Leggett-Garg macrorealism and temporal correlations

Leggett and Garg formulated macrorealist models encoding our intuition on classical systems, i.e., physical quantities have a definite value that can be measured with minimal disturbance, and with the goal of testing macroscopic quantum coherence effects. The associated inequalities, involving the statistics of sequential mea-surements on the system, are violated by quantum-mechanical predictions and experimental observations. Such tests, however, are subject to loopholes: a classical explanation can be recovered assuming specific models of measurement disturbance. We review recent theoretical and experimental progress in characterizing macrorealist and quantum temporal correlations, and in closing loopholes associated with Leggett-Garg tests. Finally, we review recent definitions of nonclassical temporal correlations, which go beyond macrorealist models by relaxing the assumption on the measurement disturbance, and their applications in sequential quantum information processing.

Automated summary

Leggett and Garg formulated macrorealist models to test macroscopic quantum coherence effects, but these tests are subject to loopholes. This review discusses recent progress in characterizing macrorealist and quantum temporal correlations, closing the loopholes associated with Leggett-Garg tests, and introduces definitions of nonclassical temporal correlations and their applications in sequential quantum information processing.