Generating quantum entanglement from sunlight

Energy consumption is on track to become a serious bottleneck in integrating quantum technologies with existing information and communication infrastructure. In photonic quantum technologies, a considerable portion of the energy overhead stems from the use of lasers, whose high coherence has long been considered indispensable for preparing quantum states of light. Here, we demonstrate that despite their low optical coherence, natural sources of light, such as sunlight, can produce quantum-entangled photon states by means of nonlinear optical effects. From spontaneous parametric down-conversion pumped by sunlight, we detect polarization-entangled photon pairs with a concurrence of C =0.905±0.053, a purity of P =0.919±0.045, and a fidelity to a Bell state of F =0.939±0.027. The resulting two-photon state violates Bell’s inequality with S =2.5408±0.2171, which exceeds the threshold value of 2 to confirm quantum behavior. More importantly, we observe a photon pair generation rate of ∼1600s −1 ( mW of pump power ) −1 , which is comparable to that of the laser-pumped SPDC when normalized for the spectral bandwidth of the pump. Our work paves the way for developing sustainable and accessible photonic quantum technologies, especially for implementation in resource-constrained areas such as the Arctic region and satellites in space for interplanetary missions.