Quantum discord of Ganssian states in an expanding universe

This paper investigates how the expansion of the universe affects the distribution of quantum discord—a measure of non-classical correlations—in continuous-variable Gaussian states. Specifically, the authors examine how the expansion rate and volume of a Robertson-Walker spacetime influence the initial quantum correlations shared between two observers (Alice and Bob) and whether the expansion generates new correlations among additional bosonic and antibosonic modes.

The researchers model the expanding universe as a Gaussian channel that acts on an initial two-mode squeezed Gaussian state. By solving the Klein-Gordon equation in a 1+1-dimensional Robertson-Walker metric, they derive Bogoliubov transformations that map the asymptotic-past vacuum state to an asymptotic-future state. They then calculate the quantum discord for various bipartitions of the resulting four-mode system (involving both particles and antiparticles) to track how these correlations evolve and redistribute as the universe expands.

The study reveals that cosmic expansion acts as a mechanism for the global redistribution of quantum correlations. While the initial quantum discord between the primary bosonic modes (Alice and Bob) decays as the expansion rate and volume increase, the expansion simultaneously induces new quantum discord in other mode pairs. This induced discord is most significant for cross-observer bosonic-antibosonic pairs, followed by same-observer bosonic-antibosonic pairs, and is weakest for antibosonic pairs. Furthermore, the authors find that quantum discord is more sensitive to the expansion rate than to the expansion volume, and that particles with specific mass and lower momentum are more effective for extracting information about the expanding spacetime.