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arXiv:2602.15655 · Optica

Generating quantum entanglement from sunlight

Cheng Li, Jasvinder Brar, Michael Küblböck, Jeremy Upham, Hanieh Fattahi, Robert W. Boyd

quant-phphysics.opticsSubmitted Feb 17, 2026• Announced Feb 18, 2026• 0 citations• Revised May 15, 2026 (v2)
v1 · Feb 17, 2026v2 · May 15, 2026
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Abstract

Energy consumption is becoming a serious bottleneck for integrating quantum technologies within the existing global information infrastructure. In photonic architectures, considerable energy overheads stem from using lasers, whose high coherence was long considered indispensable for quantum state preparation. Here, we demonstrate that natural, incoherent sunlight can successfully produce quantum-entangled states via spontaneous parametric down-conversion. We detect polarization-entangled photon pairs with a concurrence of $0.905\pm0.053$ and a Bell state fidelity of $0.939\pm0.027$. Importantly, the system violates Bell's inequality with $S=2.5408\pm0.2171$, exceeding the classical threshold of 2, while maintaining generation rates comparable to laser-based setups. These findings pave the way for sustainable quantum applications in resource-limited environments like interplanetary missions.

DOI
10.1364/OPTICA.601797

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Energy consumption is becoming a serious bottleneck for integrating quantum technologies within the existing global information infrastructure. In photonic architectures, considerable energy overheads stem from using lasers, whose high coherence was long considered indispensable for quantum state preparation. Here, we demonstrate that natural, incoherent sunlight can successfully produce quantum-entangled states via spontaneous parametric down-conversion. We detect polarization-entangled photon pairs with a concurrence of 0.905±0.0530.905\pm0.0530.905±0.053 and a Bell state fidelity of 0.939±0.0270.939\pm0.0270.939±0.027. Importantly, the system violates Bell's inequality with S=2.5408±0.2171S=2.5408\pm0.2171S=2.5408±0.2171, exceeding the classical threshold of 2, while maintaining generation rates comparable to laser-based setups. These findings pave the way for sustainable quantum applications in resource-limited environments like interplanetary missions.
Spontaneous parametric down-conversion (SPDC) is a reliable platform for entanglement generation. Routinely, a coherent laser beam is an essential prerequisite for pumping the nonlinear crystal. Here we break this barrier to generate polarization entangled photon pairs by using a commercial light-emitting diode (LED) source to serve as the pump beam. This effect is counterintuitive, as the LED source is of extremely low spatial coherence, which is transferred during the down-conversion process to the biphoton wavefunction. However, the type-II phase-matching condition naturally filters the specific frequency and wavelength of LED light exclusively to participate in SPDC such that localized polarization Bell states can be generated, regardless of the global incoherence over the full transverse plane. In our experiment, we characterize the degree of LED light-induced polarization entanglement in the standard framework of the violation of Bell inequality. We have achieved the Bell value S=2.33±0.097S=2.33\pm 0.097S=2.33±0.097, obviously surpassing the classical bound S=2S=2S=2 and thus witnessing the quantum entanglement. Our work can be extended to prepare polarization entanglement by using other natural light sources, such as sunlight and bio-light, which holds promise for electricity-free quantum communications in outer space.
Spontaneous parametric down-conversion (SPDC) can produce entangled photons from a nonlinear medium driven by a pump beam, and has thus been a key resource in photonic quantum technologies. Historically, the low efficiency of SPDC prompted the use of lasers, which are high-intensity light sources, as pump sources. However, the practical implementation of laser-driven systems faces an imminent energy bottleneck. Lasers typically have suboptimal energy efficiencies due to lasing thresholds and the technical demands of thermal management. As a result, laser-pumped entangled photon sources suffer from significant power overhead, rendering quantum devices less sustainable. Over the past few decades, light-emitting diodes (LEDs) have emerged as inexpensive and energy-efficient alternatives for high-intensity light sources. Since 2010, studies have shown that LEDs can drive SPDC and generate photon pairs. These results suggests the potential to replace lasers with ubiquitous and natural light sources to build environmentally friendly quantum devices. However, replacing lasers with other light sources entails a lack of coherence in all degrees of freedom (DoFs) of the pump beam, including spatial, temporal-spectral, and polarization, which may influence the resulting entanglement generated from SPDC. While earlier studies established that pump coherence in a specific DoF sets an upper bound for entanglement in that same DoF, the cross-influence between different DoFs has not been fully investigated. In this thesis, we study the influence of the pump's spatiotemporal coherence on the polarization entanglement generated from SPDC. First, we experimentally demonstrate that polarization-entangled two-photon states can be generated from SPDC pumped by spatiotemporally incoherent light emitted from an LED. Although the resulting entanglement appears lower than that of laser-pumped SPDC, we theoretically show that spatiotemporal incoherence does not fundamentally constrain polarization entanglement; rather, the reduction arises from a coupling between spatiotemporal and polarization DoFs introduced by the experimental setup. We determined experimentally that this cross-influence acts as a deterministic mapping from the spatial components of the pump to the two-photon polarization state. Because this mapping depends on technical aspects of the setup, we envisioned that it could be either (i) engineered to structure entanglement in higher dimensions, or (ii) compensated to reduce spatial distinguishability. Building on the latter, we show that by placing the nonlinear medium in a phase-compensated interferometric setup, SPDC pumped by spatially incoherent light can produce high polarization entanglement that violates local realism, even without postselecting the SPDC field for a single spatial mode. Finally, as a proof-of-principle demonstration of a truly environmentally friendly quantum light source, we demonstrate the generation of polarization-entangled two-photon states from SPDC pumped by a natural source - sunlight.
Spontaneous parametric down-conversion (SPDC) is a reliable platform for entanglement generation. Routinely, a coherent laser beam is an essential prerequisite for pumping the nonlinear crystal. Here we break this barrier to generate polarization-entangled photon pairs by using a commercial light-emitting diode (LED) source to serve as the pump beam. This effect is counterintuitive, as the LED source is of extremely low spatial coherence, which is transferred during the down-conversion process to the biphoton wave function. However, the type-II phase matching condition naturally filters the specific frequency and wavelength of LED light exclusively to participate in SPDC such that localized polarization Bell states can be generated, regardless of the global incoherence over the full transverse plane. In our experiment, we characterize the degree of LED light-induced polarization entanglement in the standard framework of the violation of Bell's inequality. We achieve the Bell value $S=2.33\ifmmode\pm\else\textpm\fi{}0.097$, obviously surpassing the classical bound S=2S=2S=2 and thus witnessing the quantum entanglement. Our work can be extended to prepare polarization entanglement by using other natural light sources, such as sunlight and biolight, which holds promise for electricity-free quantum communications in outer space.
This thesis investigates the concentration of solar light for the generation of entangled photons. Previous studies have demonstrated that pumping a nonlinear crystal with coherent laser light can produce entangled photons via spontaneous parametric down-conversion (SPDC). The objective of this work is to explore the feasibility of generating entangled photons using solar light as the pump source. An experimental setup is developed that efficiently collects approximately 1000 W/m² of sunlight and couples it into an optical fiber to pump a nonlinear ppKTP crystal. Primary focusing is achieved using a Fresnel lens with dimensions of 1 m × 1.4 m, while a secondary concentrator guides the focused light into a multimode optical fiber with a core diameter of 1500 µm. The efficiency of the secondary concentrator is identified as a key factor determining the usable pump power. Several concentrator designs are investigated through optical simulations and experimental testing. Initial approaches employing cascaded compound parabolic concentrators and conical segments yield low coupling efficiencies. To overcome these limitations, optimized geometries including a solid glass droplet and a single polished conical concentrator made of fused silica are developed. Simulations predict total efficiencies of up to 66.9 % for high-index materials. Outdoor experiments under real sunlight demonstrate that coupling solar light into an optical fiber is feasible, with up to 62 mW successfully coupled. The results provide a foundation for future solar-driven quantum optical systems and related free-space applications.
We experimentally demonstrate a violation of local realism with highly spatially multimode polarization-entangled two-photon states produced by spontaneous parametric down-conversion (SPDC) pumped by a spatially incoherent light source---a light-emitting diode (LED). While existing studies have observed such a violation only by postselecting the LED-pumped SPDC photons into a single spatial detection mode, we achieve a Clauser-Horne-Shimony-Holt inequality violation of $S=2.532\ifmmode\pm\else\textpm\fi{}0.069>2$ using a spatially multimode detection setup that collects nearly 4080 SPDC spatial modes. These results indicate that coherent pump sources, such as lasers, are not required for SPDC-based entanglement generation. Our work could enable novel and practical sources of entangled photons for quantum technologies such as device-independent quantum key distribution and quantum-enhanced sensing.
In single-particle or intraparticle entanglement, two degrees of freedom of a single particle, e.g., momentum and polarization of a single photon, are entangled. Single-particle entanglement is a resource that can be exploited both in quantum communication protocols and in experimental tests of noncontextuality based on the Kochen-Specker theorem. Here we show that single-particle entangled states of single photons can be produced from attenuated classical sources of light. To experimentally certify the single-particle entanglement, we perform a Bell test, observing a violation of the Clauser, Horne, Shimony, and Holt inequality. We show that single-particle entanglement can be achieved even in a classical light beam, provided that first-order coherence is maintained between the degrees of freedom involved in the entanglement. This demonstrates that cheap, compact, and low-power photon sources can be used to generate single-particle entangled photons which could be a resource for quantum technology applications.