Cosmological Expansion Induces Interference Between Communication and Entanglement Harvesting

We investigate the interplay between genuine entanglement harvesting and communication mediated correlations for local particle detectors in expanding cosmological spacetimes. Focusing on a conformally coupled scalar field in de Sitter spacetime, we analyze how spacetime expansion induces interference between these two sources of entanglement when the detectors are in causal contact. We compare two physically distinct detector models: detectors whose spatial profile expands with the Universe, and detectors whose proper size remains fixed despite cosmological expansion. We find that the lack of time-reversal symmetry in cosmological settings generically leads to constructive or destructive interference between communication mediated correlations and harvested field correlations, dramatically affecting the entanglement that detectors can acquire. In particular, rapid expansion can suppress entanglement entirely for expanding detectors through destructive interference, even when both communication and field correlations are individually large, whereas detectors that maintain a fixed proper size remain capable of acquiring significant entanglement. Our results show that cosmological expansion qualitatively reshapes the balance between communication and harvesting, and that the detector internal cohesion (whether it expands with the Universe or not) plays a crucial role in determining whether detectors’ entanglement can survive in rapidly expanding universes.

💡 Research Summary

The paper investigates how the expansion of the universe affects the entanglement that two local particle detectors can acquire from a quantum field. The authors focus on a conformally coupled, mass‑less scalar field in a spatially flat Friedmann‑Robertson‑Walker (FRW) spacetime, specializing to de Sitter expansion (scale factor a(t)=e^{Ht}). They adopt the standard Unruh‑DeWitt (UDW) detector model, with each detector interacting weakly (coupling λ) with the field through a spacetime‑smeared monopole operator. Starting from an initial product state—both detectors in their ground states and the field in a Gaussian state (typically the vacuum)—they expand the time‑evolution operator to second order in λ. This yields a reduced two‑qubit density matrix whose off‑diagonal element M encodes the correlation built between the detectors, while the diagonal elements L_{ij} describe local excitation probabilities.

A central conceptual step is the decomposition of the field’s Wightman function W(x,x′) into its symmetric (W_+) and antisymmetric (W_–) parts. The antisymmetric part is proportional to the field commutator and is independent of the field’s quantum state; it therefore represents “communication‑assisted” entanglement that can arise whenever the detectors are timelike or lightlike connected. The symmetric part carries the state‑dependent vacuum (or thermal) correlations and thus quantifies genuine entanglement harvesting from pre‑existing field fluctuations. By inserting the corresponding symmetric/antisymmetric pieces into the expression for M, the authors define two separate negativities: N_+ (harvested) and N_– (communication‑assisted). The total negativity N = max{0, V} with V built from |M|^2 and the L_{ij} terms splits naturally into N = N_+ + N_– (up to O(λ^4) corrections).

The authors then consider two physically distinct detector constructions. In the first (“expanding detectors”) the spatial profile of each detector scales with the cosmic scale factor, i.e. the proper width σ_j(t) = a(t) σ_0, so that the detector’s size stretches with the universe. In the second (“fixed‑size detectors”) the proper width is held constant, σ_j(t)=σ_0, mimicking real atoms or tightly bound probes that resist cosmological expansion. Both models share a Gaussian temporal switching χ_j(t)=exp