Our everyday life could not be imagined without the internet. Actually, you are using the internet right now to display this website to read about *quantum networks*. ![[quantum_network.excalidraw.light.svg]] While the internet connects classical computers, quantum networks are supposed to connect different quantum devices, like a **quantum internet**. The internet is based on the exchange of classical information via glass fiber, i.e. we send [[Bit|bits]] by either sending a [[Photon|photon]] or no [[Photon|photon]]. In the case of [[Quantum Information|quantum information]], we can send information by photon well, but instead of using its existence, we use a quantum property: [[Polarization|polarization]]. Polarization can encode [[Quantum Information|quantum information]] and we can use the photons to transmit [[Qubit|qubits]] from one node of the network to the next. *Quantum networks* enable applications that are not possible with classical networks. For example, we can establish [[Quantum Key Distribution|secure cryptographic connections]] between remote parties, transmit information faster than with any classical connection, [[Quantum Teleportation|teleport]] quantum states from one node to the next or perform so-called [[Blind Quantum Computing|blind quantum computations]], i.e. computing on remote [[Quantum Computer|quantum computer]] without that computer knowing which problem it is solving. Building quantum networks, however, is far from easy. Since [[Quantum Information|quantum information]] cannot be copied due to the [[No-Cloning Theorem|no-cloning theorem]], we cannot just amplify signals like in a classical network: when we send information over a long distance through glass fiber, they start to disappear with increasing length. Glass fiber is not as transparent as we would with. In the classical case, that is not a bit problem. We can just detect them and send a 'fresh' photon: we copy the information. In the quantum case, that is not possible. The alternative scheme in the case of quantum networks is called [[Quantum Repeater|quantum repeaters]]. Instead of directly sending the [[Quantum Information|quantum information]] from one end of the network to the other, it is transmitted between way stations (the repeaters) and later connected. >[!read]- Further Reading > - [[Quantum Repeater]] > - [[No-Cloning Theorem]] > - [[Quantum Information]] >[!ref]- References > - H. J. Kimble, The quantum internet, Nature **453**, 1023 (2008). > - W. J. Munro, K. Azuma, K. Tamaki, and K. Nemoto, Inside Quantum Repeaters, IEEE Journal of Selected Topics in Quantum Electronics **21**, 78 (2015). > - S.-H. Wei et al., Towards Real-World Quantum Networks: A Review, Laser & Photonics Reviews **16**, 2100219 (2022).