Scientists have shown that sunlight can be used to create quantum entanglement, a surprising advance that could eventually help make quantum technologies more energy efficient.
Quantum entanglement is a strange phenomenon in which two particles become connected so strongly that their properties remain linked, even when they are separated.
It is an important part of emerging technologies including quantum computers, highly sensitive sensors and ultra-secure communication systems.
Today, researchers typically use powerful lasers to produce entangled particles. But lasers require energy and additional equipment, raising concerns about the resources needed if quantum technologies eventually become widespread.
Researchers from the University of Ottawa in Canada and the Max Planck Institute for the Science of Light in Germany have now demonstrated another possibility: using freely available sunlight.
Reporting their results in the journal Optica, the researchers showed that sunlight could generate high-quality entanglement between pairs of photons, the tiny particles that make up light.
After differences in the bandwidth of the incoming light were considered, the results were comparable to those achieved using lasers.
The achievement challenges a long-standing assumption about the type of light needed to produce entanglement.
Lasers generate highly organized, or “coherent,” light. Their light waves are closely synchronized and usually concentrated within a narrow range of colors. Sunlight is very different. It contains a broad range of colors and travels in many directions, making it highly incoherent.
Earlier research from the University of Ottawa team had already shown that incoherent light from an LED could generate entangled photons. The new experiment pushed the idea much further by using natural sunlight.
The researchers used a process called spontaneous parametric down-conversion. In this process, incoming photons interact with a special crystal and can split into pairs of lower-energy photons. Under the right conditions, these photon pairs become entangled.
Instead of shining a laser onto the crystal, the researchers used polarized sunlight. Although the sunlight remained disordered in its colors and directions of travel, its light waves were made to oscillate in the same general direction.
The team designed the experiment so that differences in color and travel direction did not interfere with the property they wanted to entangle: polarization.
There was another practical challenge. The sunlight had to be concentrated onto a nonlinear crystal only millimeters in size.
To accomplish this, the German team developed a cone-shaped, all-glass solar concentrator. Sunlight collected by a Fresnel lens roughly the size of a household window was concentrated into an optical fiber about as thin as a human hair and then directed onto the crystal.
Outdoor experiments showed that the resulting quantum state was about 94% similar to a perfectly entangled state. The photon correlations also violated Bell’s inequality, an important test showing that their behavior cannot be explained by ordinary classical physics.
The technology is still at an early stage, and researchers are now working to improve the brightness and quality of the entangled light.
In the future, however, sunlight-powered entanglement could have intriguing applications. Satellites might use sunlight already available in space to generate quantum encryption keys without carrying energy-hungry lasers. Similar approaches could potentially reduce the energy demands of larger quantum systems.
The experiment shows that one of nature’s most abundant energy sources could also become a useful resource for the strange world of quantum technology.
