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.
Quantum technologies have promised to revolutionize modern information processing, offering computational advantages for classically hard problems (1-3), information-theoretic communication security (4)(5)(6), and sensing precision beyond classical shot-noise limits (7,8). Over the past few decades, breakthroughs in device performance have been widely celebrated in both scientific journals and the popular press, while a critical figure of merit for these advances-the energy cost-has raised concerns for their sustainability (9). In the classical domain, the global information and communication technology sector already accounts for ∼ 1.8-3.9% of worldwide greenhouse gas emissions and energy use (10,11). With the ever-growing global data traffic, any large-scale deployment of quantum-enabled information and communication technology will inevitably add to this burden.
On existing quantum platforms, a substantial share of the power consumption stems from the energy overhead required to prepare and control the physical quantum systems. Preserving quantum coherence in superconducting circuits requires millikelvin-level cooling, which is provided by cryogenic equipment that routinely consumes 5-10 kW of electrical power per unit (12,13). Ion-trap processors require energy-intensive resources such as ultra-high-vacuum infrastructure and highpower radiofrequency drive fields (14). In photonic quantum systems, generating quantum states of light often involves deploying lasers, which have suboptimal electrical-to-optical conversion efficiency due to their inherent requirement of operating above a driving current threshold. Commercial lasers typically draw watts of electrical power only to deliver optical power on the milliwatt scale, with a significant portion of the consumed energy going towards spectrum stabilization and temperature control, or dissipating as heat. Moreover, their durability and long-term reliability are limited when operating in harsh conditions such as intense radiation, high vacuum, and extreme temperature fluctuations. The energy requirements for these platforms could significantly constrain the scalability and accessibility of quantum technologies. Fortunately, operations of photonic quantum systems may not require enduring the energy burden from lasers, unlike superconducting and ion-trap systems, whose underlying physical principles necessitate the energy overheads.
A key ingredient in photonic quantum technologies is the entangled photon state (15)(16)(17)(18)(19)(20). Lasers often serve as the pump source for generating entangled photons from spontaneous parametric down-conversion (SPDC), which is a nonlinear optical process widely employed in many photonic quantum applications. In SPDC, photons from a pump beam interact with a nonlinear medium and are down-converted into photon pairs (21)(22)(23). Conventionally, lasers are employed as pump sources due to their typically high coherence across all degrees of freedom, including spatial, spectral-temporal, and polarization. The reason is that the coherence of the pump in a given degree of freedom puts an upper bound on the maximally attainable entanglement in the same degree of freedom (24)(25)(26)(27)(28)(29)(30)(31)(32)(33). However, this constraint does not apply when the target entanglement is in a different degree of freedom from that of the coherence of the pump (34). For instance, studies from the authors, as well as others, have demonstrated that a polarized but spatiotemporally incoherent light source, such as a light-emitting diode (LED), can produce polarization-entangled photon pairs via SPDC (35)(36)(37). These earlier achievements suggest that natural sources of light, such as sunlight, can replace lasers as the pump source for nonlinear optical processes and generate entangled photons, thereby enabling energy-efficient quantum technologies. Solar energy harvesting through photovoltaics offers a prominent option for green, renewable power supply. However, the efficiency of state-of-the-art solar cells remains below 50% (38), and their performance and durability are similarly constrained in severe environmental conditions. In parallel with the materials science effort in solar cell research, sunlight concentration technologies have been developed over the years to facilitate optimal utilization of solar power (39). Studies have proposed using solar concentration technology to directly drive optical amplification and build a solar laser (40)(41)(42). On the other hand, large-scale solar-powered space infrastructure, such as space-based solar power platforms (43) and orbital data centers (44), is rapidly advancing, driving interest in photonic technologies capable of operating directly on abundant, incoherent solar radiation without dependence on power-intensive laser or electrical subsystems. Within this landscape, solar-driven photonic quantum technologies represent a promising route toward energyautonomous quantum photonic fun
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