Large quantum networks

Quantum networks that allow generating entangled states between distant qubits have enormous scientific and applied potential. They can be used for secure quantum cryptography and the teleportation of quantum states between cities and countries, in high-resolution astronomy, and in distributed quantum computing. The scattering of photons in an optical fiber and the difficulties in creating full-fledged quantum nodes impede the construction of large quantum networks. We review current approaches to the creation of such networks, with the emphasis on quantum repeaters intended for ‘compensating’ losses in optical fibers. We also discuss methods for increasing the range of quantum cryptography systems without using quantum repeaters.