Quantum teleportation has been demonstrated over a fiber optic cable carrying classical traffic (like the Internet) for the first time. The work suggests it will be possible to take advantage of the benefits quantum communication offers without needing to build a whole new infrastructure in parallel to what already exists.
Quantum entanglement, famously derided by Einstein as “spooky action at a distance”, allows changes to one entangled particle to induce matching alternations in another, irrespective of their separation. This means information can be conveyed between two points without needing to travel between – in other words, information teleportation.
That doesn’t mean we can do without a transmission network, however. The entangled particles start off together and need to travel between the location of the sender and receiver. If the particles used are photons, that can be done using optical fiber cables, like those that carry the bulk of the Internet. However, previous demonstrations of quantum communication have been conducted in peace and quiet, rather than having the photons travel along optical superhighways bustling with unrelated messages.
This is what Professor Prem Kumar of Northwestern University and his team have changed. They noted previous evidence that any entangled transmissions at wavelengths close to those heavily used for ordinary Internet traffic would be easily disrupted. However, by using a wavelength far from any traffic, the delicate entanglement might survive unaffected by what else was going on.
After selecting 1290 nanometers for their quantum wavelength, Kumar and colleagues entangled photons and transmitted them over a 30.2-kilometer (18.8-mile) optic fiber, which was also used to carry 400 Gbps Internet traffic in the popular C-band (1547 nanometers). They then disrupted the photons at one end, and looked for matching changes at the other, to see if the entanglement was intact.
“We carefully studied how light is scattered and placed our photons at a judicial point where that scattering mechanism is minimized,” Kumar said in a statement. “We found we could perform quantum communication without interference from the classical channels that are simultaneously present.”
“This ability to send information without direct transmission opens the door for even more advanced quantum applications being performed without dedicated fiber,” said first author, PhD student Jordan Thomas.
Besides the choice of wavelength, the work required other methods to turn down noise, such as filters at the receivers that excluded unentangled photons that might interfere with the results.
The volume of information transmitted in this case, and the distance over which it was sent, is too small to be of practical use. In fact, the sender and receiver were on the same campus, with the fiber spooled rather than connecting sites 30 kilometers apart.
However, if the proof of principle can be scaled up, the technique would allow information to be transmitted without the risk of eavesdropping, as well as the networking of quantum computers.
One of the more advanced techniques the team hopes to demonstrate is “entanglement swapping” where photons that were previously independent at either end of the cable become entangled.
“Many people have long assumed that nobody would build specialized infrastructure to send particles of light,” Kumar said. “If we choose the wavelengths properly, we won’t have to build new infrastructure. Classical communications and quantum communications can coexist.”
The work is published open access in the journal Optica.
