Quantum-information methods for quantum gravity laboratory-based tests (2410.07262v1)

Abstract: Quantum theory and general relativity are about one century old. At present, they are considered the best available explanations of physical reality, and they have been so far corroborated by all experiments realised so far. Nonetheless, the quest to unify them is still ongoing, with several yet untested proposals for a theory of quantum gravity. Here we review the nascent field of information-theoretic methods applied to designing tests of quantum gravity in the laboratory. This field emerges from the fruitful extension of quantum information theory methodologies beyond the domain of applicability of quantum theory itself, to cover gravity. We shall focus mainly on the detection of gravitational entanglement between two quantum probes, comparing this method with single-probe schemes. We shall review the experimental proposal that has originated this field, as well as its variants, their applications, and discuss their potential implications for the quantum theory of gravity. We shall also highlight the role of general information-theoretic principles in illuminating the search for quantum effects in gravity.

• The paper presents a detailed review of how quantum-information methods enable laboratory tests of gravitational entanglement to assess gravity's non-classicality.
• The study examines dual quantum probe experiments supported by the General Witness Theorem to challenge classical gravitational models.
• The review outlines numerical estimates and technical challenges, highlighting feasible paths toward a unified quantum gravity theory.

Quantum-information methods for quantum gravity laboratory-based tests: An Overview

The paper "Quantum-information methods for quantum gravity laboratory-based tests" by Chiara Marletto and Vlatko Vedral provides a comprehensive review of the burgeoning field where information-theoretic techniques are leveraged to design laboratory tests aiming to probe quantum gravitational phenomena. This endeavor arises from extending quantum information theory (QIT) beyond its typical quantum theoretical boundaries to accommodate the gravitational domain. The central premise is the exploration of gravitational entanglement between two quantum probes, with a comparative analysis of multi-probe and single-probe methods. The discourse ranges from the inception of the experimental proposal that catalyzed this field to its various iterations, potential applications, and their implications for quantum gravity theory.

Core Experimental Proposal: Gravitationally Induced Entanglement (GIE)

The experimental methodology extensively discussed in the paper involves the use of two quantum systems (potentially macroscopic masses in superposition) that interact solely through the gravitational field. Proposed independently by S. Bose and collaborators, and by C. Marletto and V. Vedral, this approach hinges on the premise that if gravity can entangle two quantum probes under certain assumptions, it must possess non-classical properties. This potential entanglement, referred to as Gravitationally Induced Entanglement (GIE), constitutes a critical observable for identifying quantum features in the gravitational force, implying non-classicality, which is not consistent with classical theories such as general relativity.

Theoretical Foundations and General Witness Theorem

The underpinning of GIE lies in leveraging the General Witness Theorem (GWT), which extends the Local-Operation-and-Classical-Communication (LOCC) principles beyond the prevalent quantum theory framework to incorporate gravitational interactions. The GWT posits that if a system (e.g., gravity) can entangle two other quantum systems only through local interactions, it cannot be purely classical. This theorem is built on the foundational principles of locality (no action at a distance) and interoperability of information, conveying that the entanglement generation capability of gravity attests to its non-classicality. This approach facilitates an argument akin to Bell's theorem, where observing GIE enables the exclusion of a comprehensive class of classical gravitational models.

Numerical Feasibility and Current Technological Challenges

Numerical estimations suggest that GIE could be detectable with masses around nanograms interacting over microseconds and separated by micrometers. Nevertheless, the paper emphasizes the formidable technological challenges accompanying such experiments, notably the requisite precision in isolating gravitational interactions from electromagnetic forces, such as the Casimir-Polder effect and decoherence sources that might obscure the gravitational entanglement signals. Despite these hurdles, advancements in quantum control over mesoscopic systems are driving this research into the field of experimental feasibility.

Implications and Future Prospects

The detection of GIE would entail significant implications for quantum gravity, challenging classical formulations like general relativity by necessitating modifications to encapsulate genuine quantum features. Importantly, failing to observe GIE would equally yield profound insights, potentially questioning the prevailing assumptions in modeling quantum gravity or hinting at novel physics. As such, the approach not only delineates a path for experimental verification of specific quantum gravity theories but also opens inquiry into broader theoretical frameworks.

Alternative Conceptions and Proposals

The paper points to several variants of the core GIE experiment, including scenarios where other quantum mechanical degrees of freedom, beyond spatial superpositions, play a role. Furthermore, single-probe variants and quantum gravity proposals based on spin or energy superpositions are presented as complementary approaches that might simplify experimental realizations or probe different quantum gravitational aspects.

Conclusion

In sum, Marletto and Vedral's review captures an innovative frontier in quantum gravity research, indicating the potential of quantum information methodologies to provide empirical insights into the nature of gravitational interactions at quantum scales. The incorporation of information-theoretic principles into the testing of gravitational theories showcases an interdisciplinary convergence poised to advance our understanding of fundamental physics. Future practical implementations of these concepts could thus serve as a pivotal step towards resolving the longstanding quest for a unified quantum gravity theory.