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arXiv:0801.2241 · Nature Physics

Testing quantum correlations versus single-particle properties within Leggett’s model and beyond

Cyril Branciard, Nicolas Brunner, Nicolas Gisin, Christian Kurtsiefer, Antı́a Lamas-Linares, Alexander Ling, Valerio Scarani

Published Jul 6, 2008• 124 citations
arXivHTMLPDF
DOI
10.1038/nphys1020

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Quantum mechanics predicts that measurement is inherently invasive, challenging the macrorealist view that physical properties exist independently of observation. The Leggett–Garg inequality (LGI) provides a test of this worldview, but its violation in systems in collective systems is often challenging due to technical measurement disturbance. By embedding an Elitzur–Vaidman interaction-free measurement (“quantum bomb-testing”) protocol into a microwave-driven Rydberg superatom (a mesoscopically coherent collective system) interferometer—we realize a high-fidelity, sequential measurement test. We observe a clear LGI violation that exceeds the classical bound by nearly 50 standard deviations in such a mesoscopic collective system. This result demonstrates the failure of noninvasive measurability for a collective quantum observable, establishing a scalable platform for probing temporal quantum correlations beyond the single-particle limit.
Bell's theorem guarantees that no model based on local variables can reproduce quantum correlations. Also, some models based on nonlocal variables, if subject to apparently "reasonable" constraints, may fail to reproduce quantum physics. In this Letter, we introduce a family of inequalities, which use a finite number of measurement settings, and which therefore allow testing Leggett's nonlocal model versus quantum physics. Our experimental data falsify Leggett's model and are in agreement with quantum predictions.
We report on a stringent test of the nonclassicality of the motion of a massive quantum particle, which propagates on a discrete lattice.Measuring temporal correlations of the position of single atoms performing a quantum walk, we observe a 6σ violation of the Leggett-Garg inequality.Our results rigorously excludes (i.e., falsifies) any explanation of quantum transport based on classical, well-defined trajectories.We use so-called ideal negative measurements-an essential requisite for any genuine Leggett-Garg test-to acquire information about the atom's position, yet avoiding any direct interaction with it.The interactionfree measurement is based on a novel atom transport system, which allows us to directly probe the absence rather than the presence of atoms at a chosen lattice site.Beyond the fundamental aspect of this test, we demonstrate the application of the Leggett-Garg correlation function as a witness of quantum superposition.Here, we employ the witness to discriminate different types of walks spanning from merely classical to wholly quantum dynamics.