Sunlight Generates Quantum Entanglement Without Lasers

Scientists are addressing the growing energy demands of modern information technologies by exploring alternative light sources for quantum communication. Cheng Li, Jasvinder Brar and Michael Küblböck from the Max Planck Institute for the Science of Light, working with Jeremy Upham and Robert W. Boyd from the Department of Physics, University of Ottawa, and in collaboration with Hanieh Fattahi, have demonstrated the generation of quantum entanglement directly from sunlight. This research is significant because it challenges the long-held assumption that coherent laser light is essential for creating quantum states, instead successfully utilising incoherent sunlight via spontaneous parametric down-conversion to produce highly entangled photon pairs. The team’s system not only violates Bell’s inequality, confirming genuine entanglement, but also achieves generation rates comparable to those of conventional laser-based systems, potentially enabling sustainable quantum technologies for applications in energy-constrained settings such as deep-space exploration.

This achievement offers a pathway towards dramatically reducing the power consumption of future quantum technologies and opens doors for deployment in remote and challenging environments. Achieving this required overcoming a fundamental hurdle: the inherent randomness of natural light. Researchers successfully produced entangled photon pairs through spontaneous parametric down-conversion, utilising sunlight as the energy source. The resulting entangled states exhibit a concurrence of 0.905 ±0.053 and a Bell state fidelity of 0.939 ±0.027, demonstrating a high degree of quantum correlation. The system convincingly violates Bell’s inequality, registering a value of 2.5408 ±0.2171, which definitively exceeds the classical limit of 2.

This violation confirms the genuinely quantum nature of the correlation and rules out explanations based on classical physics. Maintaining comparable generation rates to laser-based systems further underscores the practicality of this approach. At the heart of this advance lies a novel optical setup. Sunlight was first concentrated and guided into a specialised interferometer, a device that splits and recombines light beams to reveal their quantum properties.

Inside, a nonlinear crystal converted the sunlight into pairs of entangled photons, and careful filtering and analysis confirmed the presence of strong quantum correlations. The polarization of these photon pairs revealed the entanglement. This breakthrough opens doors to sustainable quantum technologies, particularly for applications where energy resources are limited.

Unlike conventional quantum systems requiring substantial power for lasers or cryogenic cooling, this sunlight-driven approach offers a pathway toward self-powered quantum devices. Beyond terrestrial applications, the research holds immense promise for interplanetary missions and space-based quantum communication networks, suggesting a future where quantum technologies can operate autonomously, powered by the sun’s abundant energy.

High-fidelity entanglement and Bell inequality violation using sunlight-generated photons

A concurrence of 0.905 ±0.053 and a purity of 0.919 ±0.045 were achieved in the two-photon state generated via sunlight-driven spontaneous parametric down-conversion, demonstrating a high degree of entanglement and signal quality. The fidelity to the target Bell state reached 0.939 ±0.027, confirming the close proximity of the generated state to a maximally entangled configuration.

Bell’s inequality was violated with a value of 2.5408 ±0.2171, exceeding the classical threshold of 2 by 2.94 standard deviations. This result definitively proves the non-classical correlation between the photon pairs and rules out explanations based on local hidden variable theories. The observed violation demonstrates that the entanglement quality approaches that of laser-pumped systems.

Generation rates of entangled photons from sunlight-pumped SPDC reached approximately 1600s−1 per milliwatt of pump power, comparable to laser-pumped SPDC when normalized against the effective phase-matching bandwidth. Wavefront distortions from optical components, rather than inherent solar incoherence, account for the non-maximal entanglement and purity observed.

The sunlight concentration module employed a 1m × 1.4m Fresnel lens, coupled with colour filters and a short-pass filter, to isolate the desired spectral bandwidth and direct it into a 50m core multi-mode fibre with a numerical aperture of 0.22. A 20× microscope objective with a 0.4 NA then collimated the sunlight, followed by spectral filtering using a 1.5nm band-pass filter centred at 405nm. A 10-mm long periodically poled potassium titanyl phosphate crystal, phase-matched for Type-II SPDC, served as the nonlinear medium, positioned to ensure indistinguishable SPDC processes.

Ultraviolet spectral refinement and fibre optic beam delivery

A sunlight concentration module initiates the process, capturing ambient light and directing it towards a nonlinear medium. Sunlight passes through a series of filters, films blocking warm-coloured spectral components, a short-pass filter, and a dichroic mirror, to refine the optical power within the ultraviolet band. This filtered light enters a multimode fibre with a 50m core diameter and 0.22 numerical aperture using a custom glass conic concentrator.

This initial concentration prepares the light for efficient nonlinear interaction. Subsequently, the sunlight transmitted through the fibre is collimated by a 20× microscope objective (0.4 NA) before spectral filtering with a short-pass filter (cutoff wavelength of 550nm) and a band-pass filter (centre wavelength of 405nm, 1.5nm bandwidth). Polarization control is achieved using a polarizing beam splitter and a half-wave plate, ensuring equal amplitudes for horizontal and vertical polarization components, a requirement for generating high polarization entanglement.

A quarter-wave plate pre-compensates for potential phase delays between orthogonal polarization components, refining the polarization state. The polarized pump beam is focused into a 10-mm-long periodically poled potassium titanyl phosphate crystal, quasi-phase-matched for Type-II spontaneous parametric down-conversion from 405nm to 810nm. Placing the crystal at the centre of the polarization-sensitive interference ensures indistinguishability of SPDC processes, producing a polarization-entangled two-photon state.

Down-converted photons are then collimated by 250mm focal length lenses and analysed using a combination of quarter-wave plates, half-wave plates, and polarizing beam splitters to characterise their polarization. Avalanche photodiodes detect these photons, coupled via microscope objectives and 200m core diameter multimode fibres, while a time-to-digital converter with 81ps resolution records arrival times.

Sunlight harnesses quantum entanglement for potential low-energy communications

Scientists have long sought ways to shrink the energy footprint of data transmission, a challenge now addressed with an unexpected light source. For decades, the prevailing wisdom held that dependable quantum states demanded the precision of laser light, but a new demonstration reveals that even the diffuse glow of sunlight can generate entangled photons, the building blocks of quantum communication, with surprising efficiency.

Achieving this required overcoming a fundamental hurdle: the inherent randomness of natural light. Previous attempts to use sunlight faltered because maintaining the delicate quantum link between photons proved difficult amidst the noise. Now, researchers have shown that spontaneous parametric down-conversion works effectively even when driven by sunlight.

The resulting system not only violates a key test of quantum mechanics, confirming genuine entanglement, but does so at rates comparable to conventional laser systems. The implications extend far beyond the physics laboratory, offering possibilities for remote sensing or communication in environments where power is scarce, such as deep space probes or disaster zones.

Relying on sunlight sidesteps the need for bulky, energy-intensive lasers and their associated cooling systems. Interplanetary missions once faced limitations due to power demands for on-board communications; this approach could dramatically reduce those constraints. However, the current setup is tied to weather conditions and time of day, with peak efficiency occurring around solar noon on clear days.

Further work must focus on improving the sunlight collection and spectral filtering processes to maximise the conversion rate. Scaling up the system, creating multiple entangled photon sources, presents a significant engineering challenge. Despite these hurdles, this work opens a new avenue for sustainable quantum technologies, prompting a broader re-evaluation of what constitutes a viable light source for the future.