Quantum Entanglement Demonstrated With Incoherent Light, No Spatial Filtering

Researchers demonstrate a violation of local realism, a key principle of quantum mechanics, using entangled photons generated from a light-emitting diode. This achievement bypasses the need for laser light, previously considered essential for this type of experiment, and supports over 45,000 spatial modes, potentially advancing quantum technologies like secure communication and sensing.

The fundamental principles of local realism, which posit that an object possesses definite properties independent of observation and that influences cannot travel faster than light, continue to be rigorously tested through experiments involving quantum entanglement. Recent work challenges the conventional requirement for coherent light sources, such as lasers, in generating entangled photons via a process known as spontaneous parametric down-conversion (SPDC). Cheng Li, Jeremy Upham, and colleagues, from the University of Ottawa and the University of Toronto, demonstrate a violation of local realism using SPDC pumped by a simple light-emitting diode (LED), a spatially incoherent source. Their findings, detailed in the article “Violation of local realism with spatially multimode parametric down-conversion pumped by spatially incoherent light”, achieve a violation of the Clauser-Horne-Shimony-Holt (CHSH) inequality with a spatially multimode detection setup supporting over 45,000 spatial modes, suggesting a pathway towards more practical quantum technologies including device-independent key distribution and enhanced sensing.

LEDs Enable Simplified Entangled Photon Generation for Quantum Technologies

Quantum entanglement, a cornerstone of quantum mechanics, is central to emerging technologies such as quantum communication and sensing. Generating entangled photon pairs typically relies on spontaneous parametric down-conversion (SPDC), a process where a pump photon splits into two correlated photons within a nonlinear crystal. Traditionally, coherent laser sources have been considered essential for efficient SPDC, with the coherence of the pump beam believed to directly influence the quality of entanglement. Recent research challenges this assumption, demonstrating that light-emitting diodes (LEDs), inherently incoherent light sources, can effectively drive SPDC and generate high-quality entangled photons.

This work demonstrates a clear violation of the Clauser-Horne-Shimony-Holt (CHSH) inequality, a key indicator of entanglement, using an LED-pumped SPDC source. Crucially, this is achieved with minimal spatial filtering of the emitted photons, supporting over 45,000 spatial modes, a significant departure from previous studies requiring stringent spatial mode control. The researchers meticulously characterised the generated biphoton state, confirming the presence of entanglement despite the inherent incoherence of the LED pump. This achievement bypasses the need for complex and expensive laser systems, substantially reducing the cost and complexity associated with generating entangled photons.

The experimental setup centres on a type-I beta-barium borate (BBO) crystal, where photons from the LED undergo SPDC. The team embraced the multimode nature of the LED emission, designing a detection system capable of efficiently collecting and analysing photons across a wide range of spatial modes. This approach is vital because spatial coherence, a property of laser light, is absent in LED emission. By avoiding stringent spatial filtering, the team significantly increases the photon collection rate, a critical factor for practical applications. The robustness of this LED-pumped system to spatial variations is particularly noteworthy, offering potential advantages for free-space quantum communication where atmospheric turbulence can degrade signal quality.

These results have significant implications for the development of practical quantum technologies. The simplified and cost-effective nature of the LED-based source facilitates the creation of compact, integrated quantum sources suitable for diverse applications, including quantum key distribution (QKD) and quantum imaging. Furthermore, the ability to generate high-dimensional entanglement, with correlations across a vast number of spatial modes, enhances the security of QKD and the sensitivity of quantum sensing applications. Future work will focus on optimising the efficiency of the SPDC process with LED pumping and exploring the scalability of this approach to generate multi-photon entangled states.