Macroscopically nonlocal quantum correlations (2104.03988v2)

Abstract: It is usually believed that coarse-graining of quantum correlations leads to classical correlations in the macroscopic limit. Such a principle, known as macroscopic locality, has been proved for correlations arising from independent and identically distributed (IID) entangled pairs. In this letter we consider the generic (non-IID) scenario. We find that the Hilbert space structure of quantum theory can be preserved in the macroscopic limit. This leads directly to a Bell violation for coarse-grained collective measurements, thus breaking the principle of macroscopic locality.

• The paper demonstrates that non-IID quantum correlations preserve superposition at macroscopic levels, leading to a clear violation of Bell inequalities.
• The analysis employs variable coarse-graining quantified by a parameter, pinpointing a critical boundary at 1/2 where classical behavior fails.
• The paper confirms that macroscopic quantum correlations remain robust against noise and particle loss, highlighting new possibilities for quantum technologies.

Macroscopically Nonlocal Quantum Correlations: An Analytical Review

The paper "Macroscopically Nonlocal Quantum Correlations" presented by Miguel Gallego and Borivoje Dakić explores the challenging notion of macroscopic locality (ML) in quantum mechanics, particularly in scenarios beyond the widely assumed independent and identically distributed (IID) entangled pairs. Traditionally, quantum correlations are believed to converge to classical correlations through the process of coarse-graining in the macroscopic limit. This principle, known as macroscopic locality, is thought to ensure the transition from quantum to classical behavior as systems scale. The authors challenge this notion by presenting a non-IID framework wherein the preservation of quantum superposition in the macroscopic limit leads to a violation of Bell inequalities, thus contravening the principle of macroscopic locality.

Key Findings and Numerical Analysis

• Violation of Macroscopic Locality: The paper presents a theoretical scenario in which the Hilbert space structure inherent to quantum systems is preserved even in the macroscopic limit. This leads to a violation of Bell inequalities when collective measurements are coarse-grained, contravening ML. Notably, the analysis reveals that nonlocal correlations can be observed at scales where classical physics is traditionally expected to dominate.
• Variable Coarse-Graining Levels: The authors generalize the concept of ML to various degrees of coarse-graining, quantified by a parameter $\tilde{\epsilon} \in [0,1]$. This allows for an exploration of the delicate boundary between quantum and classical regimes, with $\tilde{\epsilon}=1/2$ demonstrated as a boundary where ML is violated, contrasting the established understanding that ML is typically preserved under these conditions with IID states.
• Macroscopic Quantum Behavior (MQB): The introduction of MQB at different coarse-graining levels specifies conditions under which the superposition principle and the Born rule are maintained in the thermodynamic limit. The paper provides an example of MQB causing a violation of ML at $\tilde{\epsilon}=1/2$, inherent to a correlated macroscopic state that tends to violate classical locality assumptions.
• Robustness Considerations: The authors analyze the impact of noise and particle losses on the persistence of macroscopic nonlocality. They find that, even with losses, the macroscopic statistics remain intact, suggesting a form of resilience against such perturbations.

Implications and Future Directions

The implications of this paper are significant both theoretically and practically. From a theoretical standpoint, the violation of ML in non-IID scenarios questions the completeness of the correspondence principle, offering insights into the pseudo-classical behavior observed in large quantum systems. The topic invites further exploration into potential quantum-to-classical transition points beyond traditional assumptions, critically analyzing the robustness of quantum coherence at macroscopic scales.

In terms of practical implications, this insight could influence the development of quantum technologies that operate across scales, ensuring nonlocal effects can be harnessed without being diminished through conventional coarse-graining practices. Future works could expand the modeling to include non-local hidden variable theories or investigate specific physical implementations in quantum optics and condensed-matter systems to experimentally validate these theoretical predictions.

Through such refined analyses, the paper moves the field towards a potential reevaluation of macroscopic classicality and opens pathways for discovering new post-quantum physics phenomena. The elucidation of macroscopic quantum mechanics intricacies feeds into broader themes of quantum information theory and the development of novel quantum algorithms, potentially redefining edge cases where quantum computing and information might operate more effectively.

In conclusion, "Macroscopically Nonlocal Quantum Correlations" provides a significant contribution to the understanding of the quantum-classical dichotomy, encouraging further discourse and experimentation on preserving quantum characteristics at increased scales or under different-than-typical conditions.