Towards long-distance quantum communication: Quantum repeaters and transportable quantum memories

The non-cloning theorem provides the unconditional security for quantum communication but forbids the use of classical amplifiers. Therefore, the inevitable channel loss prevents the long-distance quantum communication. Protocols involving quantum memories can overcome this problem through the approach of quantum repeaters and transportable quantum memories. Based on rare-earth-ion doped crystals, we recently demonstrate an elementary link of a quantum repeater based on absorptive quantum memories and extend the optical storage time to 1 h. Here we provide a brief introduction to quantum communication, quantum repeaters and transportable quantum memories. Based on this, we review our recent achievements in this field.