Quantum to classical transition

Quantum-to-classical transition via coarse-graining measurements

Coarse-grained measurements in phase space: Phase space is partitioned into cells of size Δx×Δp. A coarse-grained measurement reveals only the occupied cell, while the individual points within each cell remain unresolved. The inset under the magnifying glass illustrates that the cell area is much larger than the typical uncertainty region associated with coherent states, whose phase-space area is of the order of Planck’s constant.

Quantum and classical physics differ not only mathematically, but also conceptually. Understanding how classical behaviour emerges from quantum theory remains one of the central open questions in physics.

Our group developed an approach to this problem that is conceptually distinct from decoherence and instead emphasises the limited precision with which quantum effects can be observed. We showed that, under sufficiently coarse-grained measurements, quantum features become inaccessible and quantum states appear effectively classical, admitting an effective joint probability distribution for all coarse-grained observables. This gives rise to macroscopic realism. If, in addition, the dynamics is probed frequently enough through coarse-grained observables, then both macroscopic realism and effective Newtonian laws emerge from standard quantum theory. In separate work, we also introduced no-signalling in time as a necessary condition for violations of macroscopic realism, analogous to the role of no-signalling in Bell scenarios.