Macroscopic Quantum Theory

Pushing the Boundaries of Quantum Mechanics to the Macroscopic Realm

A major question in the foundations of physics is whether quantum theory applies universally—not only to atoms and photons, but also to macroscopic systems comprising many particles. Traditionally, it is assumed that quantum phenomena such as superposition, entanglement, and nonlocality fade away due to mechanisms like decoherence, coarse-grained measurements, and increasing system size—leading to the so-called quantum-to-classical transition.

Challenging the Classical Limit

Our research program challenges this prevailing view and demonstrates that quantum theory can, in fact, remain fully valid and operational at the macroscopic scale.

We have identified families of quantum states that are robust to classicalization mechanisms—preserving superposition, the Born rule, and even nonlocal correlations in the presence of decoherence, particle losses, and limited-resolution measurements.

  • In our foundational work [ML1], we showed that macroscopic quantum correlations survive under coarse-graining and exhibit violations of classical assumptions even at scale.
  • Building on this, we introduced a general and operationally meaningful framework for robust macroscopic entanglement [ML2], accounting for practical constraints such as probabilistic access to subsystems and imperfect measurements.
  • Most significantly, in [ML3] we develop a full conceptual and mathematical framework for quantum theory at the macroscopic scale. We demonstrate that the entire structure of quantum mechanics—including entanglement, Bell inequality violations, and Leggett-Garg-type nonclassicality—persists in this regime.

These findings suggest that classical mechanics may not emerge from quantum theory in the macroscopic limit as commonly assumed—instead, quantum theory itself can extend into the macroscopic world without modification.

Selected Publications

[ML1] M. Gallego & B. Dakić, Macroscopically Nonlocal Quantum Correlations,
Physical Review Letters 127, 120401 (2021)
https://doi.org/10.1103/PhysRevLett.127.120401

[ML2] M. Gisti, M. Gallego & B. Dakić, Robust and Indestructible Macroscopic Entanglement,
arXiv preprint arXiv:2502.15339 (2025)
https://arxiv.org/abs/2502.15339

[ML3] M. Gallego & B. Dakić, Quantum Theory at the Macroscopic Scale,
arXiv preprint arXiv:2409.03001 (2024)
https://arxiv.org/abs/2409.03001