Subsystems (in)dependence in GIE proposals

TL;DR

This paper critically examines the assumptions of subsystem independence in gravitationally induced entanglement proposals, revealing fundamental issues with Hilbert space factorization and microcausality in quantum gravity, which impact experimental interpretations.

Contribution

It provides a rigorous algebraic quantum field theory analysis of subsystem independence in quantum gravity proposals, highlighting the nontrivial effects of gravitational dressing and microcausality violations.

Findings

Subsystem independence assumptions are nontrivial due to gauge constraints.

Commutation relations between spacelike observables are affected by gravitational dressing.

Violations of microcausality could serve as a probe of quantum gravity.

Abstract

Recent proposals suggest that detecting entanglement between two spatially superposed masses would establish the quantum nature of gravity. However, these gravitationally induced entanglement (GIE) experiments rely on assumptions about subsystem independence. We sharpen the theoretical underpinnings of such proposals by examining them through the lens of algebraic quantum field theory (AQFT), distinguishing distinct operational and algebraic notions of independence. We argue that state and measurement independence of subsystems, essential to the experimental logic, is nontrivial in the presence of gauge constraints and gravitational dressing. Using gravitationally dressed fields, we recall that commutation relations between spacelike separated observables are nontrivial, undermining strict Hilbert space factorization. We further explore the implications for entanglement witnesses, investigating the Tsirelson bound when subsystem algebras fail to commute, and showing that the Tsirelson bound persists for a suitably symmetrized CHSH observable even though the operational status of such "joint" observables becomes delicate when commensurability fails. Our analysis highlights how even within linearized covariant quantum gravity, violations of microcausality may affect both the interpretation, modelling, and design of proposed laboratory tests of quantum gravity, despite remaining negligible for current experimental regimes. Although we consider GIE-style protocols as a concrete case study, the subsystem-independence issues we highlight are generic to low-energy (perturbative) quantum gravity. Finally, we derive estimates for dressing-induced microcausality violations, which suggest a complementary avenue to current proposals: in principle, bounding dressing-induced microcausality violations themselves as a probe of the quantum nature of gravity.