Gravitationally-induced Photon Entanglement in FLRW Backgrounds Demonstrates System Locality for Quantum Gravity Tests

The fundamental nature of gravity remains a profound mystery, and scientists are continually seeking new ways to probe its behaviour. Chi Zhang, from Zhejiang Ocean University, and colleagues explore a novel approach to investigating gravity by examining the entanglement of photons produced in astronomical events. This research proposes a method to detect gravity-induced entanglement, but using photons instead of massive particles, which offers a unique test of whether gravity requires a nonclassical explanation, based on principles of quantum mechanics. While the gravitational interaction between photons is incredibly weak, the team’s calculations reveal key characteristics of entanglement arising from a photon’s own gravitational field, potentially inspiring new experiments in photon entanglement and deepening our understanding of gravity itself.

Photons, unlike massive particles, simultaneously satisfy both the requirements for event and system localities, allowing for a clear test of whether the gravitational mediator must be nonclassical based on the principles of Local Operations and Classical Communication. Although the gravitationally induced entanglement between massless relativistic photons is extremely small, quantitative calculations clarify the characteristic features of the entanglement induced by the photon’s own gravitational field in astronomical processes and may inspire other photon entanglement experiments. The unification of gravity with quantum mechanics remains a central challenge in modern physics.

Entanglement and Gravity in Cosmic Photons

This paper investigates the possibility of detecting quantum entanglement and signatures of quantum gravity through cosmological observations, specifically focusing on photons. The central argument is that gravitational interactions can induce entanglement between photons, and this entanglement, while extremely fragile, might be detectable in the Cosmic Microwave Background or through other astrophysical observations. Detecting this entanglement would provide evidence for quantum effects in gravity, potentially supporting theories beyond classical general relativity. The core idea is that gravity is not simply a classical force, but a manifestation of spacetime curvature that can affect quantum states.

This curvature can create correlations, known as entanglement, between particles that would not exist in flat spacetime. The authors focus on the early universe and environments with strong gravity, such as around black holes and neutron stars, because these conditions might amplify or preserve entanglement long enough to be detectable. The paper acknowledges the challenge of decoherence, the loss of quantum coherence due to environmental interactions, and explores mechanisms that might slow it down or amplify the entanglement signal. Tools like Bell inequalities and quantum discord are used to detect and quantify entanglement.

Violations of Bell inequalities or the presence of quantum discord in cosmological data could be evidence of gravitationally induced entanglement. The research emphasizes the use of continuous variable entanglement, such as entanglement in the phase or polarization of photons, as opposed to discrete variable entanglement, because continuous variables are more robust to certain types of noise. The authors suggest that entanglement between photons in the Cosmic Microwave Background could manifest as subtle correlations in the polarization patterns, requiring extremely sensitive polarization measurements. Entanglement might also be detectable in gravitational waves, particularly those emitted from merging black holes or neutron stars.

They propose searching for violations of Bell inequalities or the presence of quantum discord in the correlations between photons observed from these sources, focusing on correlations in the phase and polarization of photons as potential signatures of entanglement. Decoherence presents the biggest challenge, but the authors discuss potential mechanisms to slow it down, such as exploiting specific symmetries or using squeezed states of light. Astrophysical observations are inherently noisy, requiring extremely sensitive detectors and sophisticated data analysis techniques to distinguish the entanglement signal from the noise. The entanglement signal is expected to be very weak, requiring long integration times and large-scale surveys.

It is crucial to rule out other possible sources of correlations, such as those arising from classical effects or instrumental artifacts. This paper attempts to bridge the gap between quantum gravity and observable cosmological phenomena, proposing new ways to search for quantum effects in gravity using existing and future astronomical observations. If gravitationally induced entanglement is detected, it would provide strong evidence for quantum gravity and could help to constrain different theoretical models. The focus on continuous variable entanglement is a novel approach that could be more robust to noise than traditional discrete variable entanglement.

Photons Entangle via Self-Gravity in Cosmology

This research presents a novel investigation into gravitationally induced entanglement using photons, building upon the established concept of quantum gravity mediated entanglement. The team demonstrates how photons, unlike massive particles, offer a unique system for testing the fundamental nature of gravity due to their relativistic properties and simultaneous satisfaction of event and system localities. Through detailed calculations, they model how photon pairs can become entangled via their own gravitational fields within the framework of a flat, homogeneous, and isotropic Friedmann, Lemaître, Robertson, Walker universe. The study clarifies the characteristics of this entanglement, accounting for cosmological factors and the specific properties of photons traveling along null geodesics.

By considering scenarios involving multi-path propagation of photons, such as those arising from anisotropic media, gravitational lensing, or astrophysical masers, the researchers establish a theoretical basis for observing entanglement induced by the photons’ own gravitational interaction. The team also evaluated decoherence and conversion rates resulting from photon-graviton coupling, and simulated the statistical properties of entanglement phase within the cosmic microwave background. The authors acknowledge that calculating entanglement in a cosmological setting presents significant challenges, and that the predicted effects are extremely small. Future research could focus on refining the models and exploring potential observational signatures of this entanglement, potentially through advanced astrophysical observations. This work provides a theoretical foundation for investigating the quantum nature of gravity using photons and opens new avenues for exploring the interplay between quantum mechanics and cosmology.