In recent years, quantum computing has taken quantum leaps in practicality, scalability, and raw computing power. However, replacing the world’s internet infrastructure with an entirely new system would likely take the best part of a century – after all, many parts of the world don’t even have access to the current internet.
One of the best ways scientists could make the quantum internet scalable would be to utilize the current infrastructure to transmit information.
Now, a research laboratory in Illinois has demonstrated long-distance transmission of quantum information using just existing fiber optic cables, pushing forward the dream of a scalable quantum internet.
“To have two national labs that are 50 kilometers apart, working on quantum networks with this shared range of technical capability and expertise, is not a trivial thing,” said Panagiotis Spentzouris, head of the Quantum Science Program at Fermilab, in a statement.
“You need a diverse team to attack this very difficult and complex problem.”
The experiment involved transporting quantum-encoded photons across a large distance while maintaining a high level of synchronization between them – in human words, particles that have been modified to carry information are transported through a network while both ends of the line are working in harmony.
Synchronization is the real difficulty here. Computers must be synchronized across a network for a number of security and functional reasons, but this cannot rely on a standard clock. If you checked your watch and your friend checked theirs, even if you intentionally set them to almost identical times, they would still differ slightly by fractions of a second. For classical computing this simply won’t do, so synchronization relies on Network Time Protocol (NTP), which synchronizes all participating computers within milliseconds of one another.
However, quantum computers are even pickier and require even smaller margins of error, so researchers must get creative to achieve synchronicity. The researchers sent a clock down the same optical fibres they were sending the quantum-encoded photons, just on different wavelengths to avoid interference, though this is no easy feat.
“Choosing appropriate wavelengths for the quantum and classical synchronization signals is very important for minimizing interference that will affect the quantum information,” said Rajkumar Kettimuthu, an Argonne computer scientist and project team member.
“One analogy could be that the fiber is a road, and wavelengths are lanes. The photon is a cyclist, and the clock is a truck. If we are not careful, the truck can cross into the bike lane. So, we performed a large number of experiments to make sure the truck stayed in its lane.”
They succeeded, with just a 5-picosecond difference between the clocks at each location. The researchers had managed to send quantum information across a long-distance network, using just current infrastructure, with incredible precision.
“This is the first demonstration in real conditions to use real optical fiber to achieve this type of superior synchronization accuracy and the ability to coexist with quantum information,” Spentzouris said.
“This record performance is an essential step on the path to building practical multinode quantum networks.”