Researchers Entangle Photons Of Different Colors In Quantum Computing Breakthrough

Light from a pump laser (purple wave) generates one visible-light wavelength photon (red patches in and around resonator) and one infrared photon (blue patches).  S. Kelley/NIST

Applying the laws of quantum mechanics to computing has the potential to solve problems we wouldn’t even know how to tackle with current devices, but before we can get there, there are several hurdles to overcome. Now, researchers seem to have found a solution to a particularly thorny issue.

Getting information out of quantum systems is usually done with visible light photons. Telecommunication via fiber optics uses infrared photons. The question is how to connect the two without losing important information. The team was able to construct a chip-based optical device that, for the first time, can entangle photons of different wavelengths. The findings are published in Nature Physics.

Entanglement is a curious quantum mechanical property where two (or more) particles can influence each other's properties instantaneously, even over a great distance. An entangled quantum system should be considered as a whole, even though its individual particles might travel a long away. This property is sought after in quantum systems. The entangled information is ideally carried over a long distance, although entangled states are often very fragile.

That’s why this new device is important. Near-infrared photons are very good at traveling via optical fiber without losing much signal. The team focused on creating visible-light/near-infrared pairs to get the best of both worlds.

The device is known as a whispering gallery resonator. Whispering galleries are the walkways found in many domes (such as St Peter’s Basilica in Rome or St Paul's Cathedral in London), where it’s possible to hear conversations from the other side of the dome as the sound waves propagate on a curved surface. The resonator is also curved and given a specific input laser light that produces a pair of optical and infrared photons.

"We figured out how to engineer these whispering gallery resonators to produce large numbers of the pairs we wanted, with very little background noise and other extraneous light," lead author Xiyuan Lu, from the National Institute of Standards and Technology, said in a statement.

The entanglement persisted even after traveling through several kilometers of optical fiber, which is a great success for the approach. The team suggests that future endeavors could focus on creating fiber optics/quantum memory relay systems, which would extend the distance that entangled photons can cover. The record is currently at 404 kilometers (251 miles), using non-commercial, high-quality optical fiber.


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