play an important role in linking smaller ground-based networks over larger distances. Repeaters Long-distance communication is hindered by the effects of signal loss and decoherence inherent to most transport mediums such as optical fiber. In classical communication, amplifiers can be used to boost the signal during transmission, but in a quantum network amplifiers cannot be used since qubits cannot be copied – known as the no-cloning theorem. That is, to implement an amplifier, the complete state of the flying qubit would need to be determined, something which is both unwanted and impossible. Trusted repeaters An intermediary step which allows the testing of communication infrastructure are trusted repeaters. Importantly, a trusted repeater cannot be used to transmit qubits over long distances. Instead, a trusted repeater can only be used to perform quantum key distribution with the additional assumption that the repeater is trusted. Consider two end nodes A and B, and a trusted repeater R in the middle. A and R now perform quantum key distribution to generate a key . Similarly, R and B run quantum key distribution to generate a key . A and B can now obtain a key between themselves as follows: A sends to R encrypted with the key . R decrypts to obtain . R then re-encrypts using the key and sends it to B. B decrypts to obtain . A and B now share the key . The key is secure from an outside eavesdropper, but clearly the repeater R also knows . This means that any subsequent communication between A and B does not provide end to end security, but is only secure as long as A and B trust the repeater R. Quantum repeaters A true quantum repeater allows the end to end generation of quantum entanglement, and thus - by using quantum teleportation - the end to end transmission of qubits. In quantum key distribution protocols one can test for such entanglement. This means that when making encryption keys, the sender and receiver are secure even if they do not trust the quantum repeater. Any other application of a quantum internet also requires the end to end transmission of qubits, and thus a quantum repeater. Quantum repeaters allow entanglement and can be established at distant nodes without physically sending an entangled qubit the entire distance. In this case, the quantum network consists of many short distance links of perhaps tens or