This technical note addresses a narrow structural question in the current gravity debate: whether the mere presence of entanglement, within a fixed experimentally accessible domain, can function by itself as a discriminator between incompatible theoretical architectures. Working within a fixed operational algebra of accessible observables, the note shows that entanglement, considered solely as a binary witness, is not by itself sufficient for such discrimination. The result is strictly diagnostic and domain-bounded: it does not deny the relevance of entanglement in quantum gravity, nor does it claim observational equivalence in principle between classical and quantum gravity. It argues instead that discrimination requires analysis of the full observable algebra and of the scaling structure of accessible correlations, not merely the presence or absence of entanglement
This technical note addresses a narrow structural question in the current gravity debate: whether the mere presence of entanglement, within a fixed experimentally accessible domain, can function by itself as a discriminator between incompatible theoretical architectures. Working within a fixed operational algebra of accessible observables, the note shows that entanglement, considered solely as a binary witness, is not by itself sufficient for such discrimination. The result is strictly diagnostic and domain-bounded: it does not deny the relevance of entanglement in quantum gravity, nor does it claim observational equivalence in principle between classical and quantum gravity. It argues instead that discrimination requires analysis of the full observable algebra and of the scaling structure of accessible correlations, not merely the presence or absence of entanglement
Probing the quantum nature of gravity is one of the most profound open challenges in physics. A promising direction is the Gravitationally Induced Entanglement (GIE) protocol, where two initially unentangled probes become entangled solely through local interactions with a mediator of interaction, the gravitational field. Such entanglement would imply that gravity must carry quantum features, namely, it must possess at least a couple of incompatible observables. This thesis further expands the GIE protocol and its Theory-Independent argument by introducing a general, dynamics-independent inequality: the quantum correlation gain between the probes is upper-bounded by the non-commutativity of the mediator’s observables and its Hilbert space dimension. This bound holds for arbitrary interactions and any finite-dimensional systems. A decoherence analysis shows the bound vanishes when the mediator becomes classical. These results provide a quantitative, operational tool for observing the non-classicality of mediators, thereby expanding the conceptual and experimental reach of GIE-based tests of gravity.
Abstract There has been a wave of recent interest in detecting the quantum nature of gravity with tabletop experiments that witness gravitationally mediated entanglement. Central to these proposals is the assumption that any mediator capable of generating entanglement must itself be nonclassical. However, previous arguments for this have modeled classical mediators as finite, discrete systems such as bits, which excludes physically relevant continuous and infinite-dimensional systems such as those of classical mechanics and field theory. In this work, we close this gap by modeling classical systems as commutative unital C ∗ -algebras, arguably encompassing all potentially physically relevant classical systems. We show that these systems cannot mediate entanglement between two quantum systems A and B , even if A and B are themselves infinite-dimensional or described by arbitrary unital C ∗ -algebras (as in quantum field theory), composed with an arbitrary C ∗ -tensor product. This result reinforces the conclusion that the observation of gravity-induced entanglement would require the gravitational field to possess inherently non-classical features.
Recently, table-top experiments involving massive quantum systems have been proposed to test the interface of quantum theory and gravity. In particular, the crucial point of the debate is whether it is possible to conclude anything on the quantum nature of the gravitational field, provided that two quantum systems become entangled solely due to the gravitational interaction. Typically, this question has been addressed by assuming a specific physical theory to describe the gravitational interaction, but no systematic approach to characterise the set of possible gravitational theories which are compatible with the observation of entanglement has been proposed. Here, we remedy this by introducing the framework of Generalised Probabilistic Theories (GPTs) to the study of the nature of the gravitational field. This framework enables us to systematically study all theories compatible with the detection of entanglement generated via the gravitational interaction between two systems. We prove a no-go theorem stating that the following statements are incompatible: i) gravity is able to generate entanglement; ii) gravity mediates the interaction between the systems; iii) gravity is classical. We analyse the violation of each condition, in particular with respect to alternative non-linear models such as the Schrödinger-Newton equation and Collapse Models.
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.
We delineate a tripartite diagnostic framework for probing the quantum character of gravitationalinteractions: (i) an entanglement witness based on gravimetrically-induced coherent phase shiftsin spatial superpositions, (ii) an effective field theory (EFT) taxonomy classifying non-universalgravitational couplings, and (iii) a generalized criterion for the breakdown of classical spacetimedescriptions at singular configurations, interpreted as an EFT failure. Central to this work is theintroduction of a dimensionless crossover parameter,Q = κΓtot(1)where κ quantifies the rate of coherent gravitational phase accumulation and Γtot denotes the totalenvironmental decoherence rate. We rigorously demonstrate that Q ≫ 1 delineates regimes ofgenuine quantum gravitational mediation and entanglement generation, whereas Q ≪ 1 characterizeschannels where quantum correlations are suppressed, yielding effectively classical, entanglement-breaking dynamics. This Q-parameter thus serves as a unified discriminant criterion for mappingout the quantum-to-classical transition in gravity.
The quantization of gravity remains one of the most important---yet extremely elusive---challenges at the heart of modern physics. Any attempt to resolve this long-standing problem seems to be doomed, as the route to any direct empirical evidence (i.e., detecting gravitons) that sheds light on the quantum aspect of gravity is far beyond the current capabilities. Recently, it was discovered that gravitationally induced entanglement, tailored in the interferometric frameworks, can be used to witness the quantum nature of gravity. Even though these schemes offer promising tools for investigating quantum gravity, many fundamental and empirical aspects of the schemes are yet to be discovered. Considering the fact that, besides quantum entanglement, the quantum uncertainty and complementarity principles are the two other foundational aspects of quantum physics, the quantum nature of gravity needs to manifest all of these features. Here, we lay out an interferometric platform for testing these nonclassical aspects of quantum mechanics in the quantum gravity setting, which connects gravity and quantum physics in a broader and deeper context. As we show in this work, all of these fundamental features of quantum gravity can be framed and thoroughly analyzed in an interferometric scheme.