Entanglement’s Foundations Win Physics’ Top Honour

Scientists have long sought to understand the bizarre phenomenon of quantum entanglement, a concept famously challenged by Albert Einstein and now recognised with the 2022 Nobel Prize in Physics. Yu Shi, working independently, provides a comprehensive review of the historical development of entanglement research, tracing its origins through key milestones such as Bell inequalities and the pioneering work of C. S. Wu, who utilised polarisation-photons from electron-positron annihilation to further understanding. This analysis is significant because it not only contextualises the recent Nobel recognition but also highlights the crucial physical ideas underpinning this complex field, demonstrating how decades of theoretical and experimental investigation have transformed our understanding of quantum mechanics and paved the way for emerging quantum technologies.

This week’s Nobel Prize recognises work confirming that tiny particles can be linked in an instantaneous way, regardless of the distance separating them, with implications extending beyond fundamental physics to potentially revolutionise technologies like secure communication and computing.

Researchers are unraveling the intricacies of quantum entanglement, a phenomenon once dismissed as a paradox but now poised to revolutionize technology. Recent work clarifies the historical development of this field, culminating in the 2022 Nobel Prize in Physics awarded to Alain Aspect, John F. Clauser, and Anton Zeilinger for their groundbreaking experiments.

These physicists demonstrated the violation of Bell’s inequality using entangled photons, solidifying the foundations of quantum information science and ushering in a new era of quantum possibilities. The research traces the evolution of understanding from initial theoretical objections to the now-verified reality of entanglement, a process where two or more particles become linked and share the same fate, no matter how far apart they are.

The study meticulously reviews the key concepts and milestones underpinning Bell’s inequality research, highlighting the pivotal role played by Albert Einstein in initiating the exploration of quantum entanglement. Initially skeptical, Einstein’s thought experiments inadvertently propelled the field forward, prompting investigations into the counterintuitive nature of quantum mechanics.

The work details how experiments have moved beyond observing entanglement to harnessing it as a powerful tool for emerging technologies, representing a significant leap beyond the first quantum revolution which focused on understanding the fundamental principles governing the microscopic world. The investigation establishes the fundamental principles of quantum mechanics, emphasizing the concept of the quantum state, a mathematical description of a particle’s properties differing significantly from classical physics.

Unlike classical descriptions defining a particle’s position and momentum with certainty, quantum states represent probabilities, encompassing multiple possibilities until a measurement is made. This inherent uncertainty is central to entanglement, where the quantum state of two particles becomes correlated, meaning the measurement of one instantaneously influences the state of the other, regardless of the distance separating them. The research then focuses on photon polarization, the direction of the electric field of light, as a key property used in experiments verifying entanglement.

Rapid light redirection validates non-local quantum entanglement

Alain Aspect’s experiments between 1981 and 1982 meticulously investigated violations of the Bell-CHSH inequality, challenging the principle of locality. Initial efforts to excite electrons to the 61S0 state relied on two-photon absorption using lasers, proving more effective than previous techniques employing 61P1 excitation. Subsequent measurements utilised a two-channel polarizer, generating robust statistical data and demonstrating a substantial breach of the Bell inequality with a precision exceeding several dozen standard deviations.

The pivotal experiment incorporated an acousto-optic switch, a device that rapidly redirects light based on sound waves, positioned before the photons reached the polarizers. This switch altered the photons’ path every 10 nanoseconds, directing them to one of two polarizer pairs, circumventing the practical difficulty of rotating the polarizers during the extremely short 20-nanosecond photon flight time over a 6-metre distance.

The experiment yielded a value of 0.101 ±0.020 for the CHSH inequality, aligning with quantum mechanical predictions and demonstrating a violation at a 5 standard deviation level. Further advancements came in 1997 with work from Zeilinger’s group, which definitively closed the locality loophole by separating the devices analysing entangled photon pairs by 400 metres, requiring 1,300 nanoseconds for light transmission.

Entangled photons were transmitted through optical fibres to the polarizers, and the direction of polarization analysis was rapidly and randomly altered using an atomic clock and a random number generator. Crucially, this research employed type-II parametric down-conversion, a nonlinear optical process achieved using β-boron borate (β-BBO) crystals initially developed by the Fujian Institute of Research on the Structure of Matter, providing a reliable source of entangled photon pairs.

Early Detection of Photon Asymmetry and Realisation of Entangled States

Initial experiments employing the γ detector developed by Wu and Shaknov demonstrated a sensitivity ten times greater than prior attempts, allowing them to measure asymmetry at 2.04 ±0.08, closely approaching the theoretical prediction of 2.00, establishing a significant early milestone in understanding quantum entanglement. The subsequent development of Bell’s inequalities provided a framework for experimentally testing local realism, a concept positing the existence of hidden variables supplementing quantum mechanics.

Bell demonstrated that local hidden variable theories must adhere to specific inequalities, while quantum mechanics predicts violations under certain entangled states. Utilising a spin-entangled state, calculations revealed that quantum mechanics predicts a correlation violating these inequalities, a crucial theoretical foundation for experimental verification.

Early attempts to verify Bell’s inequalities faced limitations due to idealized assumptions. To address this, the CHSH inequality was formulated in 1969 by Clauser, Horn, Shimony, and Holt, offering a more practical framework for real-world experiments. This inequality, expressed as 2 ≤ S ≤ 2, where S = P(a, b) + P(a, b’) + P(a’, b) − P(a’, b’), allows for experimental testing of local realism, with quantum mechanics predicting values reaching ±2√2 for Bell states. The Freedman-Clauser experiment, initiated in 1972, aimed to directly test these predictions and further refine the understanding of quantum entanglement.

Satellite quantum key distribution extends secure communication range

The persistent challenge of secure communication has driven decades of innovation, culminating in the recent advances detailed in this body of work. For years, the goal was not simply transmitting information, but doing so with a guaranteed level of security rooted in the laws of physics themselves. Early explorations into Bell inequalities and the counterintuitive nature of quantum entanglement laid the theoretical groundwork, but translating these concepts into practical technologies proved remarkably difficult.

Maintaining entanglement over significant distances, and building detectors sensitive enough to verify it, demanded breakthroughs in materials science, laser technology, and signal processing. What distinguishes this progression is the sheer scale now achievable, moving from laboratory demonstrations to satellite-based quantum key distribution and intercontinental entanglement distribution.

This fundamentally alters the landscape, suggesting a future where truly unhackable communication networks are not merely a theoretical possibility, but a developing reality. However, significant hurdles remain. Building a global quantum internet requires not only long-distance entanglement but also quantum repeaters to overcome signal degradation. Furthermore, the complexity and cost of these systems currently restrict their deployment.

The push towards “device-independent” protocols, which minimise trust in the hardware itself, is crucial but still in its early stages. Future research will likely focus on miniaturisation, cost reduction, and the development of more robust and scalable quantum technologies, ultimately determining whether this promise of absolute security can be extended beyond a select few.