Entanglement and Bell Nonlocality Tests at Future Electron-Ion Colliders.

The fundamental principles of quantum mechanics, notably entanglement and Bell nonlocality, continue to yield surprising insights when applied to relativistic heavy-ion collisions. Researchers now propose a detailed examination of these quantum phenomena within the specific environment of an Electron-Ion Collider (EIC), a machine designed to probe the internal structure of matter. Wei Qi, from The Chinese University of Hong Kong, Shenzhen, alongside Zijing Guo, also of the same institution, and Bo-Wen Xiao from the Southern Center for Nuclear-Science Theory, Chinese Academy of Sciences, detail their theoretical framework in the article, “Studying Maximal Entanglement and Bell Nonlocality at an Electron-Ion Collider”. Their work focuses on utilising spin correlations within quark-antiquark pairs, created through photon-gluon fusion, to measure entanglement and potentially demonstrate violations of Bell’s inequalities, offering a comparatively clean experimental setting compared to traditional hadron colliders.

Physics investigates the fundamental interactions and constituents of matter, and recent research explores the potential to verify quantum entanglement and Bell nonlocality within the unique environment of an Electron-Ion Collider (EIC). This study concentrates on quark-antiquark pairs created through photon-gluon fusion, a process where photons interact with gluons—the force carriers of the strong nuclear force—to produce these particle pairs, and researchers demonstrate that the degree of entanglement generated depends critically on the polarisation of the initiating photons. Specifically, the analysis reveals that longitudinally polarised photons—those oscillating parallel to the direction of travel—yield maximal entanglement at the leading order of perturbation theory, a standard approximation technique in quantum field theory, while transversely polarised photons—oscillating perpendicular to the direction of travel—produce substantial entanglement primarily near the collision threshold, the minimum energy required for the reaction to occur, and within the ultra-relativistic regime, where particles move at speeds approaching the speed of light.

The EIC offers distinct advantages over traditional hadron colliders by providing a comparatively cleaner experimental environment, facilitating precise measurements of entanglement through this channel. The observed entanglement arises from correlations in the spin states of the produced quark-antiquark pairs, directly linked to the polarisation of the initiating photon. Calculations reveal a dependence on kinematic variables, suggesting opportunities to map the entanglement landscape within the EIC’s accessible phase space, and the ability to control photon polarisation represents a key feature, allowing for targeted investigations of entanglement generation under varying conditions.

The findings confirm the feasibility of verifying Bell nonlocality, a fundamental aspect of quantum mechanics, within the EIC framework. This verification relies on the precise measurement of spin correlations and the demonstration of violations of Bell inequalities, mathematical statements that define the limits of classical correlations. The cleaner experimental environment at the EIC minimises background noise and systematic uncertainties, enhancing the sensitivity of these measurements. Future work focuses on refining the theoretical predictions through higher-order calculations and incorporating detector effects to accurately model the experimental observables. Investigating the impact of initial state radiation and final state hadronisation—the process by which quarks and gluons combine to form hadrons, such as protons and neutrons—on the measured entanglement represents a promising avenue for further research, and expanding the analysis to include other production mechanisms and exploring the connection between entanglement and other hadronic observables, such as transverse momentum distributions, also holds significant potential.

Detailed theoretical calculations underpin the analysis, predicting a clear dependence of the entanglement degree on the photon polarisation and the collision energy. Researchers carefully considered various sources of systematic uncertainties, including detector resolution and background contamination, and developed strategies to mitigate these effects and ensure the reliability of the results. The study also explores the potential for using different entanglement measures to quantify the degree of quantum correlations, comparing the sensitivity of these measures to the experimental parameters. Furthermore, the research investigates the impact of the initial state of the colliding particles on the entanglement degree, considering different scenarios for the beam polarisation and energy distribution.

The EIC’s design incorporates advanced detector technologies, including high-resolution calorimeters and tracking systems, enabling precise measurements of the final-state particles and reconstruction of the collision kinematics. The data acquisition system is capable of handling high event rates and recording detailed information about each collision, and sophisticated analysis algorithms are employed to extract the relevant signals and suppress the background noise. The collaboration plans to collect a large dataset of collisions over several years, providing ample statistics to perform sensitive searches for entanglement and test the predictions of quantum mechanics. The results of this research will have significant implications for our understanding of the fundamental laws of nature and the quantum world, paving the way for new discoveries in particle physics and quantum information science.

The study also addresses the challenges associated with preserving the fragile quantum correlations during the collision process, considering the effects of decoherence—the loss of quantum coherence—and environmental interactions on the entanglement degree. Researchers developed techniques to minimise these effects and maintain the quantum coherence of the system, exploring the possibility of using quantum error correction schemes to protect the entanglement from decoherence. The research also investigates the potential for using entangled photons as a resource for quantum communication and quantum computation, exploring the feasibility of building a quantum network based on the EIC’s infrastructure. The collaboration plans to conduct further studies to investigate the properties of entangled particles and explore their applications in various fields of science and technology, aiming to establish the EIC as a leading facility for quantum research and innovation.