access Paper: View a PDF of the paper titled No-Go Theorem for Ancilla-Assisted Gaussian Enhancement in Passive-Unitary Estimation, by Zihao Gong and Saikat GuhaView PDFHTML (experimental)TeX Source view license Current browse context: quant-ph new | recent | 2026-06 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?)