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Optimal local measurements in single-parameter quantum metrology
Stockholm University, Nordic Institute for Theoretical Physics (Nordita).ORCID iD: 0000-0002-3588-0832
Number of Authors: 42025 (English)In: Physical Review A: covering atomic, molecular, and optical physics and quantum information, ISSN 2469-9926, E-ISSN 2469-9934, Vol. 111, no 2, article id 022436Article in journal (Refereed) Published
Abstract [en]

Quantum measurement plays a crucial role in quantum metrology. Due to the limitations of experimental capabilities, collectively measuring multiple copies of probing systems can present significant challenges. Therefore, the concept of locality in quantum measurements must be considered. In this work, we investigate the possibility of achieving the quantum Cramér-Rao bound (QCRB) through local measurements (LMs). We first demonstrate that if there exists a LM to saturate the QCRB for qubit systems, then we can construct another rank-1 local projective measurement to saturate the QCRB. In this sense, rank-1 local projective measurements are sufficient to analyze the problem of saturating the QCRB. For pure qubits, we propose two necessary and sufficient methods to determine whether and how a given parameter estimation model can achieve the QCRB through LMs. The first method, dubbed the iterative matrix partition method and based on unitary transformations that render the diagonal entries of a traceless matrix vanish, elucidates the underlying mathematical structure of LMs as well as the local measurements with classical communications (LMCC), generalizing the result by Zhou et al. [Quantum Sci. Technol. 5, 025005 (2020)2058-956510.1088/2058-9565/ab71f8], which only holds for the later case. We clarify that the saturation of the QCRB through LMs for the Greenberger-Horne-Zeilinger-encoded states is actually due to the self-similar structure in this approach. The second method, dubbed the hierarchy of orthogonality conditions and based on the parametrization of rank-1 measurements for qubit systems, allows us to construct several examples of saturating QCRBs, including the three-qubit W states and the N-qubit W states (N≥3). Our findings offer insights into achieving optimal performance in quantum metrology when measurement resources are limited.

Place, publisher, year, edition, pages
2025. Vol. 111, no 2, article id 022436
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
URN: urn:nbn:se:su:diva-242127DOI: 10.1103/PhysRevA.111.022436ISI: 001447702400002Scopus ID: 2-s2.0-85218934646OAI: oai:DiVA.org:su-242127DiVA, id: diva2:1952062
Available from: 2025-04-14 Created: 2025-04-14 Last updated: 2025-10-06Bibliographically approved

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Yang, Jing

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