One of the current issues with Quantum Key Distribution (QKD) is the limitation of how far a quantum signal can be carried over a lossy fiber optic cable before it becomes unusable. With classical optical networking, this problem is solved using classical repeater technology that measures an incoming optical stream and then regenerates it to send it on to the next link in the chain. However, the approach used in these classical repeaters won’t work with a quantum signal because of the No-cloning theorem.
Nonetheless, people are testing out QKD networks in what we call metropolitan networks that cover relatively short distances so that repeaters are not needed. Researchers are working on developing quantum repeaters, but at this point no one has a device that is ready to be implemented in a long distance network. Still others are working on technical approaches that can make these metropolitan networks as long as possible without using any repeaters. And Terra Quantum has just demonstrated one of the longest distances we have seen of 1707 kilometers with their Quantum-Protected Control-Based Key Distribution (QCKD) protocol that utilizes physical control over signal losses.
Like any optical network, the bit rate will drop with distance. So although they were able transmit a QKD key over a 1707 kilometer line, the bit rate drop dramatically to 0.9 bits per second. To put this another way, at 0.9 bps, it would take about 284 seconds (or about 4.75 minutes) to transmit a 256 bit key that could be used in AES-256. But nonetheless, this is still a considerable achievement.
To read more about this experiment and see how Terra Quantum was able to achieve this, we can refer you to a technical article that has been posted here on the MDPI website.
May 25, 2024
