One of the most important applications of quantum mechanics is the ability to make secure communications.
Quantum computers will be able to break the security of current communication systems. Quantum communication, by contrast, offers an unconditional security that can be secured against quantum computers. With the development of quantum technology-based systems, now is a right time to develop a global quantum communication network. A global quantum communication network, or so-called quantum Internet, enables a variety of tasks, including quantum teleportation, quantum clock synchronization, distributed quantum computing, and distributed quantum metrology and sensing.
The goal of quantum internet is to have a quantum-connected world, similar to today's internet, where users around the world can share quantum information for a variety of tasks. But the important question is, what is the best way to achieve the quantum internet?
The main feature of the quantum network is the quantum entanglement. But this feature is very fragile and difficult to maintain. In fact, the smallest interaction of the system with the environment can break the entanglement. This is exactly what happens when physicists transmit entangled photons directly into the atmosphere or through optical fibers. Photons interact with other atoms in the atmosphere or optical fiber glass, thereby eliminating the entanglement. The maximum distance that entanglement can be shared with this method is a few hundred kilometers.
So, to build a global quantum communication network or quantum internet, we have to solve this problem. One option is to use quantum repeaters. Another option is to create entangled pairs of photons in space and send them to different stations on Earth. These stations are then entangled and messages can be transmitting between them with complete security. Photons in this scheme can travel more distances.
In a new paper, researchers have investigated the possibility of using satellites for a global quantum communication network. Their network architecture consists of a constellation of satellites in polar orbits around Earth that transmit pairs of entangled photons to ground stations. They estimated the number of satellites needed to maintain high-speed 24-hour coverage, taking into account the cost of the satellites as well as satellite-to-ground transmission loss. They proposed a constellation of at least 400 satellites about 3,000 kilometers high.
The scientists compared the rates of two ground stations with satellites and quantum repeaters and found that satellite-based schemes had much higher rates. Therefore, satellite-based network schemes are more appropriate for realizing a quantum internet, given the increasing miniaturization and fidelity of entanglement distribution sources and the decreasing cost and miniaturization of satellites.
In order to realize quantum internet, networks must be built based on local and global quantum entanglement. In this network, the major cities on different continents act as hubs that connect the local network users of one city to those of another. Hybrid networks using space-based quantum communication platforms with ground-based quantum repeaters will realize this idea.