# Regensburg 2022 – wissenschaftliches Programm

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# QI: Fachverband Quanteninformation

## QI 8: Quantum Sensors and Metrology

### QI 8.6: Vortrag

### Mittwoch, 7. September 2022, 16:45–17:00, H9

**Multicopy metrology with many-particle quantum states** — •Róbert Trényi^{1,2,3}, Árpád Lukács^{1,4,3}, Paweł Horodecki^{5,6}, Ryszard Horodecki^{5}, Tamás Vértesi^{7}, and Géza Tóth^{1,2,8,3} — ^{1}Dept. of Theoretical Physics, U. of the Basque Country UPV/EHU, Bilbao, Spain — ^{2}DIPC, San Sebastián, Spain — ^{3}Wigner Research Centre for Physics, Budapest, Hungary — ^{4}Dept. of Mathematical Sciences, Durham University, United Kingdom — ^{5}International Centre for Theory of Quantum Technologies, University of Gdansk, Gdansk, Poland — ^{6}Faculty of Applied Physics and Mathematics, National Quantum Information Centre, Gdansk University of Technology, Gdansk, Poland — ^{7}Institute for Nuclear Research, Debrecen, Hungary — ^{8}IKERBASQUE, Bilbao, Spain

We consider quantum metrology with several copies of bipartite and multipartite quantum states. We characterize the metrological usefulness by determining how much the state outperforms separable states. We identify a large class of entangled states that become maximally useful for metrology in the limit of infinite number of copies. The maximally achievable metrological usefulness is attained exponentially fast in the number of copies. We show that, on the other hand, pure entangled states with even a small amount of white noise do not become maximally useful even in the limit of infinite number of copies. We also make general statements about the usefulness of a single copy of pure entangled states. We show that the multiqubit states presented in Hyllus et al. [Phys. Rev. A 82, 012337 (2010)], which are not useful, become useful if we embed the qubits locally in qutrits.