Ludwig-Maximilians University, Munich, Germany. Max Planck Institute for Quantum Optics, Garching, Germany. Munich Centre for Quantum Science and Technology, Munich, Germany. Institute of Spintronics and Quantum Information, Faculty of Physics, Adam Mickiewicz University, 61-614 Poznań, Poland., Xiamen University Malaysia. Centre for Photonics and Quantum Communication Technology, Indian Institute of Technology Roorkee. Bell inequality violations, or Bell nonlocality, are not only key to testing the foundations of quantum physics but also an essential ingredient of numerous quantum information protocols.
Single photon pairs confirm nonlocal quantum behaviour with relaxed reproducibility demands
A Bell parameter value exceeding 2 was achieved by the team from National University of Singapore and collaborating institutions using only one detected particle pair per measurement direction, an improvement over prior work requiring many copies or exact reproducibility. Strong quantum nonlocality is possible even when experimental setups cannot be perfectly replicated; previously, such violations were assumed impossible without precise control over apparatus settings. By confirming this using polarization-entangled photons, they broadened the scope for applications needing limited control, including secure communication protocols and device certification methods.
These findings establish that imperfect repeatability does not preclude demonstrating fundamental aspects of quantum mechanics like entanglement, opening new avenues in randomized measurements within quantum information science. A violation of Bell’s theorem was demonstrated with entangled photon pairs while deliberately avoiding precise control over measurement angles.
To quantify uncertainty, researchers modelled experimental arrangements as fluctuating around ideal values on the Bloch sphere, a geometrical representation of qubit states, using both uniform “patches” and quasi-normal distributions to simulate imperfect repeatability. They even devised an inequality utilising 1728 separate Clauser-Horne-Shimony-Holt (CHSH) expressions which could all be simultaneously violated under certain conditions, showing strong performance against randomness in measurement settings.
Bell Nonlocality Achieved Despite Imperfect Control Opens Pathways To Practical Quantum Networks
This work convincingly demonstrates durability against imperfect apparatus control, a key step towards practical applications like secure communication networks; however, it leaves open questions about scaling these results to more complex scenarios. Just one measurement per setting successfully achieved Bell nonlocality, but the team acknowledges that increasing data collection at each unique direction could introduce statistical challenges not yet addressed in their analysis. Even with acknowledged limitations regarding more complex data collection scenarios, this demonstration of durability is important for practical quantum technologies, it broadens the scope of where reliable entanglement-based protocols can operate.
Strong performance against imperfections in real-world devices and expanded possibilities for quantum information protocols operating with limited control over components are now established. This flexibility will likely broaden applications of entanglement across diverse technologies; secure communication networks stand to benefit sharply from these advances. The team’s findings suggest a future where reliable quantum protocols begin operating even with less-than-ideal equipment, representing a truly new prospect.
This research demonstrated that violations of Bell inequalities, indicating nonlocality central to quantum physics and information protocols, remain possible even when measurement directions are not precisely repeatable. Using polarization-entangled photons and detecting only one pair per analyzer direction, the researchers showed this robustness against imperfect apparatus control. Their results, confirmed through analysis including 1728 CHSH expressions, establish that entanglement-based protocols can function effectively in settings with limited measurement precision. This expands possibilities for applications like device certification and communication complexity where perfect control is unrealistic.
👉 More information
🗞 Revealing Bell Nonlocality Does Not Require the Same Physical Measurement to be Performed Twice
✍️ Paweł Cieśliński (National University of Singapore); Carlotta Versmold, Eun Mi Kim, Jan Dziewior and Harald Weinfurter (Ludwig-Ludwig-Maximilians University); Jan Wójcik, Tomasz Paterek and Wiesław Laskowski (University of Gdańsk); Paweł Kurzyński (Adam Mickiewicz University)
🧠 ArXiv: https://arxiv.org/abs/2611.15871
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