Physicists Entangled Photons in the Lab With Photons From the Sun

The sun (left) and quantum dots (right).
Photo: NASA/SDO (AIA)/University of Rochester / J. Adam Fenster

In a classic physics experiment, scientists set up quantum entanglement between sunlight and light generated here on Earth.

The researchers in China, the United States, Germany, and the United Kingdom wondered whether any two particles of light, called photons, could show the spooky interactions governed by the rules of quantum mechanics, even if they originated from vastly distant sources. The experiment was mainly curiosity-driven, but it demonstrates that in the future, researchers might be able to use the Sun as a source of light for quantum mechanics-related purposes.

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As far as fundamental science goes, “it is just cool,” study author Chao-Yang Lu from University of Science and Technology of China told Gizmodo in an email. “It makes many researchers [say] ‘wow.’”

Quantum mechanics is the toolkit that scientists use to understand the smallest particles, like electrons and photons. It says that particles’ properties, like their energy, can only take on distinct values from a list. When you’re not looking at the particles, they can take on a superposition of these values, meaning multiple values at the same time, but they collapse into just one of those values upon measurement. Multiple particles can entangle, meaning when you measure the particles, the values are more correlated than they would be by chance alone. They can also interfere, meaning that quantum mechanics’ math makes certain combinations of values more likely than others.

The experiment the researchers used is one devised 30 years ago to show quantum effects between photons on Earth, based on something called the Hong–Ou–Mandel effect. In that experiment, photons enter a beam splitter from two sides. The beam splitter will either cause a photon to bounce off or to pass through. With two beams entering the beam splitter from either side, the possible results are both photons bouncing off and coming out on the same side they entered; both photons passing through and coming out on the side opposite from where they entered; or one photon bouncing off and the other passing through, in which case both photons will be on the same side.

But, according to the rules of quantum mechanics, there is no way to distinguish between identical photons. So, if identical photons enter on both sides, then the mathematics describing this experiment cancels out the cases where the photons end up on opposite sides, and the experimenters will measure beams coming out of the same side of the splitter most of the time.

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The researchers performed this same experiment‚ where one source of photons was sunlight collected with a telescope and fed into a fiber optic cable and through filters, and the other was a quantum dot, essentially an artificial atom designed to emit photons with identical properties. The test demonstrated that the Sun and the quantum dot photons mostly came out on the same side of the beam splitter. The researchers further ran a test (similar to the one described here) to confirm that the particles’ properties demonstrated the correlations indicative of entanglement, according to the paper slated to be published in Physical Review Letters.

Ultimately, it’s important for researchers to know that photons from disparate sources can entangle and interfere for various quantum technology applications, and the Sun is a very different than a photon source in a lab. But mainly, this is a neat experiment that hammers home the fact that according to the laws of physics, two particles with exactly the same properties are indistinguishable, regardless of their origin—even if their sources are 93 million miles apart.

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About the author

Ryan F. Mandelbaum

Science writer at Gizmodo | I like physics and eating