Even without interactions with the macroscopic world, quantum systems will develop classical properties. Even without interactions with the macroscopic world, quantum systems will develop classical properties.
Macroscopic quantum superpositions are widely believed to be unobservable because large systems cannot be perfectly isolated from their environments. Here, we show that even under perfect isolation, intrinsic unitary dynamics in generic many-body systems, as analyzed through the eigenstate thermalization hypothesis and random matrix theory, can suppress the observable signatures of macroscopic coherence. Using the Greenberger-Horne-Zeilinger (GHZ) state as a representative example, we demonstrate that while fully correlated measurements can initially distinguish a macroscopic superposition from its corresponding classical mixture, generic many-body evolution renders them operationally indistinguishable for most times. By analyzing both distinguishability measures and established quantifiers of macroscopic quantumness, we find that equilibration not only hides coherence from accessible observables but also suppresses the corresponding signatures of macroscopic quantumness, in particular within the additive-local framework considered here. These results identify unitary thermalization, independent of environmental decoherence, as a fundamental mechanism that limits the observation of macroscopic quantum effects.
Closed quantum systems with quenched randomness exhibit many-body localized regimes wherein they do not equilibrate, even though prepared with macroscopic amounts of energy above their ground states. We show that such localized systems can order, in that individual many-body eigenstates can break symmetries or display topological order in the infinite-volume limit. Indeed, isolated localized quantum systems can order even at energy densities where the corresponding thermally equilibrated system is disordered, i.e., localization protects order. In addition, localized systems can move between ordered and disordered localized phases via nonthermodynamic transitions in the properties of the many-body eigenstates. We give evidence that such transitions may proceed via localized critical points. We note that localization provides protection against decoherence that may allow experimental manipulation of macroscopic quantum states. We also identify a ``spectral transition'' involving a sharp change in the spectral statistics of the many-body Hamiltonian.
Both classical and quantum principles coexist in the mesoscopic world, something between the microscopic and the macroscopic worlds.More precisely, mesoscopic physics studies how the "coherence", which governs the wave property and especially the interference effect of matter, disappear gradually as the scale increases from the microscopic to the macroscopic scale.Another important issue in mesoscopic physics is many-body effects, which originate from the interaction among electrons.Many-body effects are a fundamental issue in all modern physics, but mesoscopic physics is particularly well adopted for their study because mesoscopic systems allow for tunability of various parameters, which is not possible in bulk systems.Further, the interplay between coherence and many-body effects leads to rich physical phe-