Negativity of nondemolition quasiprobability distribution as a necessary and sufficient condition for macrorealism violation

Year: 2025

Authors: Solinas P., Gherardini S.

Autors Affiliation: Univ Genoa, Dipartimento Fis, Via Dodecaneso 33, I-16146 Genoa, Italy; INFN, Sez Genova, Via Dodecaneso 33, I-16146 Genoa, Italy; CNR, Ist Nazl Ottica, Largo Enrico Fermi 6, I-50125 Florence, Italy; Univ Firenze, European Lab Nonlinear Spect, I-50019 Sesto Fiorentino, Italy.

Abstract: Quantum nondemolition measurements define a noninvasive protocol to extract information from a monitored quantum system. This protocol exploits an additional quantum system that is sequentially coupled to the system and eventually measured to reconstruct temporal correlations developed by the system. In reference to a given measurement observable, the protocol’s outcomes are well described by a quasiprobability distribution, which can be negative. In this paper, we prove that the presence of negative regions in this distribution defined at three times is a necessary and sufficient condition for the violation of macrorealism. This approach falls within the category of sequential noninvasive measurability and automatically satisfies all no-signaling-in-time conditions at multiple times by design. We show that reconstructing the negativity is a much stronger condition than the violation of the Leggett-Garg inequalities, commonly used for the same task, as they may be satisfied even if the macrorealism is violated. Quantum nondemolition measurements provide a clear and operational tool to identify violations of macrorealism directly. Thus, our result could also be applied to the study of the quantum-to-classical transition and in all those fields where quantumness certification is desirable.

Journal/Review: PHYSICAL REVIEW A

Volume: 111 (5)      Pages from: 52217-1  to: 52217-10

More Information: The authors acknowledge fruitful discussions with Andrea Smirne. P.S. acknowledges financial support from INFN. S.G. acknowledges PRIN Project No. 2022FEXLYB, Quantum Reservoir Computing; PNRR MUR Project No. PE0000023-NQSTI funded by the European Union-Next Generation EU; and the MISTI Global Seed Funds MIT-FVG Collaboration grant Revealing and exploiting quantumness via quasiprobabilities: from quantum thermodynamics to quantum sensing. P.S. designed the research and performed the calculations. P.S. and S.G. discussed the results and wrote the paper.
KeyWords: Counting Statistics; Quantum; Flux
DOI: 10.1103/PhysRevA.111.052217