Storing Quantum Information

In the science of quantum communication, the challenge has always been prolonging the entangled state that the particles are in. As quantum information is carried by these entangled particles, the length of time the entanglement is sustained affects the distance that the information can travel.

Quantum communication systems do this using direct optical-fiber connections, which are rather limited because the way that fibers absorb light can disrupt the entanglement needed to carry quantum information.

Building a quantum internet, which is essentially a network of quantum entangled routers linked by fiber that can store quantum information, requires a function of routers that can store and send entangled particles. A team of researchers from the University of Vienna in Austria, led by Ralf Riedinger, supposedly built such a router.

This device is a nanomachine capable of receiving and storing quantum information sent through ordinary fiber optic cables. It contains a pair of nanofabricated silicon resonators that use electron-beam lithography and plasma reactive-ion etching, which are tiny silicon beams that vibrate like a guitar string.

The inner workings of this nanomachine. Image Credit: Ralf Riedinger

In order for the machine to store quantum information, the silicon beams needed to vibrate at a precise frequency. Riedinger's team arrived at the exact frequency, which is 5.1 gigahertz (or a wavelength of about 1,553.8 nanometers), after fabricating around 500 of these silicon resonators and testing each chip to find identical pairs. “We find a total of 5 pairs fulfilling this requirement within 234 devices tested per chip,” the researchers wrote in the paper published online. Both chips were placed in a fridge, while they remained connected to each other by 70 meters of optical cable fiber, covering a distance of 20 centimeters. The two resonators were then successfully entangled. "We create and demonstrate entanglement between two nanomechanical devices across two chips that are separated by 20 cm.," the researchers wrote.

Making the Quantum Internet Real

In their proof-of-principle experiment, the researchers first cooled the resonators to almost absolute zero to keep them in a quantum ground state. To generate entanglement, they then connected the resonators by a fiber-optic cable that contained photons at the identified resonance frequency.

Although their tests were only done over 20 centimeters, this setup could be significantly expanded. "We do not see any additional restrictions to extend this to several kilometers and beyond,” Riedinger's team wrote. “The system presented here is directly scalable to more devices and could be integrated into a real quantum network."

In short, what they've built is essentially a working quantum router — a device that could be crucial in realizing a quantum internet. Even better, it could be modified to carry information over microwave frequencies, according to the MIT Technology Review, and therefore be connected with quantum computers that operate on these frequencies.

“Combining our results with optomechanical devices capable of transferring quantum information from the optical to the microwave domain could provide a backbone for a future quantum internet using superconducting quantum computers," Riedinger and his colleagues wrote.

Just like how quantum computers would change our problem-solving abilities, a quantum internet is expected to completely revolutionize communication. This is partially because it promises to be more secure, thanks to quantum cryptography that renders messages potentially un-hackable. Experts believe that we're only a decade away from realizing a working, secure quantum network.

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