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arXiv:2012.03940 · Physical Review A

Experimental violation of the Leggett-Garg inequality using the polarization of classical light

Wenlei Zhang, Ravi K. Saripalli, Jacob M. Leamer, Ryan T. Glasser, Denys I. Bondar

Published Oct 28, 2021• 6 citations
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Abstract

In contrast to Bell's inequalities, which test the correlations between multiple spatially separated systems, the Leggett-Garg inequalities test the temporal correlations between measurements of a single system. We experimentally demonstrate the violation of the Leggett-Garg inequality in a classical optical system using only the polarization degree of freedom of a laser beam. Our results show maximal violations of the Leggett-Garg inequality.

DOI
10.1103/physreva.104.043711

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In contrast to Bell's inequalities, which test the correlations between multiple spatially separated systems, the Leggett-Garg inequalities test the temporal correlations between measurements of a single system. We experimentally demonstrate the violation of the Leggett-Garg inequality in a classical optical system using only the polarization degree of freedom of a laser beam. Our results show maximal violations of the Leggett-Garg inequality.
Abstract Leggett-Garg (LG) inequalities have been derived to clarify the validity of generalizing quantum mechanics to macroscopic systems for more than thirty years. They have been used as tools to investigate the existence of quantum coherence in macroscopic systems. Here, we show that such inequality violation can also be observed in classical optical systems. Using the linear polarization degree of freedom as the dichotomic observables, in a classical optical experiment, we have observed the violation of LG inequality by implementing the non-invasive measurement. Furthermore, the coherence witness, which is analogous to the quantum witness, has also been observed in the classical optical experiment. Our results not only provide new physical insights into the macroscopic quantum correlation and quantum witness, but also show their potential applications in correlated information processes.
In contrast to the spatial Bell's inequalities which probe entanglement between spatially separated systems, the Leggett–Garg inequalities test the correlations of a single system measured at different times. Violation of a genuine Leggett–Garg test implies either the absence of a realistic description of the system or the impossibility of measuring the system without disturbing it. Quantum mechanics violates the inequalities on both accounts and the original motivation for these inequalities was as a test for quantum coherence in macroscopic systems. The last few years has seen a number of experimental tests and violations of these inequalities in a variety of microscopic systems such as superconducting qubits, nuclear spins, and photons. In this article, we provide an introduction to the Leggett–Garg inequalities and review these latest experimental developments. We discuss important topics such as the significance of the non-invasive measurability assumption, the clumsiness loophole, and the role of weak measurements. Also covered are some recent theoretical proposals for the application of Leggett–Garg inequalities in quantum transport, quantum biology and nano-mechanical systems.
Leggett-Garg inequalities are tests of macroscopic realism that can be violated by quantum mechanics. In this letter, we realise photonic Leggett-Garg tests on a three-level system and implement measurements that admit three distinct measurement outcomes, rather than the usual two. In this way we obtain violations of three- and four-time Leggett-Garg inequalities that are significantly in excess of those obtainable in standard Leggett-Garg tests. We also report violations the quantum-witness equality up to the maximum permitted for a three-outcome measurement. Our results highlight differences between spatial and temporal correlations in quantum mechanics.
Leggett-Garg inequalities are tests of macroscopic realism that can be violated by quantum mechanics. In this letter, we realise photonic Leggett-Garg tests on a three-level system and implement measurements that admit three distinct measurement outcomes, rather than the usual two. In this way we obtain violations of three- and four-time Leggett-Garg inequalities that are significantly in excess of those obtainable in standard Leggett-Garg tests. We also report violations the quantum-witness equality up to the maximum permitted for a three-outcome measurement. Our results highlight differences between spatial and temporal correlations in quantum mechanics.