Long-Distance Quantum Correlations Secured Against Photon Loss with Tunable States.
Researchers demonstrated a method for sustaining Bell inequality violations—a key requirement for secure device-independent communication—over extended distances despite photon loss. By heralding a specifically prepared quantum state, they achieved loss-tolerance at the theoretical Eberhard limit, scaling optimally with transmission distance and simplifying practical implementation.
The quest to definitively demonstrate quantum non-locality – the phenomenon where entangled particles exhibit correlations stronger than any classical explanation allows – faces a persistent challenge: signal loss. Maintaining entanglement over significant distances inevitably diminishes photon counts, potentially creating a ‘detection loophole’ that invalidates experimental results. Researchers at Imperial College London have developed a protocol that substantially improves the tolerance to such losses, enabling Bell tests – experiments designed to test the foundations of quantum mechanics – to be performed reliably over extended distances. Yazeed K. Alwehaibi, Ewan Mer, Gerard J. Machado, Shang Yu, Ian A. Walmsley, and Raj B. Patel detail their approach in a new paper, “A Tractable Protocol for Detection-Loophole-Free Bell Tests over Long Distances”, published this week, outlining a method that leverages the vacuum component of the prepared quantum state to achieve a post-selection-free violation of Bell inequalities at the established Eberhard limit.
Enhanced Bell Inequality Violation with Vacuum-Engineered States
Recent research details a method for strengthening demonstrations of quantum non-locality – specifically, violations of Bell inequalities – even in the presence of substantial photon loss. This addresses a significant challenge in photonic quantum communication: maintaining robust entanglement over distance despite inevitable signal attenuation. Researchers achieve this by engineering a specific quantum state, denoted |ψ⟩, which exploits properties of the quantum vacuum to mitigate loss effects.
Bell inequalities define limits on correlations achievable by any local realistic theory. Quantum mechanics predicts, and experiments confirm, violations of these inequalities, demonstrating the non-local nature of quantum correlations – a key resource for quantum technologies.
The innovation centres on the design of the |ψ⟩ state, contrasting it with the conventionally used entangled state |ϕ⟩. Calculations utilising Q-functions – quasi-probability distributions representing quantum states – reveal that photon loss impacts both the vacuum and single-photon components of |ψ⟩. In contrast, loss primarily affects the single-photon components of |ϕ⟩, leaving its vacuum component unaffected. The vacuum component refers to the zero-photon contribution to the quantum state, a consequence of the uncertainty principle.
This protocol offers a practical advantage for long-distance quantum applications by simplifying experimental setup. It requires only interference at a central station followed by single-photon detection, eliminating the need for complex entanglement swapping or quantum repeaters. Entanglement swapping extends entanglement over longer distances by creating entanglement between previously independent entangled pairs. Quantum repeaters overcome signal loss by dividing a long distance into shorter segments with entanglement distribution and swapping. This streamlined approach preserves an optimal square-root scaling with channel transmittance – the fraction of photons successfully transmitted through a channel – ensuring signal strength degrades more slowly with distance.
The team’s calculations reveal that |ψ⟩’s resilience stems from its balanced response to photon loss. Researchers optimised measurement settings (parameters α and β) to further minimise loss impact, achieving a significant improvement over conventional approaches.
Meticulous quantification of both states under various loss conditions demonstrated that |ψ⟩ consistently reaches the Eberhard limit – a benchmark for loss tolerance – while |ϕ⟩ falls short under comparable conditions. This improvement arises from the loss-independence of the vacuum component amplitude within the |ψ⟩ state, effectively providing a resource that sustains entanglement despite signal attenuation.
This work opens avenues for future research in long-distance quantum communication. Scientists can explore the potential of |ψ⟩ in various quantum communication protocols, such as quantum key distribution and quantum teleportation. Further investigation into different quantum states to enhance the resilience of quantum communication systems is also warranted. The findings have significant implications for developing secure and reliable quantum networks.
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🗞 A Tractable Protocol for Detection-Loophole-Free Bell Tests over Long Distances
🧠 DOI: https://doi.org/10.48550/arXiv.2506.05048