Some of the most advanced communication systems currently under development rely on the properties of quantum science to store and transport information. However, researchers who design quantum communication systems that use light rather than electric power to transmit information face a dilemma: the optical components that store and process quantum information usually require photons of visible light (particles of light). light) to work. However, only photons close to the infrared (whose wavelengths are about 10 times longer) can carry this information over miles of optical fibers.
Researchers from the National Institute of Standardization and Technology (NIST) have come up with a new way to solve this problem. For the first time, the team created quantum correlation pairs composed of a visible photon and a near infrared photon, using optical chips-based components that can be series products. These pairs of photons combine the best of both worlds: visible light partners can interact with atoms, ions or other trapped systems that serve as a quantum version of a computer's memory, while the near infrared members of each couple are free to spread over long distances. through the optical fiber.
This achievement promises to enhance the ability of light-based circuits to securely transmit information to distant destinations. NIST researchers Xiyuan Lu, Kartik Srinivasan and their colleagues at the University of Maryland, NanoCenter at College Park, have demonstrated quantum correlation, known as entanglement, using a specific pair of photons in visible light and near infrared. However, the design methods of researchers can be easily applied to create many other pairs of visible / near infrared light suitable for specific systems of interest. In addition, the miniature optical components that created the entanglements are manufactured in large numbers.
Lu, Srinivasan and their colleagues recently described their work in Physical Nature.
One of the most counterintuitive properties of quantum mechanics, quantum entanglement occurs when two or more photons or other particles are prepared in a way that makes them so intrinsically linked that they behave as a single unit . A measurement that determines the quantum state of one of the entangled particles automatically determines the state of the other, even if the two particles lie on either side of it. # 39; universe. Entanglement is at the heart of many quantum information systems, including quantum computing and encryption.
In many situations, the two entangled photons have similar wavelengths or colors. But NIST researchers have deliberately sought to create strange couples: an entanglement of photons whose colors are very different.
"We wanted to connect visible light photons, which are good for storing information in atomic systems, and telecommunication photons, which are in the near infrared and traveling well in optical fibers with low signal loss," Srinivasan said.
In order for photons to interact with most quantum information storage systems, the team also needed the light to reach a specific peak at a particular wavelength rather than have a wider and more diffuse distribution .
To create the entangled pairs, the team built a specially designed optical "whisper gallery" – a nanometer-sized silicon nitride resonator orienting the light around a tiny running track, similar to the way the sound waves move unhindered around a curving wall like the Cathedral Dome Saint Paul in London. In such curved structures, called acoustic whispering galleries, a person standing near a part of the wall easily hears a low noise coming from any other part of the wall.
When a selected wavelength of laser light was directed into the resonator, entangled pairs of photons in visible and near infrared light emerged. (The specific type of entanglement used in the experiment, called energy-time entanglement, relates the energy of the photon pairs at the moment they are generated.)
"We imagined how to design these whispering gallery resonators to produce a lot of the pairs we wanted, with very little background noise and other stray light," Lu said. The researchers confirmed that the # Tangle persisted even after the telecommunication photons had traveled several kilometers of fiber optics.
In the future, by combining two of the entangled pairs with two quantum memories, the entanglement inherent in the photon pairs can be transferred to quantum memories. This technique, known as entanglement exchange, allows memories to entangle each other over a much longer distance than would normally be possible.
"Our contribution was to determine how to create a quantum light source with adequate properties, which could allow such entanglement at great distances," Srinivasan said.
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Xiyuan Lu et al., Pair of tangled photons in the visible and the telecom integrated into the chip for quantum communication, Physical Nature (2019). DOI: 10.1038 / s41567-018-0394-3