The Quantum Internet Will Require Quantum Repeaters
The record distance for QKD with direct transmission is 421km. This result, produced by the team over at ID Quantique in Switzerland, impressive as it may be, is already close to the possible limit that can be achieved with direct qubit transmission. The setup included state of the art single-photon detectors with low noise rates, and the usage of ultra-low loss optical fiber. Even in an ideal scenario, where the detectors noise rate is assumed to be zero, the generated key bit-rate will eventually fall off to several kilobits per day, making its practical usage negligible. In its own study, ID Quantique provides the result of a close to ideal simulation at a distance of 600km, which shows a key rate of 2.2 kb per day.
Overcoming these limitations is crucial to extend QKD networks beyond metropolitan areas. Utilizing quantum repeaters in the communication line becomes necessary. Amplifying a quantum signal in the manner used traditionally in quantum networks isn’t a possibility here due to the no-cloning theorem. Given that many of the viable applications for quantum networks will certainly be long haul, we think that there will be a growing opportunity to create commercial quantum repeaters in the not too distant future.
Quantum Repeater Evolution and Challenges
Quantum repeaters base their technology on distributing pairs of entangled particles to the sender and the receiver. Quantum repeaters can be concatenated, increasing the maximum distance for entangled particles distribution farther still. However, quantum repeaters are a new area of development in quantum communication, with most devices currently being used in labs for different proof-of-concept experiments. There is no clear vision on what specific technology to use, and despite the relative simplicity of the physics behind quantum repeaters, they are still far away from being commercially available. Some of the companies and academic groups that are currently involved in research work on quantum repeaters include the University of Toronto, Nippon Telegraph and Telephone Corporation, Max Planck Institute of Quantum Optics and ID Quantique.
Currently, one of the main obstacles to developing quantum repeaters is the lack of availability of high-performing quantum memories. Such memories would play a major role in the ability to synchronize devices in the network, storing and retrieving qubits on demand. Achieving a balance between high-frequency rates, decent coherence times, and storing not one but many qubits still lies in the future. Delay-line fiber at a fixed length has been helpful for proof-of-concept experiments but will not be suited for commercial products.
As a temporary workaround, so-called “trusted repeaters” can be used. SK Telecom has successfully tested its trusted repeater at a distance of 112km, with plans to connect Seoul to Busan, covering 460km by connecting five repeaters together. These repeaters are based on performing QKD as many times as there are nodes in the channel. After that, the sender can transfer his main key, which is encrypted and deciphered along the way with the auxiliary keys. Of course, all the nodes know exactly what key is being transferred, adding the restriction that they should be “trusted”.
To learn more about quantum repeaters, quantum memory and quantum technology in general, visit the Inside Quantum Technology Conference, which will be held at the Hynes Convention Center, Boston, March 19-21. Also note that Inside Quantum Technology will be publishing a report on QKD Markets in March.