Quantum Communication: Fiber vs. Satellite
For general data and voice communications, fiber won the fiber versus satellite war back in the 1980s. For quantum, communications to be scaled globally it looks like both fiber and satellite will be needed. Both can transmit qubits, but for now in the world of the “Quantum Internet”, fiber and satellites serve two different markets and represent two different opportunities.
Fiber, QKD and Distance
Just as they did in the early days of digital fiber optics, optical losses set a restriction on the distance at which fiber QKD can work effectively. For now, the record is 421 km, which was set by ID Quantique in November 2018. Simulations show that fiber-transmitted QKD at greater than 500km will make the results of little practical use.
Today, most QKD devices are optimized to show high performance at distances up to about 50km and a wide range of companies and academic groups have built or are building their own QKD devices for this distance. These include ID Quantique, Toshiba, MagiQ Technologies, the University of Cambridge and the University of Montreal. These are all initially focused on the metropolitan environment, where data must be secured. Examples of metropolitan networks where QKD might be used include connecting bank branches in a city or secure locations at a military base.
Metropolitan QKD is improving. For example, the standard rate has upgraded to about 100 Kbps, with QKD devices providing 1 Mbps at 50 km appearing in the next couple of years. The restriction on distance will eventually be overcome by using quantum repeaters. However, these are still far from being commercially available. Experiments are showing a lot of promise for progress in the next 3-5 years.
Until quantum repeaters become a commercial reality, long haul QKD will be satellite based. The dominant scientific reality here is that even in the best-case scenarios fiber has losses around 0.2dB/km, while the atmosphere is practically transparent to photons. This is the main benefit of establishing a free-space satellite connection. Groups involved in the development of satellite QKD include the Institute for Quantum Computing and the University of Vienna, with the Chinese Academy of Science apparently ahead of the game.
China’s interest in QKD — and the government support it has garnered — reflects both Chinese industrial policy (creating a quantum technology industry and a domestic market) and military policy. A few years back, the Chinese satellite Micius, paired with ground-based fiber lines, and using QKD enabled a videoconference between Beijing and Vienna, located 7500 km apart. Situated on a 500km orbit, the satellite generated keys at rates close to 1kbps with the ground stations when passing near them, and then acting as a trusted relay to host the conference.
All that said, satellite QKD does come with its disadvantages. Secure keys can be generated only during uninterrupted ground-to-sky contact, limiting the amount of time during the day when a satellite can be active. This is somewhat resolvable by launching satellites at higher orbits, increasing their maximum area of coverage. Sunlight also has to be taken into account since single-photon detectors are very sensitive to noise and operate best at night.
In addition, it is not yet obvious how to guarantee that the satellite is itself a secure system – obviously necessary for the concept of QKD networks. Despite these drawbacks, satellite QKD for now looks to be a promising technology for safe communication between cities and countries, when a free-space connection can be established.
To learn more about the potential of fiber and satellite QKD, 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, please note that Inside Quantum Technology will be publishing a report on QKD Markets in March.