Natural Sunlight Could Power Quantum Entanglement, Replacing Traditional Lasers
In a first-of-its-kind experiment, researchers have shown that sunlight can produce quantum-entangled photon pairs. The proof-of-principle finding upends the traditional view that lasers are the sole viable source of entanglement.
In photonic quantum technologies, the high optical coherence of lasers is viewed as essential to generating quantum correlations through spontaneous parametric down-conversion (SPDC). Natural sunlight is incoherent. Also, lasers are seen as the only light sources capable of delivering the optical power densities needed to drive nonlinear optical processes efficiently. Sunlight is much less intense than laser light.
Despite the perceived drawbacks of solar light, researchers from the Max Planck Institute for the Science of Light (MPL), Max Planck Center for Extreme and Quantum Photonics (MPC), and the University of Ottawa developed an approach to using sunlight to produce quantum-entangled photon states by means of nonlinear optical effects.
Hanieh Fattahi, Group Leader at the Max Planck Institute for the Science of Light (MPL)
Using SPDC pumped by sunlight, the researchers detected polarization-entangled photon pairs that violated Bell’s inequality, confirming quantum behavior. The results could help clear a path to sustainable, accessible photonic quantum technologies, especially for resource-constrained areas.
Contrary to conventional presumptions regarding coherence and entanglement, the researchers recognized that entangled photons can be generated from incoherent sources, because light can behave coherently in one degree of freedom while remaining disordered in another.
For example, a beam can be fully polarized, yet incoherent in space and time. “As long as the pump beam is perfectly polarized, its spatial or temporal incoherence should not preclude the generation of polarization entanglement,” researcher Cheng Li said.
When generating quantum entanglement through SPDC, the pump’s incoherence in one degree of freedom only limits entanglement in that same degree of freedom. As an example, a type of spatial entanglement known as position-momentum entanglement will vanish when the pump beam loses its spatial coherence.
However, this constraint does not apply when the target entanglement is in a degree of freedom that is different from the incoherence of the pump. “The trick to harnessing sunlight is to keep different degrees of freedom of light from influencing each other during the process,” Li said. “This means that sunlight is perfectly capable of generating entangled photons, as long as one can concentrate enough sunlight into a nonlinear crystal to induce SPDC.”
To direct sunlight into a clear aperture of a nonlinear optical crystal, which is usually only a few millimeters (mm) wide, a team led by Hanieh Fattahi built a sunlight concentration system. This system collects light over 1.4 square meters (m2) and funnels it down into a µm-thick fiber.
The device is cone-shaped and made of glass. The researchers place the base of the cone at the focal spot of a large Fresnel lens, which is mounted on a solar-tracking motor. As the focused sunlight propagates toward the tip of the cone, it becomes more concentrated through total internal reflections. At the tip of the cone, the sunlight is coupled into a multimode fiber. The fiber guides the concentrated sunlight into a nonlinear crystal to drive entanglement generation via SPDC.
Measurements taken with quantum state tomography showed that photon pairs generated using sunlight-pumped SPDC had a fidelity of nearly 94%, which is close to the performance of laser-pumped SPDC. In addition to demonstrating a very high degree of entanglement, the sunlight-pumped SPDC photon pairs displayed correlations that could violate Bell’s inequality, indicating that the correlations cannot be modeled using classical physics and are hallmarks of quantum entanglement.
The team further observed that when the photon production rate was normalized against the pump power and effective bandwidth of the nonlinear process, the efficiency of sunlight-driven entanglement generation was on par with laser-driven entanglement. This finding suggests that lasers may not have as many advantages over sunlight as previously thought, and that practical, sunlight-driven quantum light sources can be achieved by optimizing sunlight collection and bandwidth use. Sunlight’s broad spectrum could offer access to entangled photons across a wide range of wavelengths, especially in areas where lasers are unavailable.
“Sunlight is an abundant and reliable resource in many environments, especially in space,” Fattahi said. “Being able to generate quantum-entangled photons directly from sunlight could enable simpler and more resilient quantum systems for satellites and future deep-space missions.”
Harnessing sunlight to directly pump the SPDC process, instead of using a local laser or LED, could significantly reduce the need for a local energy supply by removing the electrical-to-optical power conversion process and associated waste heat management.
The temperature control of the nonlinear crystal is electrical, and the tracking system of the sunlight collector uses electrical motors. However, it offers the future possibility of superior overall power efficiency compared with laser-based quantum light sources, and shows that an entangled-photon source can be achieved with less complexity, and thus fewer points of potential failure.
A less complex means to access quantum entanglement would be particularly useful in strategically important but remote environments like space and the Arctic region. For example, a spacecraft in a Sun-synchronous orbit could have nearly uninterrupted access to its solar pump source.
“The best part of this research is that it is only a beginning,” professor Robert Boyd said. “In addition to SPDC, there are many other nonlinear optical approaches to generate entangled photons — four-wave mixing is one good example.
“For each of these nonlinear interactions, there are ways to make it more efficient. We believe this work can inspire much new research in nonlinear and quantum optics, and these researches may, in turn, make sunlight-driven quantum technology more practical.”
Source: photonics
