[[Entanglement]] is an important resource for [[Quantum Communication|quantum communication]]. It enables protocols like [[Blind Quantum Computing|blind quantum computing]] and [[Quantum Teleportation|quantum teleportation]]; but how do we entangle two parties, let's call them [[Alice and Bob]]. One option is that Alice and Bob meet, entangle two particles and leave again. While that is feasible, it somewhat defeats the purpose of communication technology: if you have to meet before sending an email, why send the email at all. We need a way to distribute [[Entanglement|entanglement]] without being in physical contact. Luckily, there are carriers of [[Quantum Information|quantum information]] that move with the [[Speed of Light|speed of light]], [[Photon|photons]]! They are already currently in use to transmit [[Classical Information|classical information]] through glas fibers, e.g. when you connect to the internet. However, sending [[quantum information]] over large distances is more difficult then sending [[Classical Information|classical information]]: the longer [[Light|light]] travels in fiber, the weaker it becomes. Glas fiber is not perfectly transparent. Think of the deep sea: below approximately $300\,\text{m}$, the sea is pitch black since water is not perfectly transparent. ![[quantum_repeater_decay.excalidraw.light.svg]] When dealing with [[Classical Information|classical information]] this loss of light is not a big deal. We just install an amplifier before the signal gets to weak. We measure what comes in and send it out again with more strength. This process requires the duplication of information. And that is the problem with [[Quantum Information|quantum information]]! While we can copy [[Classical Information|classical information]], we cannot duplicate [[Quantum Information|quantum information]]. The [[No-Cloning Theorem|no-cloning theorem]] forbids it. ![[quantum_repeater.excalidraw.light.svg]] Luckily, there is a solution for this problem in the form of *quantum repeaters*. Instead of entangling Alice and Bob directly, we build a chain of quantum repeaters in between the two and entangle neighboring repeaters. By performing local operations on the repeaters, we can swap the entanglement from entanglement between neighbors to [[Entanglement|entanglement]] between Alice and Bob. >[!read]- Further Reading > - [[Quantum Network]] > - [[Entanglement]] >[!ref]- References > - H.-J. Briegel, W. Dür, J. I. Cirac, and P. Zoller, Quantum Repeaters: The Role of Imperfect Local Operations in Quantum Communication, Phys. Rev. Lett. **81**, 5932 (1998) > - 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).