Figure 1:
A schematic illustration of a process for realizing a long-distance quantum channel by using a quantum repeater. The first step is to create two pairs of entangled photons, using, for example, parametric down conversion. The quantum information in the photons in each pair is transferred to two atomic quantum memories. The two outer quantum memories can be entangled with measurements of Bell states between the two adjacent quantum memories at the center. This doubles the distance over which entanglement is established. Repeating this processâ€”ideally in a nested fashion for higher fidelityâ€”can increase the distance almost arbitrarily.