Certification and Quantification of Multilevel Quantum Coherence

Martin Ringbauer, Thomas R. Bromley, Marco Cianciaruso, Ludovico Lami, W. Y. Sarah Lau, Gerardo Adesso, Andrew G. White, Alessandro Fedrizzi, Marco Piani

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60 Citations (Scopus)
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Quantum coherence, present whenever a quantum system exists in a superposition of multiple classically distinct states, marks one of the fundamental departures from classical physics. Quantum coherence has recently been investigated rigorously within a resource-theoretic formalism. However, the finer-grained notion of multilevel coherence, which explicitly takes into account the number of superposed classical states, has remained relatively unexplored. A comprehensive analysis of multilevel coherence, which acts as the single-party analogue to multipartite entanglement, is essential for understanding natural quantum processes as well as for gauging the performance of quantum technologies. Here, we develop the theoretical and experimental groundwork for characterizing and quantifying multilevel coherence. We prove that nontrivial levels of purity are required for multilevel coherence, as there is a ball of states around the maximally mixed state that do not exhibit multilevel coherence in any basis. We provide a simple, necessary, and sufficient analytical criterion to verify the presence of multilevel coherence, which leads to a complete classification of multilevel coherence for three-level systems. We present the robustness of multilevel coherence, a bona fide quantifier, which we show to be numerically computable via semidefinite programming and experimentally accessible via multilevel coherence witnesses, which we introduce and characterize. We further verify and lower bound the robustness of multilevel coherence by performing a semi-device-independent phase discrimination task, which is implemented experimentally with four-level quantum probes in a photonic setup. Our results contribute to understanding the operational relevance of genuine multilevel coherence, also by demonstrating the key role it plays in enhanced phase discrimination - a primitive for quantum communication and metrology - and suggest new ways to reliably and effectively test the quantum behavior of physical systems.

Original languageEnglish
Article number041007
JournalPhysical Review X
Issue number4
Early online date10 Oct 2018
Publication statusPublished - Dec 2018

ASJC Scopus subject areas

  • General Physics and Astronomy


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