Quantum networks: prospects and difficulties of implementation

    According to German researchers from the Max Planck Society , the global quantum network will be implemented in the next few years. We will tell you what difficulties are there. / The Flickr / Officers Mike Seyfang / CC

    What are quantum networks?

    A quantum network is a data transmission system that works according to the laws of quantum mechanics. In such networks, data is exchanged using qubits. These are polarized photons transmitted via an optical communication channel. In order to deploy global quantum networks covering the entire planet, like the Internet, developers and researchers have to solve a number of difficulties. For example, a certain difficulty is the transmission of photons over long distances because of their "fragility". We will talk more about this and other problems later on, but first, let's talk about why quantum networks should be created at all.

    How can they be useful

    The phenomenon of quantum entanglement links quantum particles in such a way that when measuring the characteristics of one of them, we automatically recognize the characteristics of the second. Moreover, this connection persists even at large distances.

    If you establish a connection between two points, you can generate sequences of random numbers at its two ends. In cryptography, this feature is used to generate encryption keys.

    Another advantage of quantum networks is the inability to read broadcast photons twice. The laws of quantum mechanics prohibit the "cloning" of the state of light particles. When interceptedqubit, it changes its meaning. It turns out that when trying to “eavesdrop” on a data transfer channel, attackers will not be able to extract any valuable information. At the output, they get a random set of numbers.

    Thus, quantum networks are almost absolute cryptographic protection. Almost absolute, as scientists from Sweden proved that it is still possible to “eavesdrop” on such a network . To do this, simulate a quantum cipher. Photon detectors ignore unpolarized particles of light called zeros. If you simulate these zeros at a certain point in time and send them to the receiver, then he will consider the signal quantum (although this is not so).

    You can solve the problem, but you have to change the principles of the receivers. One option is to add an indicator of signal strength (since it will change with external intervention). But this will increase the cost of scanning quantum networks.

    Why is it difficult

    The “fragility” of qubits, which makes quantum communication reliable, also introduces disadvantages. Single photons change their states or are simply absorbed by the medium due to interference. For this reason, it can be difficult to transmit a quantum over a fiber optic cable over a distance of over 100 km.

    / Flickr / Alexandre Delbos / CC

    Fiber-optic quantum networks are now being built using repeaters. They decode the information, encode it again and transmit it to other nodes in a chain. However, in this way, intermediaries also find out the content of the message, which can lead to a leak if one of them is compromised. There is a problem with the cost - such repeaters use expensive magnets and rare minerals .

    It is important to consider the environment in which these networks will be deployed. There is a significant difference between laboratory and “combat” conditions. In the city, fiber optic cables are affected by temperature changes. This can lead to phase shifts of the photon and cause errors in data transmission.

    To solve the problem with transmission over long distances will allow quantum teleportation. Researchers can optionally introduce two qubits into a state of quantum entanglement. A project from the Delft University of Technology in the Netherlands is engaged in such a project . Researchers are building a ten-kilometer quantum network between the city of Delft and The Hague.

    Such technologies are still in the early stages of development. The fact is that maintaining “connectedness” for a long time is difficult because of the destructive effect (called decoherence ) that the environment has on quanta. It is possible to maintain the state of quantum entanglement for a split second.

    Where can I use quantum networks

    As we have said, quantum networks are highly resistant to wiretapping. Therefore, they allow you to build reliable cryptographic key distribution systems. Such technologies already exist. For example, at the beginning of the year, China launched a cryptographic key distribution system in which data is transmitted via satellites and laser beams. A similar system was proposed by German researchers.

    Also, quantum networks must integrate quantum computers into networks. Clusters of quantum machines are expected to accelerate physical and chemical simulations, for example, when developing new drugs.

    There are also cases outside science, such as voting. Such a project was implemented in Switzerland - a few years ago CERNhelped organize a quantum network for elections. According to experts from the Harvard-Smithsonian Center for Astrophysics, in addition to reliability, quantum networks make it possible to implement new strategic voting schemes that are not available today. For example, people will be able to choose not one candidate, but two at once (the second option).

    The development of quantum networks involves many institutions and organizations. Therefore, more and more such projects have recently appeared. We will tell about foreign and Russian developments in this area in our next materials on Habré.

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