A technology that will bring quantum networks closer

    Physicists from the Universities of Toronto, Osaki, and Toyama presented the concept of a quantum follower that does not use quantum memory cells and is capable of operating at room temperature. In the future, it will allow to implement large-scale quantum networks.

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    / PxHere / the PD

    What is the difference of a new quantum repeater

    Information in quantum networks is encoded in photons. However, it is quite difficult to send them over long distances via fiber. More than 90% of particles are lost in the cable, the length of which exceeds 50 km. To increase the effective transmission distance, scientists and engineers are working on the creation of quantum repeaters. They help prevent loss when transmitting photons via fiber optic cable. However, existing devices use quantum memory cells, such as ion traps, which are stable only with strong cooling.

    Such solutions first decode the transmitted information, and then encode it again for transmission further down the chain. Such an approach creates a potential vulnerability for attackers who can intercept information on a compromised node.

    A team of physicists and engineers under the guidance of Professor Hoi-Kwong Lo managed to solve this problem. They demonstrated the possibility of implementing a photon repeater, which does not require "intermediate" transformations. Note that the researchers proposed the device concept in 2015, and in early 2019 they were able to prove it experimentally.

    In this case, the quantum memory is replaced by a graph-state (photonic graph state ). Two computers located at different ends of a fiber-optic connection generate quantum entanglement between their photons. After they send immediately many photons to the repeater. In the repeater, these particles are represented as a graph in which each qubit- this is the top. Then a Bell state measurement is performed on the photons . The result of the measurement is the design of the particles in an entangled state.

    Benefits and Challenges

    Optical repeaters are capable of transmitting photons over much longer distances — the sender and receiver can be located 800 kilometers from each other. In this case, repeaters remain operable at room temperature.

    To achieve such results in the network requires the use of highly sensitive optical detectors, the quantum efficiency of which will exceed 60%. Of the existing devices, few are able to produce such indicators, and those that are capable are expensive .

    Despite the shortage, the developers expect that the new optical repeaters will become the link that will finally allow to unite individual quantum computers and create a protected quantum Internet. According to the principles of quantum mechanics, when measuring the characteristics of a photon, it changes its state. If someone tries to overhear a quantum network, then this attempt will be immediately noticed and the photon will “collapse”.

    Note that with the advent of the quantum Internet, scientists of the world will have to solve a number of other problems. According to a recent study by employees of George Washington University, hackers can disrupt the transmission of information in quantum networks, “mixed” third-party traffic into the system of entangled photons. There is no protection against this type of attack yet, but engineers are planning to work in this direction.

    / Flickr / Nick Harris / CC BY-ND

    Что ещё делают для реализации квантового интернета

    A number of researchers are working on ion trap repeaters. New materials are being developed for them, for example, artificial diamonds , which are used to store and transfer qubits. Synthetic diamond can serve as a quantum repository due to a defect in the carbon lattice. In it, two carbon atoms are replaced by an arbitrary atom and "empty space".

    Work is also underway on error correction algorithms. They encode quantum states of photons so that if one or several particles are lost during signal transmission, information from them can be recovered. For error correction, several methods are suggested, for example , Shor's algorithm , Stein coding , the principle of quantum parity.other.

    All of the technologies presented are still in the early stages of development. Therefore, it is too early to say that some of them will definitely be used in the quantum Internet. However, test networks are already beginning to appear: in the next five years, stations with quantum repeaters are planned to be built in the UK. Probably, the initiative will be picked up by other countries.

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